HK1055530B - An improved method to embed thick film components - Google Patents

Description

Improved method for embedding thick film components

Technical Field

The present invention relates to such a process and the product produced by such a process is characterized by a reinforcing layer between the substrate and the thick film resistor package, and optionally a further sealing layer covering the thick film resistor package. The reinforcement layer reduces cracking of the assembly when embedded, typically by lamination. The enhancement layer also enables laser trimming of the resistor assembly.

Background

Passive components are now available as carrier substrates, while built-in passive components are usually derived from resistors or dielectric thick film technology, the terminals of these components being connected with metal conductors. The components are mounted one at a time on the surface of a Printed Wiring Board (PWB) using pick and place equipment and connected to the circuitry by one of several complex methods including adhesives, fluxes, solder compositions, wave soldering or reflow.

As the demand for miniaturized electronic devices has increased, the circuit density and component density per unit area have increased dramatically. The number of components rises exponentially, forcing the size of the components to be smaller. As smaller designs and therefore denser circuit boards are approaching the practical limits of the prior art, circuit designers have maximized the real-area (real-area) and are unable to add any components unless they become much smaller for surface mounting or embedded in an internal layer, i.e., a vertically stacked assembly. It is desirable to have components from thick film compositions in the embedded layer. Thick film resistor and dielectric compositions are known in the art such that a viscous thick film composition is screen printed in the desired pattern and then fired at a temperature to burn off the organic components and sinter the inorganic materials. The result is a thick film component that is embedded into the circuit.

Although thick film resistors are thin enough to be embedded, they cannot be printed directly onto a printed circuit board because of the firing step during manufacture. The resistor composition may be printed and fired on a sinterable substrate and then laminated to a circuit board. However, the fired assembly is easily broken at the time of lamination, thereby reducing productivity. There is a need for a method of laminating thick film assemblies to printed circuit boards with reduced cracking.

Another obstacle to the use of thick film resistors on printed circuit boards is that they cannot be laser trimmed using existing techniques. Laser trimming is one method of adjusting the resistance of a fired thick film resistor. The resistance is measured first and then the width change required to achieve the appropriate resistance is calculated. A laser may be used to cut through the thick film resistor and a portion through the circuit via. This cutting reduces the effective width of the thick film resistor, increasing the resistance to the desired value. When trimming is performed on a printed circuit board, the laser cuts through the thick film resistor and burns the circuit board. The charred material can form a conductive carbon bridge across the cut path, thereby reducing the resistance of the thick film resistor and/or causing a drift in resistance (doping). The present invention solves the problem of laser trimming printed thick film resistors on organic substrates such as printed circuit boards and also solves the problem of cracking during lamination.

Disclosure of Invention

The invention relates to a method for embedding a thick film component into a circuit board, comprising the steps of:

(a) applying a reinforcing composition over the resistor composition on the metal substrate to form a part, wherein the resistor composition is at least partially coated with the reinforcing composition;

(b) processing the above components;

(c) applying the above-mentioned component to at least one side of an organic substrate to form an assembly, wherein the organic substrate is at least partially coated with an adhesive layer, the reinforcing composition side of the above-mentioned component being embedded in the adhesive layer.

Drawings

FIG. 1 is a schematic of the general process of the present invention.

Detailed Description

The invention relates to a method of embedding a thick film assembly into a circuit board, wherein a reinforcement layer is used. The reinforcement layer increases the strength of the thick film assembly, thereby reducing cracking during lamination. It is advantageous when laminating soft metal substrates to carry the assembly on a circuit board, and when laminating the circuit layer on top of a thick film assembly. The use of a sealing layer and a reinforcing layer (sandwiching the thick film assembly) can also reduce cracking.

The enhancement layer may also act as a barrier to protect the underlying substrate when the thick film resistor assembly is laser trimmed. This layer prevents burning of the underlying substrate and formation of thick film resistor assembly carbon bridges. Laser trimming may be performed with or without the presence of a sealing layer.

Thick film compositions

Thick film compositions are useful as the resistor and medium, the non-conductive reinforcing and sealing layer, and the conductive underlayer (undercoat) of the present invention. Thick film resistor, conductor and dielectric compositions are well known in the industry and are commercially available. Generally, there are two main types of thick film compositions used in the present invention. Are common products sold in the electronics industry. First, a thick film composition (where the organics of the composition can burn or fire off upon processing) is referred to as a "burnable thick film composition". They generally comprise a conductive, resistive or dielectric powder and an inorganic binder dispersed in an organic medium. Before firing, the desired treatments include optional heat treatments such as: drying, curing, reflow, soldering and other techniques known in the art for thick films. Second, a thick film composition (typically comprising a conductive, resistive or dielectric powder and dispersed in an organic medium, wherein the composition cures during processing and the organic remains) is referred to as a "polymer thick film composition". The sinterable thick film compositions and polymer thick film compositions are generally referred to herein as "thick film compositions" unless otherwise specified. "organic" includes the polymer component of the thick film composition.

In conductor applications, such as when a conductive underprinting layer is used in the process of the present invention, the functional phase of the thick film composition consists of a fine conductor powder with electrical functionality. In a thick film composition, the powder having an electrical function may comprise a single type of powder, a mixture of powders, an alloy or compound of several powder elements. Examples of such powders include: gold, silver, copper, nickel, aluminum, platinum, palladium, molybdenum, tungsten, tantalum, lanthanum, gadolinium, ruthenium, cobalt, titanium, yttrium, europium, gallium and alloys and combinations thereof and other powders commonly used in existing thick film compositions. Generally, the conductor powder present in the underlying sinterable thick film composition is compatible with the metal component present in the metal foil substrate which is used in the process of the present invention.

In the resistorIn thick film compositions, the functional phase is typically a conductive oxide powder, such as RuO₂. Examples of the conductor phase in commercially available thick film resistor compositions are selected from RuO₂、SnO₂TaN and LaB₆. Other examples include ruthenium pyrochlore oxide, which is a Ru⁺⁴、Ir⁺⁴Or mixtures of these (M'), said compounds being represented by the following general formula:

(M_xBi_2-x)(M’_yM″_2-y)O_7-z

wherein M is selected from the group consisting of yttrium, thallium, indium, cadmium, lead, copper and rare earth metals,

m' is selected from the group consisting of platinum, titanium, chromium, rhodium and antimony,

m' is ruthenium, iridium or mixtures thereof,

x represents 0 to 2, but x.ltoreq.1 for monovalent copper,

y represents 0 to 0.5, but when M' is rhodium or two or more of platinum, titanium, chromium, rhodium and antimony, y represents 0 to 1,

z represents 0 to 1, but when M is divalent lead or cadmium, z is at least equal to about x/2.

These ruthenium pyrochlore oxides have been described in detail in U.S. patent 3583931. A preferred ruthenium pyrochlore oxide is bismuth ruthenate (Bi)₂Ru₂O₇) And lead ruthenate (Pb)₂Ru₂O₆)。

In dielectric or non-conductive compositions, the functional phase that imparts non-conductive properties is typically a glass, ceramic or non-conductive filler. The dielectric thick film composition is a non-conductive composition or an insulator composition that separates charges and is capable of storing charges. They are used as the non-conductive reinforcing and sealing composition of the present invention. The thick film dielectric composition typically comprises a ceramic powder, an oxide and/or non-oxide glass frit, a crystallization initiator or inhibitor, a surfactant, a colorant, an organic medium and other common components in thick film dielectric compositions. Ceramic materialSome examples of solids include: alumina, titanates, zirconates, stannates, BaTiO₃、CaTiO₃、SrTiO₃、PbTiO₃、CaZrO₃、BaZrO₃、CaSnO₃、BaSnO₃、Al₂O₃Glass and glass-ceramic. Precursors of these materials, i.e. solid materials which convert to dielectric solids and mixtures thereof upon firing, may also be suitable.

The powders are finely dispersed in an organic medium and optionally accompanied by inorganic binders, metal oxides, ceramics and fillers such as other solid powders. The inorganic binder in the thick film composition serves to bind the particles to each other and to the fired substrate. Examples of inorganic binders include glass binders (frits), metal oxides, and ceramics. Glass binders for thick film compositions are common binders in the art. Some examples include borosilicate and aluminosilicate glasses. Examples also include combinations of oxides, such as: b is₂O₃、SiO₂、Al₂O₃、CdO、CaO、BaO、ZnO、SiO₂、Na₂O, PbO and ZrO, which may be used alone or in combination to form the glass binder. Typical metal oxides for the thick film composition are those commonly used in the art and may be, for example, ZnO, MgO, CoO, NiO, FeO, MnO and mixtures thereof.

The functional phase and other powders are typically mixed with the organic medium by mechanical mixing methods to form a gel-like composition of consistency and rheology suitable for printing. A wide variety of inert liquids may be used as the organic medium. The organic medium must be such that the solid can be dispersed in the medium with sufficient stability. The rheology of the media must be such that they provide good application properties to the composition. These properties include: the solids are dispersed with sufficient stability, the composition is well coated, the viscosity is appropriate, thixotropy is appropriate, the wetting of the substrate and solids is appropriate, the drying rate is good, firing is good and the dry film strength is sufficient to withstand roughening. The organic medium is a medium commonly used in the art, and is typically a solution of the polymer in a solvent. Among the sinterable thick film compositions, the most frequently used polymer is ethyl cellulose. Other examples of polymers include ethyl hydroxyethyl cellulose, wood rosin, mixtures of ethyl cellulose and phenolic polymers, lower alkyl polymethacrylates, and the monobutyl ether of ethylene glycol monoacetate can also be used. The most widely used solvents in the sinterable thick film composition are ethyl acetate and terpenes such as alpha-or beta-terpineol or mixtures with other solvents such as kerosene, dibutyl terephthalate, diethylene glycol monobutyl ether, dibutyl carbitol and other glycol ethers, diethylene glycol-butyl ether acetate, hexylene glycol and high boiling alcohols and alcohol esters. Various mixtures of these compounds and other solvents are formulated to give the desired viscosity and volatility.

In addition, the thick film composition may also include other metal particles and inorganic binder particles to enhance various properties of the composition, such as adhesion during processing, sinterability, processability, solderability, safety, and the like. Oxalic acid catalyzed alkyl t-butyl/amyl phenolic polymer is an example of an adhesion promoter that may be used to improve adhesion between the thick film composition and a support (described in more detail below).

In the sinterable thick film composition, the bond between the thick film composition and the substrate is typically achieved by wetting the substrate with a molten glass frit when fired at a temperature in the range of 300-1000 ℃. The inorganic binder (glass frit, metal oxide and other ceramics) portion of the thick film composition is bonded to the center of the substrate. For example, during firing of conventional thick film conductor compositions, the sintered metal powder is wetted or bonded with the inorganic binder, and at the same time, the inorganic binder wets or bonds the substrate, thereby creating a bond between the sintered metal powder and the substrate. Thus, for thick film functionality, it is important that the patterning technique deposit a well-dispersed thick film composition having all the necessary components within the specified ranges. For firing temperatures in excess of 1000 ℃, in addition to the inorganic binder wetting/bonding adhesion mechanism, other interactions and compound formation may give adhesion mechanisms.

The polymer thick film composition consists essentially of conductive, resistive or dielectric powders, such as the powders mentioned above, dispersed in an organic medium containing polymer and/or natural and/or synthetic resin (referred to herein as "polymer") and solvent (typically volatile solvents and polymers). They generally do not include glass frits because they are cured, not fired. Useful polymers are well known in the industry. Polyimides and polyacrylates are suitable. The binder may also be a crosslinkable polymer. This causes the non-conductive composition to harden upon curing. The crosslinkable polymer may be an epoxy resin. Some examples of typical polymers used in polymer thick film compositions are polyesters, acrylics, vinyl chloride, vinyl acetate, urethanes, polyurethanes, epoxies, phenolic resin systems, or mixtures thereof. The organic medium is preferably formulated to give adequate wetting of the particles and substrate, good drying rates, dry film strength sufficient to withstand roughening. Good appearance of the dried composition is also important.

Suitable solvents must dissolve the polymer. Some examples of solvents are as follows: propylene glycol monomethyl ether acetate, methyl propanol acetate, 1-methoxy-2-propanol acetate, methyl cellosolve acetate, butyl propionate, primary amyl acetate, hexyl acetate, cellosolve acetate, amyl propionate, diethylene oxalate, dimethyl succinate, dimethyl glutarate, dimethyl adipate, methyl isoamyl ketone, methyl N-amyl ketone, cyclohexanone, diacetone alcohol, diisobutyl ketone, N-methylpyrrolidone, butyrolactone, isophorone, methyl N-isopropyl ketone. Various mixtures of these and other solvents are formulated to give the desired viscosity and volatility for processing (to which the polymer thick film composition is to be applied).

In polymer thick film compositions, an organic medium is required to impart the necessary adhesion to the desired substrate; it also provides the desired surface hardness, resistance to environmental changes, and flexibility of the composition. Additives well known to those skilled in the art may be used in the organic medium to fine tune the viscosity at the time of printing.

After the polymer thick film composition is applied to the substrate, the composition is typically dried by heating at a temperature of up to about 150 ℃ to volatilize or dry the volatile solvent. After drying, the composition is subjected to a curing process, depending on the application, wherein the polymer binds the powder, forms a circuit pattern or other desired result. To achieve the desired final properties, it is important for those skilled in the art to include each of the desired components in the optimum amounts to meet the final result. The optimum amount of each component is important to obtain the desired thick film conductor, resistor and dielectric properties. The desired properties may include coverage, density, uniform thickness and circuit pattern dimensions, electrical properties (e.g., resistivity, current-voltage-temperature characteristics, microwave, radio-high frequency characteristics, capacitance, inductance, etc.), interconnect characteristics (e.g., solder or braze wetting, compression and wire bonding, adhesion and bonding characteristics), optical properties (e.g., fluorescence), and other desired initiation and burn-in/stress testing properties.

Typically, in formulating thick film compositions, the solids are mixed together with the organic medium by mechanical mixing using a planetary mixer and then dispersed in a three-roll mill to form a composition having a consistency and rheology suitable for screen printing. The latter can be printed in "thick film" on the substrate by conventional methods. Dispersion methods other than three-roll milling are also possible, including powder mixing. These dispersion methods are well known in the industry.

The ratio of binder to solid in the dispersion varies widely depending on the method of application of the dispersion and the type of organic medium used. Good coverage is obtained with dispersions containing 30-91% solids and 70-9% binder, as described above. The compositions of the invention may of course be modified by the addition of other materials which do not affect their advantageous properties. These formulations are well within the skill of the art.

The compositions are typically prepared in a three-roll mill or powder mixer. The viscosity of the composition, when measured with a viscometer at low, medium, and high shear rates, can be in the following range:

reinforced composition

The reinforcing composition may be a sinterable or polymer thick film composition as described above, depending on the desired application and use. One component of the composition is a non-conductive solid or refractive filler.

In the reinforcing composition, the purpose of the non-conductive solid is two: (1) the laser beam is refracted as quickly as possible to prevent it from cutting deeply into the substrate as the laser trims the resistor. (2) Providing additional strength and rigidity to the metal foil with fired resistors and/or other thick film components when laminated onto the central portion of a printed circuit board or in a multi-layer board.

The ability to diffract light limits the number of fillers that can be selected. The light is refracted by the filler in the coating and is proportional to the difference between the refractive index of the filler used in the coating and the refractive index of the polymer matrix. If the refractive indices of the filler and the organic matrix are the same, the coating is transparent to the naked eye. Since most organic polymers have a refractive index of about 1.35-1.6, it is necessary that the refractive index be significantly higher than that of the organic matrix. Examples of preferred fillers are as follows:

among them, lead oxide is toxic and zirconium oxide is expensive. The refractive indices of zinc oxide and aluminum oxide are significantly less than the first three. Zinc is used in sunscreen lotion for preventing sun burn. Titanium dioxide is non-toxic and relatively inexpensive, so it is the most commonly used white pigment in paints and coatings; it can also be preferably used as a refractive filler for reinforcing layers in reinforcing compositions. It prevents the laser from completely ablating the reinforcement layer and completely removes the surrounding adhesive, thereby not creating carbon bridges and dirt or unstable trim. The term carbon bridge refers to the carbon remaining as a laser trimming region of the conductor and may change the resistance of the thick film resistor. It can also cause the resistance value to drift significantly during post-laser trimming stability testing and other subsequent processing.

Mixtures of fillers are also useful. For example, a mixture of alumina and titania may be used. Additionally, mixtures of particle sizes may be used to increase the solids content of the composition. Increasing the solids content is advantageous because it reduces the amount of organic material that eventually burns off upon laser trimming.

The amount of non-conductive filler should be sufficient to reduce the occurrence of cracks in the thick film assembly. It should also be sufficient to prevent damage to the underlying substrate during laser trimming. Suitable amounts of non-conductive filler range from about 35 to about 75 weight percent based on the total composition.

The creation of cracks in thick film assemblies can also be achieved by using high glass transition temperatures (T)_g) Further reduced. Higher T_gIt is ensured that the reinforcement layer remains rigid despite the high temperatures used for lamination. The rigidity enhancing layer prevents the stress from being transferred to the thick film assembly so that the thick film assembly is not deformed and cracked. The polyimide has a high T of over 200 DEG C_gWhereas lamination is generally carried out at temperatures of 150 ℃, they are particularly suitable as adhesives for non-conductive compositions. High T_gIt is also suitable for sealing layers where other layers are laminated to the substrate on top of the thick film assembly. For example, bisphenol a/formaldehyde/epichlorohydrin is a suitable epoxy resin. Such polymers asPurchased from Shell.

The non-conductive composition component may include a cross-linkable polymer and a cross-linking agent. The crosslinking agent crosslinks and hardens the crosslinkable polymer upon curing. Stiffening makes the reinforcement layer or the sealing layer more durable. Not all cross-linkers can cross-link all cross-linkable polymers. Two components are selected as one system. Such two-component systems are well known in the art. Some systems may be cured by heating, while others may be cured by ultraviolet light irradiation. For example, cyanoguanidine is a suitable crosslinking agent that can be used to prepare epoxy crosslinked polymers. Cyanoguanidines were purchased from SKF, Inc. Such systems can be cured by heating. The amount of cross-linking agent used should be sufficient to make the non-conductive composition rigid when cured. A suitable amount of cross-linking agent is 2 to 4 weight percent based on the weight of all components.

Some crosslinkable polymer/crosslinker systems cure very slowly. The cure rate is too slow for an industrially viable process. Where a crosslinkable polymer is used, an accelerator may be included in the non-conductive composition component to accelerate curing. The choice of accelerator used depends on the crosslinkable polymer and/or the crosslinker and the curing process used. Suitable combinations are well known in the art. Urea compounds are suitable thermal accelerators for epoxy systems.Is a thermal accelerator suitable for epoxy/cyanoguanidine systems and is available from SKF, Inc. The accelerator is used in an amount sufficient to reduce the desired cure time of the crosslinkable polymer to the desired time. Suitable amounts of accelerator are 0.1 to 2% by weight, based on the total amount of all components.

A solvent may be included in the non-conductive composition component to obtain the desired rheology for print thickness. Diethylene glycol dibutyl ether is an example of a suitable solvent for an epoxy system.

Other non-conductive compositions are also included within the scope of the invention, which may be cured or fired as described above. The term "treating" includes curing or firing. The reinforcing composition is typically applied by screen printing, but other coating methods in the thick film composition art are also suitable.

Method

The method of the present invention is as follows. FIG. 1 is a schematic diagram illustrating a process. The first step is to obtain a metal substrate (101). The metal substrate should be soft to enable lamination to the substrate. The metal substrate may be a metal foil. Metal foils are available from the electronics industry. For example, a wide variety of metal foils (e.g., copper foils) are available and widely used in the printed circuit board industry, the foils typically having different adhesive properties for different uses. For example, there are now reverse-treated copper foils and double-side-treated copper foils that provide a rougher surface, thereby improving adhesion in printed circuit boards. Electroplated copper foils may also have improved adhesion in some applications. Copper foil subjected to single-sided drum treatment and roll annealing will have a smoother surface, which is not expected to provide adequate adhesion to most thick film compositions, with the mechanical bonding mechanism being preferred to the bonding process. There are also many coatings applied to commercial grade copper foil used in the printed circuit board industry to improve adhesion, reduce rust or for other reasons. These foils include nickel-plated and zinc-plated copper foils. Various copper foils for printed circuit boards are available from companies such as Gould and Oak-Mitsui. In addition to the copper foil mentioned above, other suitable metal foils may include silver, gold, aluminum, nickel, or iron foils. The foil thickness is typically 5-250 microns. A thickness of 10 to 150 microns is preferred, with 15 to 50 microns being more preferred. In some processes, such as coating using a resistor composition, it is known in the art to bond a fired thick film resistor composition to a substrate, which is affected by a number of factors, including the characteristics of the resistor composition and the substrate surface. Depending on the resistor composition, the key factor is the type and volume content of the glass frit in the thick film composition, with more frit generally resulting in better adhesion. Other additives known in the art are also present in the thick film composition which are capable of forming a reactive bond with the substrate surface by forming new compounds at the interface between the thick film composition and the substrate during the firing process. Finally, the usual adhesion mechanism of the thick film composition to the substrate is a simple mechanical bonding mechanism, which depends on the roughness of the substrate surface.

In one example, as shown in fig. 1, an underlying layer (110) must be formed on the metal substrate. The underlying layer may adhere the resistor composition to the metal substrate. The underlying printed layer must have good adhesion to both the metal substrate and the fired thick film assembly. A suitable method of preparing the underlying layer is to apply a layer of the thick film conductor composition to a metal substrate and then fire the metal substrate.

The metal present in the composition of the underlying print layer matches the metal present in the metal substrate. For example, if copper foil is used, a copper composition may be used as the composition for the fireable underprintable layer. Examples of other applications may be silver, gold and nickel foils paired with similar metal thick film base print layer compositions.

The thick film undercoat printing layer composition may be applied as a sparse coating (open coating) over the entire surface or over selected areas of the metal substrate. Screen printing methods can be used to apply the thick film primer composition. The regions may also be selected by printing and etching methods. When using copper foil and the foil is fired in an oxygen-infused atmosphere, the entire surface of the foil should be coated. The glass in the copper composition may retard the oxidative corrosion of the copper foil.

The applied thick film conductor composition may be dried to remove the solvent and then fired at high temperature to burn off the organics and sinter the remaining components. Firing may be performed below the softening point or melting point of the metal. When using the copper composition and the copper foil, the copper composition may be dried at a temperature of 120-130 ℃ and fired at a temperature of 900 ℃ in a nitrogen atmosphere.

Silver foil may be substituted for the copper foil. When using silver foil, air-fired resistors can be used because silver is stable in air firing. Air-fired resistors have excellent electrical properties. Silver is more costly, is prone to migration in the presence of moisture in the electric field, and etching methods are well known in the industry. The metal present in the metal substrate may become the terminal end of the component during processing and optionally the circuit traces on this layer. Such a process is also described in U.S. patent 6317023, which is incorporated herein by reference.

The next step is to apply one or more thick film resistor compositions and/or thick film dielectric compositions to the metal substrate. If an under-print layer is present, the composition should be applied to the under-print layer side of the foil substrate. The composition may be applied using screen printing, ink jet, or other methods known in the art for thick films. These wet compositions are then dried, the solvent removed and fired. The firing is usually performed at a softening point or a melting point of the metal or less. The fired composition is referred to as a "thick film assembly". This term "thick film assembly" is a generic term and may refer to, for example, a "thick film resistor assembly" or a "thick film dielectric assembly". The present invention utilizes a reinforcing composition (fig. 1(b) (103)) which is at least partially coated with a thick film component. The reinforcing composition forms a reinforcing layer. One method of applying the reinforcing composition is screen printing. The enhancement layer allows laser trimming of the thick film resistor without damaging the organic substrate. The reinforcement layer may also prevent cracking of the thick film assembly. In addition, the reinforcement layer is coated on the thick film assembly to have laser trimming or to prevent cracking or both. Generally, an adhesive may be used in the reinforcing layer to improve adhesion, the reinforcing composition being chemically compatible with the organic substrate/adhesive layer. For example, the use of a cross-linkable epoxy polymer can improve adhesion between the polymer thick film composition and the epoxy pre-preg adhesive layer of the organic substrate (104). As shown in fig. 1(b), the reinforcing composition (103) completely covers the thick film assembly (102). In one example, the reinforcing composition may extend beyond the edges of the thick film assembly. In another example, the reinforcing composition may coat the entire metal substrate. The reinforcing composition is then cured to form the reinforcing layer. The curing method may be a heating or ultraviolet irradiation method. When used as a reinforcing composition, the sinterable thick film composition can be fired. When the binder is a polymer, the non-conductive composition may be cured by heating to remove the solvent and allow the composition to set. When the binder is a crosslinkable polymer, the curing step can crosslink the polymer, allowing the composition to set.

The metal substrate, thick film assembly and reinforcing layer optionally having an underprinting layer refer to the laminated part. The laminate component may be applied to a substrate 104, such as a Printed Wiring Board (PWB). The substrate is impregnated with an adhesive or at least partially coated with at least one layer of adhesive (prepreg) (105) such that the thick film assembly and the reinforcement layer sink into the adhesive to render the surface suitable for etch termination. The reinforcement layer increases the strength of the thick film assembly and reduces the amount of cracking (which occurs during lamination). The laminated member may be laminated to both sides of the substrate or one side of the substrate. The laminate member may also be laminated to a substrate already having one or more layers of circuitry.

Examples of substrates for use in the present invention may be of the type of circuit board mentioned below. In general, substrates used in the electronics industry and heat sensitive are useful in the present method. All types of printed circuit boards, such as high voltage laminates, can be used. By definition, a laminate consists of several layers of fibrous material bonded together with a thermosetting polymer under heat and pressure. Typically electrical grade paper bonded with phenolic or epoxy polymers, or continuous filament glass cloth bonded with epoxy polymer systems. More specifically, some examples include:

XXXPC, made from high quality electrical paper impregnated with phenolic polymer;

FR-2, similar to XXXPC grade, except for flame retardancy;

FR-3 is a self-extinguishing paper epoxy resin;

g-10 is a high quality laminate made of glass cloth sheets bonded with epoxy polymer;

FR-4 is very similar to G-10 except that self-extinguishing properties are increased. G-11 is glass cloth-epoxy resin;

FR-5 is a flame retardant modified product of G-11.

The adhesive layer should be electrically insulating. Some adhesives are commonly called prepregs. Examples include epoxy, polymer, acrylic or ceramic type adhesives. The thickness of the applied adhesive may be from about 0.04 to about 0.2 mmAnd (4) rice. Some commercially available adhesives include DuPontAnd PYRALUXAnd (3) an adhesive.

Optionally, circuit traces other than resistor terminations may also be patterned and etched on the metal substrate. Patterning and etching techniques are well known in the art. More specifically, photoresists (e.g. DuPont)(106) May be laminated on the metal side of the substrate. The photoresist is then exposed to UV light (109) through a patterned photolithographic mask plate (107) to form a pattern of metal terminals and optionally other circuit traces. The exposed photoresist is then developed so that the exposed metal is etched away, thereby creating the terminals (and optional circuit traces) shown (fig. 1 (g)). The photoresist is then removed, leaving the desired terminals and circuit traces (as shown in fig. 1 (h)).

The exposed thick film assembly can then be laser trimmed. Laser trimming techniques are well known in the industry. When the thick film resistor is laser trimmed, the reinforcement layer prevents damage to the circuit board and formation of carbon bridges. Eliminating the carbon bridge improves the electrical performance and stability of the resistor. Because the power of the laser beam is tunable, the power can be set high enough to cut completely through the thick film resistor, but not the enhancement layer (as shown in fig. 1 (i)). It may also cut through the reinforcement layer if the laser power is too high, damaging the circuit board. Therefore, the laser power used should be minimal. The use of minimal power (capable of cutting completely through the resistor) is preferred because excessive power can produce carbonized material that causes resistance drift.

When the thick film dielectric is laser trimmed, the metal terminations are trimmed. The thick film dielectric then blocks the laser light and prevents damage to the underlying substrate. The reinforcement layer need not be protected from damage. However, the reinforcing layer may also prevent cracking of the thick film dielectric.

Optionally, at least one non-conductive composition (fig. 1(j)) (108) may be at least partially coated on top of the thick film assembly in the form of a sealing composition. This will further protect the thick film assembly from cracking during subsequent lamination and reduce electrical performance drift due to stresses generated by lamination. Screen printing is one suitable method of applying the sealing composition.

The cured sealing composition applied to the assembly may be in one or more layers. The sealing layer may cover part or all of the thick film assembly, or all of the underlying substrate. As shown in fig. 1(k), the thick film assembly can be laser trimmed even after coating with the top-sealing layer.

Suitable compositional formulations, coating methods and processing conditions of the sealing composition and the reinforcing composition are the same. Non-conductive fillers (not capable of scattering the laser) should be used when laser trimming the thick film assembly through the sealing layer. Alumina is a suitable non-conductive filler. Alumina may also be used in the reinforcing composition when the thick film assembly is laser trimmed.

Sealing layers without a reinforcing layer may also be used. This helps to prevent cracking in the subsequent lamination step.

The above method can be applied to a double-sided configuration. The laminated components are then laminated to both sides of the substrate. Additional circuit layers may also be laminated or applied on top of the laminated part.

The present invention is illustrated in further detail by the examples given below. However, the scope of the present invention is not limited to these examples.

Example 1

The metal substrate was a 1 oz. copper foil. The conductor composition is a copper composition having the following composition:

15% by weight

Ethyl cellulose 0.75% by weight

Glass A0.60% by weight

Spherical copper 83.50% by weight

Phosphate wetting agent^*0.15% by weight

Phosphoric acid tridecyl ester

Composition of glass A:

silica 9.4% by weight

B₂O₃12.2% by weight

Cadmium oxide 65.9% by weight

Calcium oxide 6.7% by weight

3.2% by weight of sodium fluoride

0.2% by weight of alumina

The conductor composition may be applied by screen printing and covers almost the entire foil. It may be dried at a temperature of 130 c and then fired in a belt furnace for 10 minutes in a nitrogen atmosphere with a maximum temperature of 900 c. The total residence time in the furnace was 1 hour. The resistor composition is DuPont QP602, which is LaB₆A base thick film resistor composition. Thick film resistor compositions were screen printed onto the same metal substrate in small rectangles of two sizes (20 x 50mil and 30 x 55 mil). There are 96 thick film resistors of each size on the substrate. The resistor composition was dried at a temperature of 150 c and fired in a nitrogen atmosphere at 900 c for a total residence time in the furnace as described aboveThe sample was 1 hour.

The reinforcing composition may be formed by mixing the following components:

862 epoxy Polymer (Shell) 35.5% by weight

Cyanoguanidine 2.4% by weight

UR500 Urea (Shell) 1.3% by weight

Titanium dioxide powder (0.3 micron) 55.9% by weight

Diethylene glycol dibutyl ether 4.9% by weight

The first three components were mixed for 10 minutes using an aerodynamic powder mixer equipped with an impeller. Titanium dioxide powder was gradually added and the powders were mixed. When the viscosity is too high for a well-mixed composition, diethylene glycol dibutyl ether can be added before more titanium dioxide powder is added. The reinforcing composition may be screen printed in large rectangles that completely cover and overlap the rows of resistors. Each rectangle covered 16 thick film resistors. The reinforced composition was cured at a temperature of 150 ℃ for 1 hour to crosslink the epoxy polymer.

A vacuum laminator may be used to laminate the laminated part to the underlying substrate with the resistor side down. The underlying substrate was an 8mil FR-4 core and the adhesive was a 1.5mil woven glass filled epoxy FR-4 prepreg. Lamination conditions were 208psi and lamination at 165 deg.C under vacuum for 1 hour. Then DuPontA dry film photoresist is laminated to the foil. Photoresist is covered with a graphic mask plateAnd exposed to UV light. The patterned mask plate is removed, the photoresist is developed to expose a portion of the copper foil to etch it away. The copper is etched away and the remaining photoresist is removed. The remaining copper forms the test pads and circuit traces leading to the resistor terminals. The effective sizes of the resistors are 20 by 20 mils and 30 by 30 mils.

The resistance of each thick film resistor was measured. The thick film resistor was then laser trimmed and measured again. Six different power settings of the laser were used: 0.6, 0.8, 1.0, 1.2, 1.5 and 1.8 watts. The Q value (rate) was set to 2000 pulses/sec, and the bite size (bite size) was set to 0.1 mil. The thick film resistor is trimmed to 45 or 60 ohms. Laser trimming does not damage the circuit board and the cutting of thick film resistors is complete and adhesive. The stability of the circuit board after trimming was then checked by heating at 120 ℃ for 15 minutes. The thick film resistor was measured again. In the stability test after trimming, the average percent change in resistance was 1.5%.

Example 2

The procedure of example 1 was repeated with the following changes: QP601 (manufactured by The DuPont Company and is a nitrogen-ignitable 10 ohm/sq LaB₆Base resistor composition) is used as a thick film resistor composition; power settings were 1.2, 1.5 and 1.8 watts; the target resistance is 14 ohms. Laser trimming does not damage the circuit board and the cutting of the thick film resistor is complete and accurate. In the stability test after trimming, the average percent change in resistance was 1.2%.

Claims (14)

1. A method of embedding a thick film resistor composition into a printed circuit board comprising:

(a) applying a reinforcing composition to a resistor composition disposed on a metal substrate to form a part, said resistor composition at least partially coating said reinforcing composition;

(b) processing the part; and

(c) the component is applied to at least one side of an organic substrate to form an assembly, the organic substrate is at least partially coated with an adhesive layer, and the reinforcing composition side of the component is embedded in the adhesive layer.

2. The method of claim 1 further comprising the step of applying a primer layer to the metal substrate prior to applying the resistor composition, wherein the primer layer and the substrate are treated.

3. The method of claim 1, further comprising the steps of:

(d) applying a photoresist to a metal substrate;

(e) processing the photoresist; and

(f) the metal substrate is etched.

4. The method of claim 1, further comprising the step of laser trimming the thick film assembly.

5. The method of claim 4, further comprising the step of applying a sealing composition to at least partially cover the assembly.

6. The method of claim 1 wherein said metal substrate is copper foil.

7. The method of claim 2, wherein the base layer comprises the same metal as the metal substrate.

8. The method of claim 1, wherein the organic substrate is a high pressure laminate.

9. The method of claim 1, wherein the reinforcing composition comprises titanium dioxide.

10. The method of claim 1, wherein said treating step comprises firing.

11. The method of claim 1, wherein said treating step comprises curing.

12. The method of claim 5, wherein said sealing composition is a polymer thick film composition.

13. The method of claim 5, wherein the sealing composition comprises aluminum oxide.

14. An article formed according to the method of claim 1.