HK1081049B - Process for fabricating circuit assemblies using electrodepositable dielectric coating compositions - Google Patents

Description

Method for making circuit assemblies with electrodepositable dielectric coating compositions

Technical Field

The present invention relates to methods of forming metallized vias and making multilayer circuit assemblies including dielectric coatings, particularly dielectric coatings applied by electrodeposition.

Background

Electrical components such as resistors, transistors, and capacitors are commonly mounted on circuit panel structures such as printed circuit boards. Circuit panels typically comprise a generally planar sheet of dielectric material having electrical conductors disposed on one or both major planar surfaces of the sheet. The conductor is generally made of a metal material such as copper or the like and is used for connecting electrical components mounted on a circuit board. When the conductors are on both major surfaces of the panel, the panel may have via conductors extending through holes (or vias) in the dielectric layer to connect the conductors on the opposite surfaces. Multi-layer circuit panel assemblies have heretofore been prepared that include a multi-layer stacked circuit panel in which additional layers of dielectric material separate the conductors on the mutually facing surfaces of adjacent panels in the stack. These multi-layer assemblies typically include interconnections extending between conductors on the various circuit panels in the stack as needed to provide the desired electrical connections.

In microelectronic circuit wiring packages, wires and units are fabricated at a high-specification packaging level. Typically, the minimum specification package level is typically a semiconductor chip enclosing a plurality of microcircuits and/or other components. These chips are typically made of ceramic, silicon, or the like. An intermediate package level (i.e., "chip carrier") comprising a multi-layer substrate may have attached thereto a plurality of small-scale chips encapsulating a plurality of microelectronic circuits. Furthermore, these intermediate package levels themselves may be connected to larger format line cards, motherboards, etc. The intermediate package level serves a number of purposes throughout the circuit assembly, including structural support, conversion of small scale microcircuits and circuits into larger scale boards, and heat dissipation from the circuit assembly. Substrates used at conventional intermediate package levels include a variety of materials such as ceramics, glass fiber reinforced polyepoxides, and polyimides.

The above-described substrates, while providing sufficient rigidity to provide structural support to the circuit assembly, typically have a coefficient of thermal expansion that is quite different from the microelectronic die to which they are attached. As a result, there is a risk of failure of the circuit assembly after repeated use due to failure of the adhesive points between the assembly layers.

In addition, dielectric materials used on substrates must meet a number of requirements including conformability, flame retardancy, and compatible thermal expansion properties, with conventional dielectric materials including, for example, polyimides, polyepoxides, phenolic resins, and fluorocarbons. These polymeric dielectric materials generally have a coefficient of thermal expansion that is higher than the coefficient of thermal expansion of the adjacent layers.

There is a continuing need to provide high density, complex interconnected circuit panel structures. This need can be addressed by multi-layer circuit panel structures, however, there are serious disadvantages to producing such multi-layer circuit panels.

Typically, multi-layer panels are prepared by providing a single double-sided circuit panel that includes appropriate conductors. The panels are then laminated on top of each other with one or more layers of uncured or partially cured dielectric material (commonly referred to as "prepregs") between each pair of adjacent panels. The stack is typically cured under heat and pressure to form a single block. After curing, holes are typically drilled through the stack at locations where electrical connections between different boards are desired. The resulting hole or "via" is then coated or filled with a conductive material, typically by plating the inner walls of the hole to form a plated via. Drilling holes with high depth to diameter ratios is difficult, and therefore the holes used in these assemblies must be quite large and consume a lot of space in the assembly.

US 6,266,874B 1 discloses a method of making a microelectronic component: providing a conductive substrate or "core"; disposing a resist at a selected location on the conductive core; and electrophoretically depositing an uncured dielectric material on the conductive core except where it is covered by the resist. The document suggests that the electrophoretic deposition material may be a cationic acrylic-or cationic epoxy-based composition such as is known to those skilled in the art and is commercially available. The electrophoretically deposited material is then cured to form a conformal dielectric layer, and the resist is then subsequently removed so that the dielectric layer has openings extending to the conductive core at a location covered by the resist. The holes so formed and extending to the coated substrate or "core" are commonly referred to as "blind holes". In one embodiment, the structural conductive element is a metal sheet comprising continuous through holes from one major surface to the opposite major surface. When the dielectric material is applied by electrophoresis, the dielectric material is deposited to a uniform thickness on the conductive element surfaces and the hole walls. However, it has been found that the electrophoretically deposited dielectric materials suggested by these references can be flammable and therefore do not meet general flame retardant requirements.

US 5,224,265 and 5,232,548 disclose methods of making multilayer thin film wire structures for circuit assemblies. The dielectric material to be applied to the core substrate is a fully cured and annealed thermoplastic polymer such as polytetrafluoroethylene, polysulfone or polyimide-siloxane, preferably applied by lamination.

US 5,153,986 discloses a method of preparing a metal core layer for a multilayer circuit board. Suitable dielectric materials include vapor-depositable conformal polymer coatings. The method uses a perforated solid metal core, which reference describes the general circuit arrangement (circuitization) of the substrate.

US4,601,916 suggests that deposition of an insulating coating directly onto the metal wall portions of the holes can form a uniform film of resin on the hole walls without resulting in thinning of the coating at the top and bottom edges of the holes, subsequent metal deposition without adhesion to the hole walls, further having insufficient electrical insulating properties. Accordingly, this document relates to an improved method of forming plated through holes in metal core printed circuit boards by electrophoretically depositing a coating thereon, said coating comprising an electrodepositable resinous coating containing a solid inorganic filler in the form of a fine powder. Suitable fillers include clays, silica, alumina, silicates, clays, and the like. The composition exhibits a volume resistance between the printed circuit conductor and the metal core of greater than 10⁴Mega ohm-cm. The method comprises electrophoretically depositing the above composition onto the metal wall portions of the pores; curing the resin coating; it has a thickness of at least 0.025 mm; forming a hydrophilic microetched surface on the coating with an aqueous oxide solution to promote adhesion; a metal layer is deposited on the surface of the resin coating on the hole walls and on the insulating surface layer, the metal layer adhering to the coating with a prescribed peel strength, and a printed circuit is formed on the insulated metal substrate by a standard printed circuit process.

US4,238,385 discloses a coating composition for electrophoretic application to a conductive substrate for printed circuits. The composition comprises a finely divided pigment-containing synthetic resin powder wherein the resin comprises an epoxy resin and the pigment comprises 2 to 10 parts by weight finely divided silica in admixture with a cationic resin. The composition forms an insulating film on a conductive substrate suitable for providing printed circuits with desired properties such as dimensional stability and mechanical strength.

The circuit layout at the intermediate package level is generally performed by: a positive-or negative-working photoresist (hereinafter collectively referred to as "resist") is applied to the metallized substrate, exposed, developed, etched, and stripped to form the desired circuit pattern. The resist composition is typically applied by lamination, spray coating or dipping techniques. The resist layer thus coated has a thickness of 5 to 50 micrometers.

In addition to the above substrates, conventional substrates for intermediate packaging levels may also include solid metal sheets, such as those disclosed in US 5,153,986. These solid structures must be perforated during circuit assembly fabrication to provide vias for alignment purposes.

Based on the prior art approaches, there remains a need in the art for a multi-layer circuit panel structure that provides high density and complex interconnections, and a method of making that overcomes the disadvantages of the prior art circuit assemblies.

Disclosure of Invention

In one embodiment, the present invention is directed to a method of forming a metallized via in a substrate. The method comprises the following steps: (I) electrophoretically coating an electrically conductive substrate to electrophoretically coat an electrodepositable coating composition onto all exposed surfaces of the substrate to form a conformal dielectric coating thereon, the electrodepositable coating composition comprising a resinous phase dispersed in an aqueous phase, the resinous phase comprising (a) an ungelled active hydrogen ion group-containing resin; and (b) a curing agent reactive with the active hydrogens of resin (a), the resin phase having a covalently bonded halogen content of at least 1 weight percent, based on the total weight of resin solids present in the resin phase; (II) etching the conformal dielectric coating surface in a predetermined pattern to expose a cross-section of the substrate; and (III) applying a layer of metal on all surfaces to form a metallized via in the substrate.

In another embodiment, the present invention is directed to a method of making a circuit assembly comprising the steps of: (I) providing a conductive core; (II) electrophoretically coating the electrodepositable coating composition described above onto all exposed surfaces of the core to form a conformal dielectric coating thereon; (III) etching the conformal dielectric coating surface in a predetermined pattern to expose a core cross-section; (IV) applying a layer of metal on all surfaces to form metallized holes in the core; and (V) applying a resinous photosensitive layer to the metal layer.

The invention also relates to a method of manufacturing a circuit assembly comprising the steps of: (I) providing a conductive core; (II) disposing a photoresist at a predetermined position on the surface of the core; (III) electrophoretically coating the electrodepositable coating composition onto the core of step (II), wherein the coating composition is electrophoretically deposited onto all surfaces of the core except for the region having the photoresist thereon; (IV) curing the electrocoated coating composition to form a cured conformal dielectric layer on all surfaces of the core except for the areas having the photoresist thereon; (V) removing the photoresist to form a circuit assembly having a via extending to the core at the location covered by the resist; and (VI) optionally applying a layer of metal to all surfaces of the circuit assembly of step (V), thereby forming metallized vias extending to the core.

The invention further relates to substrates and circuit components coated by the respective above-described methods.

Detailed Description

Other than in the operating examples, or where otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term "about". Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the present invention. Each parameter should at least be construed based on the numerical value of the stated significant digit and based on conventional rounding techniques, at the very least and not as an attempt to limit the application to the doctrine of equivalents to the scope of the claims.

Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements.

Moreover, it should be understood that any numerical range recited herein is intended to include all sub-ranges subsumed therein, for example, the range "1 to 10" is intended to include all sub-ranges therebetween and includes the lowest value described as 1 and the highest value described as 10, i.e., having a minimum value equal to or greater than 1 and a maximum value equal to or less than 10.

As noted above, in one embodiment, the present invention is directed to a method of forming a metallized via in a substrate. The method comprises the following steps: (I) electrophoretic coating of a conductive substrate: electrophoretically applying an electrodepositable coating composition (described in detail below) to all surfaces of the substrate to form a conformal dielectric coating thereon; (II) etching the surface of the dielectric coating in a predetermined pattern to expose a cross-section of the substrate; and (III) applying a layer of metal to all surfaces of the substrate of step (II) to form metallized holes in the substrate. Optionally, the method further comprises the step (IV) of applying a photosensitive layer described below on the metal layer.

In a further embodiment, the present invention relates to a method of making a multilayer circuit assembly. In one embodiment, the present invention relates to a method of making a circuit assembly comprising the steps of: (I) providing a conductive core, typically a metal core as described below; (II) electrophoretically coating the electrodepositable coating composition described above onto all exposed surfaces of the core to form a conformal dielectric coating thereon; (III) etching the conformal dielectric coating surface in a predetermined pattern to expose a core cross-section; (IV) applying a layer of metal, such as copper, on all surfaces to form metallized holes in the core; and (V) applying a resinous photosensitive layer to the metal layer.

The substrate or core may comprise any of a variety of electrically conductive substrates, particularly metal substrates, such as untreated or galvanized steel, aluminum, gold, nickel, copper, magnesium, or alloys of any of the foregoing, as well as electrically conductive carbon coating materials. The core furthermore has two main surfaces and edges and may have a thickness of 10 to 100 microns, typically 25 to 100 microns.

It will be understood that for purposes of the method of the present invention, forming metallized holes "in the core" will include forming "through holes" (i.e., the formed metallized holes extend from the primary surface through the core to the other surface) to thereby provide a through connection, and forming "blind holes" (i.e., the formed metallized holes extend only through the dielectric coating, but not through the core) to provide a connection, for example, to ground or a power source. Further, for the present invention, forming metallized holes that extend "through the core" is intended to include forming only through holes. Also, forming metallized holes that extend "to the core" includes forming only blind holes.

In a particular embodiment of the invention, the core is a metal matrix selected from an open-pored copper foil, an iron-nickel alloy or a combination thereof. In one embodiment of the invention, the core comprises an iron-nickel alloy containing about 64 wt% iron and 36 wt% nickel, commercially available as INVAR (trade mark of Imphy s.a., 168 Rue de Rivoli, Paris, France). The alloy has a low coefficient of thermal expansion that is comparable to the coefficient of thermal expansion of silicon materials typically used to make chips. This property is desirable for the adhesive joints between successive larger or smaller scale layers of a chip scale package to prevent failure due to thermal cycling during normal use. When a nickel-iron alloy is used as the metal core, a layer of metal, usually copper, is applied to all surfaces of the nickel-iron alloy core to ensure optimum electrical conductivity. The metal layer, and the layer applied in step (IV), may be applied in a conventional manner, for example by electroplating, chemical deposition and metal vapour deposition techniques, typically having a thickness of 1 to 10 microns.

"perforated metal core" means a mesh sheet having a plurality of holes arranged at regular intervals. The diameter of the pores is typically about 200 microns, but may be as large or small as desired, so long as the diameter is large enough to fit all layers applied in the method of the invention without filling the pores. The center-to-center spacing of the holes is typically about 500 microns, but may be as large or small as desired. The cell density can be 500 to 10,000 cells per square inch (77.5 to 1,550 cells per square centimeter).

Any of the electrodepositable coating compositions described in detail below can be electrophoretically applied to the conductive core. The voltage applied for electrodeposition can vary and can be, for example, as low as 1 volt up to several thousand volts, but is typically 50 to 500 volts. The current density is typically 0.5 to 5 amps per square foot (0.5 to 5 milliamps per square centimeter) and tends to decrease during electrodeposition, indicating the formation of an insulating conformal thin film on all exposed surfaces of the core. As used herein and in the specification and claims, a "conformal" film or coating means that the film or coating has a substantially uniform thickness that conforms to the topography of the substrate surface, including the surface (but preferably not closed) within the pores. After the coating is applied by electrodeposition, it is thermally cured, typically at an elevated temperature of 90 to 300 ℃ for 1 to 40 minutes to form a conformal dielectric coating on all exposed surfaces of the core.

It is to be understood that for the process of the present invention, any of the electrodepositable coating compositions (described in detail below) can be applied by various coating techniques other than electrodeposition, such as by roll coating or spray coating techniques, which are well known in the art. In such cases, it may be desirable to prepare compositions having higher resin solids content. Further, for these coatings, the resin binder may or may not include solvating or neutralizing acids and amines to form cationic and anionic salt groups, respectively.

The dielectric coating has a uniform thickness and often does not exceed 50 microns, typically does not exceed 25 microns, and typically does not exceed 20 microns. Lower film thicknesses are also suitable for various reasons. For example, dielectric coatings with lower film thicknesses can be used for small scale currents. In addition, a coating with a low dielectric constant (as discussed above) allows the dielectric coating to have a lower film thickness and also minimizes capacitive coupling between adjacent signal traces.

One skilled in the art will recognize that the core surface may be pretreated or prepared to be coated with a dielectric layer prior to electrophoretic coating of the dielectric coating. For example, it may be suitable to wash, rinse and/or treat with an adhesion promoter prior to applying the dielectric layer.

After the dielectric coating is applied, the surface of the dielectric coating is etched in a predetermined pattern to expose one or more pore sections. The etching is typically performed with a laser or by conventional techniques such as mechanical drilling and chemical or plasma etching techniques.

Metallization is performed after the etching step by applying a metal layer to all surfaces, forming metallized holes in the core. Suitable metals include copper or any metal or alloy having sufficient conductive properties. The metal is typically applied by electroplating or any other suitable method known in the art to provide a uniform metal layer. The thickness of the metal layer may be 1 to 50 microns, typically 5 to 25 microns.

To enhance the adhesion of the metal layer to the dielectric polymer, prior to the metallization step, all surfaces may be treated with ion beam, electron beam, corona discharge, or plasma bombardment followed by application of an adhesion promoter to all surfaces. The adhesion promoter layer may be 50 to 5000 angstroms thick and is typically a metal or metal oxide selected from chromium, titanium, nickel, cobalt, cesium, iron, aluminum, copper, gold, tungsten, and zinc, and alloys and oxides thereof.

After metallization, a resinous photosensitive layer (i.e., a "photoresist" or "resist") may be applied over the metal layer. Optionally, prior to coating the photoresist, the metallized substrate may be cleaned and/or pretreated, for example, treated with an acid etchant to remove oxidized metal. The resinous photosensitive layer may be a positive-or negative-tone photoresist. The photoresist layer may have a thickness of 1 to 50 microns, typically 5 to 25 microns, and may be applied by any method known to those skilled in the art of photolithographic processing. Increased or decreased processing methods may be used to form the desired circuit pattern.

Suitable positive-working photosensitive resins include any known to those skilled in the art. Examples include dinitrobenzyl functional polymers such as those described in US 5,600,035, columns 3-15. These resins have high photosensitivity. In one embodiment, the resinous photosensitive layer is a composition comprising a dinitrobenzyl functional polymer, typically applied by spray coating.

In another embodiment, the resinous photosensitive layer comprises an electrodepositable composition comprising a dinitrobenzyl-functional polyurethane and an epoxy-amine polymer, as described in examples 3-6 of US 5,600,035.

Negative-acting photoresists include liquid or dry film type compositions. Any of the foregoing liquid compositions may be coated by spray coating, roll coating, spin coating, curtain coating, screen coating, dip coating, or electrodeposition coating techniques.

In one embodiment, the liquid photoresist is coated by electrodeposition, typically by cationic electrodeposition. Electrodepositable photoresist compositions comprise an ionic polymeric material (which may be cationic or anionic) and may be selected from polyesters, polyurethanes, acrylics, and polyepoxides. An example of a photoresist applied by anionic electrodeposition is given in US3,738,835. Photoresists applied by cationic electrodeposition are described in US4,592,816. Examples of dry film photoresists include those disclosed in US3,469,982, 4,378,264 and 4,343,885. Dry film photoresists are typically laminated to a surface, for example, by hot roll coating.

It is noted that after the photosensitive layer is applied, the multilayer substrate can be packaged at the site where transportation is performed and packaged at a remote location for any subsequent steps.

In one embodiment of the present invention, after the photosensitive layer is coated, a photomask having the desired pattern can be placed on the photosensitive layer and the layered substrate exposed to a sufficient amount of a suitable radiation source, typically an actinic radiation source. The term "sufficient amount of radiation" as used herein means an amount of radiation that will polymerize the monomer in the exposed areas of the radiation (for a negative-working resist), or depolymerize the polymer or render the polymer more soluble (for a positive-working resist). This results in a different solubility between the radiation exposed and the radiation masked areas.

The photo-masking layer may be removed after exposure to a radiation source and the layered substrate developed with a conventional developing solution to remove the more soluble portions of the photosensitive layer and expose selected areas of the underlying metal layer. The exposed metal is then etched with a metal etchant which converts the metal to a water-soluble metal complex. The soluble complex is removed by water spraying.

The photosensitive layer protects the underlying substrate during the etching step. The remaining photosensitive layer, which is impervious to the etchant, is then removed by a chemical lift-off process, thereby providing a circuit pattern connected by metallized holes.

After the circuit pattern is prepared on the multi-layer substrate, other circuit components may be connected to form a circuit assembly. Additional components include, for example, one or more small scale components such as semiconductor chips, interposer layers, large scale circuit cards or motherboards, and active or passive components. It is noted that an interposer for use in the preparation of a circuit assembly may be prepared by suitable steps of the method of the present invention. These components are prepared using conventional adhesives, surface mount techniques, wire bonding or flip chip techniques. The high hole density in the multilayer circuit assemblies prepared by the present invention can allow for more electrical connections from the high-function chips to the packages in the assembly.

In another embodiment, the present invention relates to a method of making a circuit assembly comprising the steps of: (I) providing a core, such as any of the metal cores described above; (II) disposing a photoresist, such as any of the above photoresist compositions, at a predetermined location on the surface of the core; (III) electrophoretically applying to the core of step (II) any of the electrodepositable coating compositions described in detail below, wherein the coating composition is electrophoretically deposited on all surfaces of the core except for the regions having the photoresist thereon; (IV) curing the electrocoated coating composition under the curing conditions described above to form a cured conformal dielectric layer on all surfaces of the core except the areas having the photoresist thereon; (V) removing the photoresist to form a circuit assembly having a hole extending to the metal core at the location covered by the resist; and (VI) optionally applying a layer of metal, typically copper metal, to all surfaces of the circuit component of step (V) by any of the foregoing metallization methods, thereby forming metallized holes extending to the core. In one embodiment of the invention, a layer of metal, typically copper, is applied to the metal core before the resist of step (II) is provided at predetermined locations on the surface of the metal core.

The electrodepositable coating composition suitable for use in any of the foregoing processes comprises a resinous phase dispersed in an aqueous medium. The resinous phase comprises (a) ungelled resin containing active hydrogen-containing ionic salt groups; and (b) a curing agent that reacts with the active hydrogens of the resin (a).

In one embodiment of the invention, the resinous phase has a covalently bonded halogen content, based on the total weight of resin solids present in said resinous phase, such that when the composition is electrodeposited and cured to form a cured film, the cured film passes the flame retardant test according to IPC-TM-650, and the resinous phase has a dielectric constant of less than or equal to 3.50. It is to be understood that for the purposes of the present invention, "covalently bonded halogen" refers to a covalently bonded halogen atom as opposed to a halide ion, such as chloride ion in an aqueous solution.

The resinous phase of the electrodepositable coating composition of the present invention may have a covalently bonded halogen content of at least 1 weight percent, typically at least 2 weight percent, often at least 5 weight percent, and typically at least 10 weight percent. Meanwhile, the resinous phase of the electrodepositable coating composition of the present invention may have a covalently bonded halogen content of less than 50 weight percent, typically less than 30 weight percent, often less than 25 weight percent, and typically less than 20 weight percent. The resinous phase of the electrodepositable coating composition may have a covalently bonded halogen content ranging between any combination of these values, inclusive of the recited values, so long as the covalently bonded halogen content is sufficient to provide a flame retardant test according to IPC-TM-650, as described above.

Further, it is noted that the covalently bonded halogen content of the resinous phase can be derived from a halogen atom covalently bonded to one or both of the resin (a) and the curing agent (b), or a halogen atom covalently bonded to a compound (c) that is different from the resin (a) and the curing agent (b) and is present as a component in the electrodepositable coating composition in addition to the resin (a) and (b).

As described above, for the present invention, flame retardance was measured according to IPC-TM-650 Test, Test methods Manual, Number 2.3.10, "flexibility of amine", Revision B, available from Institute of Interconnecting and Packaging electronics circuits, 2215 Sanders Road, Northbrook, Ill.

When the above electrodepositable coating composition is electrophoretically deposited and cured to form a cured film (described in detail below), the cured film can have a dielectric constant of no greater than 3.50, often no greater than 3.30, typically no greater than 3.00, and generally no greater than 2.80. In addition, the cured film typically has a dielectric loss factor of less than or equal to 0.02, typically less than or equal to 0.15, and may be less than or equal to 0.01.

The dielectric substance is a non-conductive substance or an insulator. The "dielectric constant" is an index or measure of the ability of a dielectric substance to store charge. The dielectric constant is directly proportional to the capacitance of the substance, which means that if the dielectric constant of the material is reduced, the capacitance value is reduced. Low dielectric materials are desirable for high frequency, high speed digital applications where the capacitance of the substrate and coating is important for reliable function of the circuit. For example, current computer operation is limited by the coupling capacitance between circuit paths on the multilayer assembly and the integrated circuits, as the computational speed between the integrated circuits is reduced by this capacitance and the power required to run increases. See Thompson, Larry F., et al, Polymers for Microelectronics, presented at the203rd national Meeting of American Chemical Society, April 5-10, 1992.

The "dielectric loss factor" is the power dissipated by the dielectric substance because the friction of the molecules of the dielectric substance opposes the molecular motion created by the alternating electric field. See, I.Gilleo, Ken, Handbook of Flexible Circuits, at p.242, VanNostrandReinhold, New York (1991). For a detailed discussion of the dielectric materials and constants, see also James J.Licari and LauraA.Hughes, Handbook of Polymer coatings for Electronics, pp.114-18, 2nd ed., Noyes Publication (1990).

For the purposes of the present invention, the dielectric constant of the cured electrodepositable coating composition is determined using electrochemical impedance spectroscopy at a frequency of 1 megahertz as follows.

The coated samples were prepared by applying an electrodepositable composition to a steel substrate, followed by curing to provide a cured dielectric coating having a film thickness of 0.85mil (20.83 micrometers). The 32 cm square free film of the cured dielectric coating was placed in an electrochemical cell containing 150 ml of electrolyte solution (1M NaCl) and allowed to equilibrate for 1 hour. 100mV AC potential was applied to the sample and the impedance was measured at 1.5 MHz to 1 Hz. The method uses a drawn mesh counter electrode of platinum on niobium and a single-junction silver/silver chloride reference electrode. The dielectric constant of the cured coating was determined by calculating the capacitance at 1 megahertz, 1 kilohertz, and 63 hertz and solving the following equation for E:

C＝EoEA/d

where C is the measured capacitance (in farads) at the discontinuous frequency; eo is the vacuum permittivity (8.854187817)¹²) (ii) a A is the sample area (32 square centimeters); d is the thickness of the coating; e is the dielectric constant. It should be noted that the dielectric constant values used in the specification and claims are the dielectric constant as determined at a frequency of 1 megahertz as described above. The dielectric loss factor as used in the specification and claims refers to the difference between the dielectric constant measured at a frequency of 1 mhz as described above and the dielectric constant of the same material measured at a frequency of 1.1 mhz.

The electrodepositable coating composition useful in the process of the present invention comprises an ungelled, active hydrogen-containing, ionic group-containing electrodepositable resin (a) as the primary film-forming precursor. Various electrodepositable film-forming polymers are known and can be used in the electrodepositable coating compositions of the present invention, so long as the polymer is "water dispersible," i.e., suitable for solvation, dispersion or emulsification in water. The water-dispersible polymer is itself an ionic polymer, i.e., the polymer may contain anionic functional groups that impart a negative charge or cationic functional groups that impart a positive charge. In one embodiment of the invention, the resin (a) comprises cationic salt groups, typically cationic amine salt groups.

By "ungelled" is meant that the resin is substantially uncrosslinked and has an intrinsic viscosity value, for example as determined in accordance with A STM-D1795 or ASTM-D4243, when dissolved in a suitable solvent. The intrinsic viscosity of the reaction product is an indication of its molecular weight. Gelled reaction products, on the other hand, have an intrinsic viscosity too high to measure, due to their essentially infinitely high molecular weight. As used herein, a "substantially non-crosslinked" reaction product refers to a reaction product having a weight average molecular weight (Mw) of less than 1,000,000 (as determined by gel permeation chromatography).

The term "polymer" as used herein refers to oligomers as well as homopolymers and copolymers. Molecular weight unless otherwise indicated, the number average molecular weight of the polymeric material, denoted as "Mn", was obtained by gel permeation chromatography using styrene as a standard in a manner known in the art.

Non-limiting examples of film-forming resins suitable for use as resin (a) in the anionic electrodepositable coating composition include alkali-solvating carboxylic acid group-containing polymers, such as the reaction products or adducts of drying oils or semi-drying fatty acid esters with dicarboxylic acids or anhydrides; and the reaction product of a fatty acid ester, an unsaturated acid or anhydride and any additional unsaturated modifying material that is further reacted with a polyol. Also suitable are hydroxyalkyl esters of unsaturated carboxylic acids, at least partially neutralized interpolymers of unsaturated carboxylic acids with at least one other ethylenically unsaturated monomer. Other suitable electrodepositable resins include alkyd-aminoplast vehicles, i.e., vehicles comprising an alkyd resin and an amine-aldehyde resin. Another suitable anionic electrodepositable resin composition comprises mixed esters of resinous polyhydroxy compounds, such compositions being described in detail in US3,749,657, columns 9, lines 1-75 and column 10, lines 1-13, which are incorporated herein by reference in their entirety. Other acid functional polymers may also be used, such as phosphorylated polyepoxides or phosphorylated acrylic polymers and the like known to those skilled in the art. In addition, those resins that include one or more pendant carbamate functional groups, such as those described in US 6,165,338, are also suitable for use as the resin of resin (a).

In one embodiment of the present invention, the active hydrogen-containing ionic electrodepositable resin (a) is a cationic resin and is capable of being deposited on a cathode. Non-limiting examples of such cationic film-forming resins include amine salt group-containing resins, such as acid-solubilized reaction products of polyepoxides with primary or secondary amines, such as US3,663,389; 3,984,299; 3,947,338 and 3,947,339. Typically these amine salt group-containing resins are used in combination with a blocked isocyanate curing agent as will be described in detail below. The isocyanate may be fully blocked as described in US3,984,299 above, or the isocyanate may be partially blocked and reacted with the resin backbone, as described in US3,947,338. Further, one-component compositions as described in U.S. Pat. No. 4,134,866 and DE-OS No.2,707,405 may be used as the resin (a) in the electrodepositable coating composition of the present invention. In addition to the epoxy-amine reaction products discussed immediately above, resin (a) may be selected from cationic acrylic resins such as those described in U.S. Pat. Nos. 3,455,806 and 3,928,157.

In addition to the amine salt group-containing resin, a quaternary ammonium salt group-containing resin may also be used. Examples of such resins include those formed from the reaction of an organic polyepoxide with a tertiary amine salt, such resins are described in US3,962,165; 3,975,346 and 4,001,101. Examples of other cationic resins are tertiary sulfonium salt group-containing resins and quaternary phosphonium salt group-containing resins, such as those described in U.S. Pat. Nos. 3,793,278 and 3,984,922, respectively. Film-forming resins which cure by transesterification reactions, such as those described in European patent application 12463, may also be used. Furthermore, cationic compositions prepared from mannich bases can be used, as described in US4,134,932.

In one embodiment of the present invention, resin (a) may comprise one or more positively charged resins containing primary and/or secondary amine groups. These resins are described in US3,663,389; 3,947,339 and 4,116,900. In US3,947,339, polyketimine derivatives of polyamines such as diethylenetriamine or triethylenetetramine and the like are reacted with polyepoxides. When the reaction product is neutralized with an acid and dispersed in water, free primary amine groups are generated. The same product is also formed when the polyepoxide is reacted with an excess of polyamines such as diethylenetriamine and triethylenetetramine and the excess of polyamine is vacuum stripped from the reaction mixture. Such products are described in US3,663,389 and 4,116,900.

It may also be advantageous to use mixtures of the above mentioned ionic resins. In one embodiment of the invention, resin (a) comprises a polymer having cationic salt groups and is selected from polyepoxide-based polymers having primary, secondary and/or tertiary amine groups (such as those described above) and acrylic polymers having hydroxyl and/or amine functional groups.

As noted above, in one embodiment of the present invention, resin (a) includes cationic salt groups. In such cases, such cationic salt groups are typically formed by solvating the resin with an inorganic or organic acid such as those typically used in electrodepositable compositions. Suitable examples of solvating acids include, but are not limited to, sulfamic acid, acetic acid, lactic acid, and formic acid. Sulfamic acid and lactic acid are the most commonly used.

Further, as described above, the covalently bonded halogen content of the resin phase of the electrodepositable coating composition may be derived from halogen covalently bonded to the resin (a). In this case, these covalently bonded halogen contents can be attributed to the reactants used to form any of the film-forming ionic resins described above. For example, for anionic group-containing polymers, the resin may be the reaction product of a halogenated phenol, such as a halogenated polyhydric phenol (e.g., chlorinated or brominated bisphenol a, etc.), reacted with an epoxy compound, and then reacted with phosphoric acid, or the reaction product obtained by reacting an epoxy compound with a halogenated carboxylic acid, followed by reacting any residual epoxy groups with phosphoric acid. The acid group may then be solvated with an amine. Similarly, for cationic salt group-containing polymers, the resin can be the reaction product of a diglycidyl ether of bisphenol a reacted with a halogenated phenol, followed by reaction of any residual epoxy groups with an amine. The reaction product may then be solubilized with an acid.

In one embodiment of the invention, the covalently bonded halogen content of resin (a) may be derived from halogenated phenols, trichlorobutylene oxide and mixtures thereof. In another embodiment of the invention, the covalently bonded halogen content of resin (a) is derived from halogenated polyhydric phenols, for example chlorinated bisphenol a such as tetrachlorobisphenol a and the like, or brominated bisphenol a such as tetrabromobisphenol a and the like. In another embodiment of the invention, the covalently bonded halogen content of resin (a) may be derived from a halogenated epoxy compound, such as a diglycidyl ether of halogenated bisphenol a.

The active hydrogen-containing ionic electrodepositable resin (a) described above can be present in the electrodepositable coating composition of the present invention in an amount of from 5 to 90 weight percent, typically from 10 to 80 weight percent, often from 10 to 70 weight percent, and typically from 10 to 30 weight percent, based on the total weight of the electrodepositable coating composition.

As noted above, the resinous phase of the electrodepositable coating composition of the present invention further comprises (b) a curing agent suitable for reacting with the active hydrogens of the ionic electrodepositable resin (a) described immediately above. Both blocked organic polyisocyanates and aminoplast curing agents are suitable for use in the present invention, although blocked isocyanates are typically used for anodic electrodeposition.

Aminoplast resins, which are commonly used as curing agents for anionic deposition, are the polycondensation products of amines or amides with aldehydes. Examples of suitable amines or amides are melamine, benzoguanamine, urea and similar compounds. The aldehyde used is typically formaldehyde, although the product may be prepared from other aldehydes such as acetaldehyde and furfural. The polycondensation product contains methylol or similar hydroxyalkyl groups depending on the particular aldehyde used. These methylol groups are preferably etherified by reaction with an alcohol. The various alcohols used include monohydric alcohols containing from 1 to 4 carbon atoms, such as methanol, ethanol, isopropanol and n-butanol, with methanol being preferred. Amino resins are available under the trademark CYMEL from American Cyanamid Co and under the trademark RESIMENE from Monsanto chemical Co.

Aminoplast curing agents are generally used in combination with the active hydrogen-containing anionic electrodepositable resin in amounts of from about 1 to 90 percent by weight, often from 5 to 60 percent by weight, and preferably from 20 to 40 percent by weight, based on the total weight of resin solids in the electrodepositable coating composition.

Curing agents commonly used in cathodic electrodeposition compositions are blocked polyisocyanates. The polyisocyanate may be fully blocked, as described in U.S. Pat. No. 3,984,299 column 1 lines 1 to 68, column 2 and column 3 lines 1 to 15, or partially blocked and reacted with the polymer backbone, as described in U.S. Pat. No. 3,947,338 column 2 lines 65 to 68, column 3 and column 4 lines 1 to 30, which are incorporated herein by reference. By "blocked" is meant that the isocyanate groups have been reacted with a compound such that the resulting blocked isocyanate groups are stable to active hydrogens at ambient temperatures, but react with the active hydrogens in the film-forming polymer at elevated temperatures, typically 90 to 200 ℃.

Suitable polyisocyanates include aromatic and aliphatic polyisocyanates, including cycloaliphatic polyisocyanates, representative examples include diphenylmethane 4,4 ' -diisocyanate (MDI), 2, 4-or 2, 6-Toluene Diisocyanate (TDI), including mixtures thereof, p-phenylene diisocyanate, butylene and hexylene diisocyanate, dicyclohexylmethane 4,4 ' -diisocyanate, isophorone diisocyanate, mixtures of phenylmethane 4,4 ' -diisocyanate and polymethylene polyphenylisocyanate. Higher polyisocyanates, such as triisocyanates, can be used. One example includes triphenylmethane-4, 4', 4 "-triisocyanate. Isocyanate prepolymers with polyols such as neopentyl glycol and trimethylolpropane and with polymeric polyols such as polycaprolactone diols and triols (NCO/OH equivalent ratio greater than 1) may also be used.

The polyisocyanate curing agent is generally used in combination with the active hydrogen-containing cationic electrodepositable resin (a) in an amount of from about 1 to 90 percent by weight, typically from 1 to 80 percent by weight, often from 1 to 70 percent by weight, and typically from 1 to 15 percent by weight, based on the total weight of the electrodepositable coating composition.

Beta-hydroxyurethane curatives such as those described in US4,435,559 and 5,250,164 are also suitable. These beta-hydroxy urethane curatives are formed from an isocyanate compound such as any of the 1, 2-polyols described immediately above and/or a conventional blocking agent such as a monohydric alcohol. Secondary amine blocked aliphatic and cycloaliphatic isocyanates as described in U.S. Pat. Nos. 4,495,229 and 5,188,716 are also suitable.

As noted above, in one embodiment of the invention, the covalently bonded halogen content of curing agent (b) can be as high as up to 60 weight percent, typically 1 to 50 weight percent, often 2 to 80 weight percent, typically 5 to 25 weight percent, and can be 10 to 20 weight percent, based on the total resin solids present in curing agent (b). In these cases, the covalently bonded halogen content present in the curing agent (b) can be derived from a halogen-containing blocked isocyanate, which can be prepared, for example, by at least partially blocking 4-chloro-6-methyl-1, 3-phenylene diisocyanate with a suitable blocking agent, such as 2-butoxyethanol, if partially blocked, any residual isocyanate groups can react with a polyol, such as trimethylolpropane, thereby increasing the molecular weight of the curing agent.

In another embodiment of the present invention, the covalently bonded halogen content present in the resin phase of the electrodepositable coating composition can be derived from component (c), which is different from and present in addition to resin (a) and curing agent (b). In this case, the component (c) is generally a covalently bonded halogen-containing compound selected from any of halogenated polyolefins, halogenated phosphoric acid esters, halogenated phenols, such as the above-mentioned halogenated phenols.

As described above, the covalently bonded halogens present in the resin phase of the electrodepositable coating composition may be derived from the resin (a), the curing agent (b), the component (c), or any combination thereof, so long as the covalently bonded halogen content is sufficient to ensure that the resulting electrodeposition coating, when electrophoretically applied and cured, passes the flame retardant test according to IPC-TM-650, as described above. The amount of covalently bonded halogen present in the resin phase of the electrodepositable coating composition should also be insufficient to adversely affect the electrodeposition process and/or the properties of the resulting dielectric coating.

In one embodiment of the present invention, the electrodepositable coating composition may further comprise a rheology modifier that may aid in the deposition of a smooth and uniform thickness of the dielectric coating on the surface of the walls of the vias and at the edges of the vias. Any of a variety of rheology modifiers well known in the coatings art can be used for this purpose.

One suitable rheology modifier includes a cationic microgel dispersion, which is prepared by: a mixture of a cationic polyepoxide-amine reaction product containing amine groups (typically primary amine groups, secondary amine groups, and mixtures thereof) and a polyepoxide crosslinking agent is dispersed in an aqueous medium and the mixture is heated to a temperature sufficient to crosslink the mixture, thereby forming a cationic microgel dispersion. Such cationic microgel dispersions and methods for their preparation are described in US 5,096,556, column 1, line 66 to column 5, line 13, incorporated herein by reference. Other suitable rheology modifiers include cationic microgel dispersions having a core-shell morphology as described in detail in EP 0272500B 1. Such microgels are prepared by emulsifying a cationic film-forming resin and a thermosetting crosslinking agent in an aqueous medium and heating the resulting emulsion to a temperature sufficient to crosslink the two components.

The cationic microgel is present in the electrodepositable coating composition in an amount sufficient for proper rheology control and to maintain edge coverage, but insufficient to adversely affect the flow of the electrodepositable composition or the surface roughness of the cured coating when applied. For example, the cationic microgel described immediately above can be present in the resin phase of the electrodepositable coating composition in an amount of from 0.1 to 30 weight percent, typically from 1 to 10 weight percent, based on the weight of total resin solids present in the resin phase.

The electrodepositable coating composition is in the form of an aqueous dispersion. The term "dispersion" is believed to be a two-phase transparent, translucent or opaque resin system in which the resin is in the dispersed phase and water is the continuous phase. The average particle size of the resinous phase is generally less than 1.0 micron, usually less than 0.5 micron, typically less than 0.15 micron.

The concentration of the resin phase in the aqueous medium is at least 1, usually from 2 to 60% by weight, based on the total weight of the aqueous dispersion. When the compositions of the present invention are in the form of resin concentrates, they generally have a resin solids content of 20 to 60 percent by weight, based on the weight of the aqueous dispersion.

The electrodepositable coating composition of the present invention is typically supplied in two components: (1) a transparent resinous material which generally comprises an active hydrogen-containing ionic electrodepositable resin, i.e., a primary film-forming polymer, a curing agent, and any additional water-dispersible non-tinting component; and (2) mill bases, which generally include one or more pigments, a water dispersible grind resin, which may be the same as or different from the main film-forming polymer, and optionally additives such as catalysts, and wetting or dispersing aids. The electrodepositable coating compositions (1) and (2) are dispersed in an aqueous medium comprising water and typically a coalescing solvent to form an electrodeposition bath. Or alternatively, the electrodepositable composition of the present invention may be supplied as a one-component composition. In one embodiment of the present invention, the electrodepositable coating composition may be provided as a one-component composition that is substantially free of pigments.

It should be noted that there are a variety of methods by which component (c), if used, can be added to the electrodepositable coating composition in the form of an electrodeposition bath. Component (c) may be added in "pure" form, i.e. component (c) or an aqueous solution thereof may be added directly to the dispersed electrodeposition composition components (1) and (2) or, if appropriate, to the dispersed one-component electrodeposition composition. Or alternatively, component (c) may be blended or dispersed into the clarified resin feed (or any single clarified resin feed component, e.g., film-forming resin or curing agent) before components (1) and (2) are dispersed in the aqueous medium. In addition, component (c) can be blended or dispersed into the mill base, or one of the various mill base components, such as the pigment grinding resin, before component (1) and component (2) are dispersed into the aqueous medium. Finally, component (c) can be added directly to the electrodeposition bath in-line.

The electrodepositable coating composition in the form of an electrodeposition bath generally has a resin solids content of 5 to 25 weight percent, based on the total weight of the electrodeposition bath.

As mentioned above, the aqueous medium may comprise a coalescing solvent in addition to water. Useful coalescing solvents include hydrocarbons, alcohols, esters, ethers, and ketones. Conventional coalescing solvents include alcohols, polyols and ketones. Specific coalescing solvents include isopropanol, butanol, 2-ethylhexanol, isophorone, 2-methoxypentanone, ethylene glycol, and propylene glycol, and glycol ethers such as the monoethyl, monobutyl, and monohexyl ethers of ethylene glycol. The amount of coalescing solvent is generally from about 0.01 to 25%, preferably from about 0.05 to about 5% by weight when used, based on the total weight of the aqueous medium.

Although generally substantially free of pigment, if desired, a pigment composition and/or various additives such as surfactants, wetting agents or catalysts may be included in the dispersion. The pigment composition may be of conventional type, including pigments such as iron oxide, strontium chromate, carbon black, titanium dioxide, talc, barium sulfate, and colored pigments well known in the art. The electrodeposition bath may desirably be a substantially chromium and/or lead free pigment.

The pigment content of the dispersion is generally expressed in terms of the ratio of pigment to resin. In the practice of the present invention, when pigments are used, the ratio of pigment to resin is generally in the range of about 0.02 to 1: 1. The other additives mentioned above are generally used in the dispersion in amounts of from 0.01 to 10% by weight, based on the weight of resin solids.

The following examples are intended to illustrate the invention without specifically limiting it. All parts and percentages in the examples, as well as throughout the specification, are by weight unless otherwise specified.

Examples

The preparation of circuitized substrates by the method of the present invention is described below. Example a describes the preparation of a resin binder consisting of tetrabromobisphenol a for use in the electrodepositable coating composition of example 1. The electrodepositable coating composition of example 1 in the form of an electrodeposition bath was used to provide a conformal dielectric coating on a perforated substrate, which was then metallized, photoimaged, developed, and stripped as described below.

Example A

This example describes the preparation of an electrodepositable coating composition for use in example 1 below. These resin binders were prepared from the following components as described below. The values listed represent parts by weight (unit: g).

Components	Example A
		Crosslinking agent1	1882
Diethylene glycol monobutyl ether formal	108.78
		EPON_8282	755.30
Tetrabromobisphenol A	694.90
		TETRONIC 150R13	0.33
Diethanolamine (DEA)	51.55
		Aminopropyl diethanolamine	113.2
Removing the distillate	(67.66)
		Sulfamic acid	45.17
Deionized water	2714
		Lactic acid4	1.70
Resin intermediate5	244.7
		Pine needleIncense stick5	27.49
Deionized water	2875

¹The polyisocyanate curing agent is prepared from the following components

Components	Parts by weight (g)
		Ethanol	92.0
Propylene glycol	456.0
		Polyhydroxy compoundsa	739.5
Methyl isobutyl ketone	476.5
		Diethylene glycol monobutyl ether formalb	92.8
DESMODUR LS2096c	1320.0
		Methyl isobutyl ketone	76.50

^aBisphenol A/ethylene oxide adduct available as MACOL 98B from BASF corporation

^bAvailable as MAZON 1651 from BASF Corporation

^cIsocyanates available from Bayer Corporation

The first 5 components were charged under argon into a suitably equipped reactor. When the temperature reached about 25 ℃, the addition of DESMODUR LS2096 was started. The temperature was raised to 105 ℃ at which time the final methyl isobutyl ketone addition was made. The temperature was maintained at 100 ℃ while monitoring the reaction and when NCO disappearance by infrared spectroscopy was determined, the temperature was lowered to 80 ℃.

²Diglycidyl ether of bisphenol A, available from Shell Oil and Chemical Company.

³Surfactants, available from BASF Corporation.

⁴88% aqueous solution

⁵Cationic resin prepared from

Components	Parts by weight (g)
		MAZEEN 35570a	603.34
Acetic acid	5.99
		Dibutyl tin dilaurate	0.66
Toluene diisocyanate	87.17
		Sulfamic acid	38.79
Deionized water	1289.89

a amine diol, available from BASF Corporation.

The first two components were charged to a suitably equipped reaction vessel and stirred for 10 minutes at which time dibutyltin dilaurate was added. Toluene diisocyanate was added slowly and the reaction was allowed to exotherm to a temperature of 100 ℃ and held at that temperature until all NCO was lost as monitored by infrared spectroscopy. The resin thus prepared was solvated by adding sulfamic acid and deionized water with stirring. The final dispersion was measured to have a resin solids content of 26 wt%.

The crosslinker is added to a suitably equipped reactor. The next four components were added to the reactor with gentle stirring and the reaction mixture was heated to 75 ℃ at which time diethanolamine was added. The reaction mixture was held at 75 ℃ for 30 minutes. Aminopropyldiethanolamine was then added and the reaction mixture was allowed to exotherm to 132 ℃ and held at this temperature for 2 hours. The distillate was removed. To perform the solvation, the reaction product is added to a mixture of sulfamic acid, deionized water, lactic acid solution and cationic resin intermediate under mild agitation. The rosin solution was then added to the solvated resin followed by two successive additions of deionized water. Excess water and solvent were removed by extraction at a temperature of 60-65 ℃ under vacuum. The final reaction product had a resin solids measurement of about 40 wt%.

Example 1

The following examples describe the preparation of an electrodepositable coating composition comprising the cationic resinous binder of example a above in the form of an electrodeposition bath of the present invention. The electrodepositable coating composition is prepared from the following components as described below. All values given represent parts by weight (unit: g).

Components	Example 1
		Resin Binder of example A	704.9
Hexyl cellosolve	28.5
		E62781	13.2
Deionized water	3053.4

¹Catalyst slurry, available from PPG Industries, Inc.

The above listed components were combined and mixed with gentle stirring. The composition was ultrafiltered 50% and reconstituted with deionized water.

Preparation of circuitized substrates

A perforated monolayer of INVAR metal (50 micron thick, supplied by a division of Buckbee-meas, bmc industries, inc.) containing 200 micron diameter pores spaced 500 microns apart (center to center) in a square grid arrangement was cleaned and microetched to remove unwanted dirt, oil and oxides. The pre-cleaned perforated substrate was then electroplated to provide a copper metal layer with a thickness of 9 microns.

The electrodepositable coating composition of example 1 above was electrophoretically applied to the electroplated substrate in an electrodeposition bath at a temperature of 105 f (41 c) over a period of 2 minutes at 90 volts. The electrocoated substrate is rinsed with deionized water and air dried so that all of the pores of the perforated substrate are anhydrous. The electrocoated substrate was heated to a temperature of 350 f (177 c) for 30 minutes to cure the electrodepositable coating, thereby providing a cured dielectric film having a thickness of 20 microns.

Metallizing the electrocoated substrate. The substrate was heated to a temperature of 50 ℃ for 30 minutes to remove all moisture that may have been adsorbed during the operation. The thus dried substrate was immediately put into a vacuum chamber subjected to plasma treatment with argon ions to activate the coated surface. The substrate surface was then sputter coated with 200 angstroms of nickel followed by 3000 angstroms of copper. The metal layer thus formed was electroplated with 9 μm copper.

The metallized substrate was cleaned and microetched to remove dirt, oil or oxides from the metal surface and then electrophoretically coated with an ELECTROIMAGE PLUS photoresist (available from PPG industries, Inc.) at a temperature of 84 ° f (29 ℃) at 80 volts over 90 seconds. The coated substrate was then rinsed with deionized water and heated to a temperature of 250 f (120 c) for 6 minutes to remove any residual solvent and/or water. A photoresist coating having a dry film thickness of 5 microns was obtained. The coated substrate was then exposed to a uv light source through a phototool on each side and developed with an electoramage _ Developer EID-523 photoresist developing solution (available from PPG Industries, Inc.) to expose the copper in pre-selected areas. The exposed copper areas were etched with a copper chloride acid etchant and the remaining photoresist was removed with an electroimaging _ stripper EID568 photoresist stripper solution (available from ppginindustries, Inc.) to provide a circuitized substrate.

It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.

Claims (51)

1. A method of forming a metallized hole in a substrate, comprising the steps of:

(I) electrophoretic coating of a conductive substrate: the electrodepositable coating composition is electrophoretically coated onto all exposed surfaces of the conductive substrate to form a conformal dielectric coating thereon,

the electrodepositable coating composition includes a resinous phase dispersed in an aqueous phase, the resinous phase comprising:

(a) ungelled resin comprising active hydrogen-containing ionic groups; and

(b) a curing agent which reacts with active hydrogen of the resin (a),

the resin phase has a covalently bonded halogen content of at least 1 wt%, based on the total weight of resin solids present in the resin phase;

(II) etching the conformal dielectric coating surface in a predetermined pattern to expose one or more pore sections of the substrate to form pores in the substrate; and

(III) applying a metal layer on all surfaces to form metallized holes in the substrate.

2. The method of claim 1, wherein the pores comprise blind pores extending to the surface of the substrate.

3. The method of claim 1, wherein the hole comprises a through hole extending through the substrate.

4. The method of claim 1, wherein the substrate comprises a metal selected from the group consisting of an open-cell copper foil, an iron-nickel alloy, and combinations thereof.

5. The process of claim 1, wherein the conformal dielectric coating is heated prior to step (II) at a temperature and for a time sufficient to cure the dielectric coating.

6. The process of claim 5 wherein the cured dielectric coating passes the flame retardant test according to IPC-TM-650 and has a dielectric constant of no more than 3.50.

7. The method of claim 6 wherein the cured dielectric coating has a dielectric constant of no more than 3.30.

8. The method of claim 6 wherein the cured dielectric coating has a dielectric constant of no more than 3.00.

9. The method of claim 6 wherein the cured dielectric coating has a dielectric loss factor of no more than 0.01.

10. The method of claim 1, wherein the resinous phase of the electrodepositable coating composition has a covalently bonded halogen content in the range of from 1 to 50 weight percent, based on the weight of total resin solids present in the resinous phase.

11. The method of claim 1, further comprising the step of (IV) applying a resinous photosensitive layer to the metal layer of step (III).

12. The method of claim 5 wherein the cured dielectric coating has a film thickness of no more than 25 microns.

13. A method of making a circuit assembly comprising the steps of:

(I) providing a conductive core;

(II) electrophoretically coating an electrodepositable coating composition onto all exposed surfaces of the core, thereby forming a conformal dielectric coating thereon,

the electrodepositable coating composition includes a resinous phase dispersed in an aqueous medium, the resinous phase comprising:

(a) ungelled resin comprising active hydrogen-containing ionic salt groups; and

(b) a curing agent which reacts with active hydrogen of the resin (a),

the resin phase has a covalently bonded halogen content of at least 1 wt%, based on the total weight of resin solids present in the resin phase;

(III) etching the conformal dielectric coating surface in a predetermined pattern to expose one or more pore sections of the core;

(IV) applying a metal layer on all surfaces to form metallized holes in the core; and

(V) coating a resin photosensitive layer on the metal layer.

14. The method of claim 13, wherein the core is a metal core selected from the group consisting of an open-celled copper foil, an iron-nickel alloy, and combinations thereof.

15. The method of claim 14, wherein the metal core comprises an open-cell copper foil.

16. The method of claim 14, wherein the metal core comprises an iron-nickel alloy.

17. The method of claim 13, wherein resin (a) comprises a polymer derived from at least one of a polyepoxide polymer and an acrylic polymer.

18. The method of claim 17, wherein resin (a) comprises cationic salt groups selected from amine salt groups and/or onium salt groups.

19. The process of claim 13 wherein resin (a) has a covalently bonded halogen content in the range of from 1 to 50 weight percent based on the total weight of resin solids present in resin (a).

20. The method of claim 19, wherein the covalently bonded halogen content of resin (a) is derived from a halogenated polyhydric phenol selected from at least one of a chlorinated polyhydric phenol and a brominated polyhydric phenol.

21. The method of claim 19, wherein the covalently bonded halogen content of resin (a) is derived from tetrabromobisphenol a.

22. The method of claim 13, wherein the curing agent (b) is selected from at least one of a blocked polyisocyanate and an aminoplast resin.

23. The method of claim 13, wherein the curing agent (b) has a covalently bonded halogen content in the range of 1 to 50 weight percent, based on the total weight of resin solids present in the curing agent (b).

24. The method of claim 13, wherein the covalently bonded halogen content in the resin phase of the electrodepositable coating composition is at least partially derived from component (c), which is different from resin (a) and curing agent (b).

25. The process of claim 24 wherein component (c) comprises a covalently bonded halogen-containing compound selected from the group consisting of halogenated polyolefins, halogenated phosphate esters, halogenated phenols, and mixtures thereof.

26. The method of claim 13, wherein the electrodepositable coating composition further comprises a rheology modifier.

27. The method of claim 26, wherein the rheology modifier comprises a cationic microgel dispersion prepared by: dispersing a mixture of a cationic polyepoxide-amine reaction product containing amine groups selected from primary amine groups, secondary amine groups, and mixtures thereof, and a polyepoxide crosslinking agent in an aqueous medium, and heating the mixture to a temperature sufficient to crosslink the mixture, thereby forming the cationic microgel dispersion.

28. The method of claim 13, wherein prior to step (III), the conformal coating is heated at a temperature and for a time sufficient to cure the coating.

29. The method of claim 28 wherein said cured conformal coating has a dielectric constant less than or equal to 3.50.

30. The method of claim 28 wherein said cured conformal coating passes the flame retardant test according to IPC-TM-650.

31. The method of claim 29, wherein the cured conformal coating has a dry film thickness of less than or equal to 25 microns.

32. A method of making a circuit assembly comprising the steps of:

(I) providing a conductive core;

(II) disposing a photoresist at a predetermined position on the surface of the core;

(III) electrophoretically applying an electrodepositable coating composition to the core of step (II), wherein the coating composition is electrophoretically deposited on all surfaces of the core except for the regions having the photoresist thereon, the electrodepositable coating composition comprising a resinous phase dispersed in an aqueous phase, the resinous phase comprising:

(a) ungelled resin comprising active hydrogen-containing ionic salt groups; and

(b) a curing agent which reacts with active hydrogen of the resin (a),

the resin phase has a covalently bonded halogen content of at least 1 wt%, based on the total weight of resin solids present in the resin phase;

(IV) curing the electrocoated coating composition to form a cured conformal dielectric coating on all surfaces of the core except for the areas having the photoresist thereon;

(V) removing the photoresist to form a circuit assembly having blind vias extending to the core at locations where the resist has been covered; and

(VI) applying a metal layer to all surfaces of the circuit component of step (V), thereby forming metallized blind vias extending to the core.

33. The method of claim 32, wherein a copper metal layer is applied to the core prior to disposing a photoresist at predetermined locations on the surface of the core in step (II).

34. The method of claim 32, wherein the core comprises a metal core selected from the group consisting of an open-cell copper foil, an iron-nickel alloy, and combinations thereof.

35. The method of claim 34, wherein the metal core comprises an iron-nickel alloy.

36. The method of claim 34, wherein the metal core comprises an open-cell copper foil.

37. The method of claim 32, wherein resin (a) comprises a polymer derived from at least one of a polyepoxide polymer and an acrylic polymer.

38. The method of claim 37, wherein resin (a) comprises cationic salt groups selected from amine salt groups and/or onium salt groups.

39. The process of claim 32 wherein resin (a) has a covalently bonded halogen content in the range of from 1 to 50 weight percent based on the total weight of resin solids present in resin (a).

40. The method of claim 39, wherein the covalently bonded halogen content of resin (a) is derived from a halogenated polyhydric phenol selected from at least one of a chlorinated polyhydric phenol and a brominated polyhydric phenol.

41. The method of claim 40, wherein the covalently bonded halogen content of resin (a) is derived from tetrabromobisphenol A.

42. The method of claim 32, wherein the curing agent (b) comprises at least one selected from the group consisting of blocked polyisocyanates and aminoplast resins.

43. The method of claim 32, wherein the curing agent (b) has a covalently bonded halogen content in the range of 1 to 50 weight percent, based on the total weight of resin solids present in the curing agent (b).

44. The method of claim 32, wherein the covalently bonded halogen content in the resin phase is derived at least in part from component (c), which is different from resin (a) and curing agent (b).

45. The process of claim 44 wherein component (c) comprises one or more covalently bonded halogen-containing compounds selected from the group consisting of halogenated polyolefins, halogenated phosphate esters, halogenated phenols, and mixtures thereof.

46. The method of claim 32 wherein said cured conformal dielectric coating has a dielectric constant of less than or equal to 3.50.

47. The method of claim 46 wherein said cured conformal dielectric coating has a dielectric constant less than or equal to 3.30.

48. The method of claim 46, wherein the cured conformal coating has a film thickness of less than or equal to 25 microns.

49. A substrate having metallized holes formed by the method of claim 1.

50. A circuit assembly formed by the method of claim 13.

51. A circuit assembly formed by the method of claim 32.