WO1998005067A1

WO1998005067A1 - Electronic packages containing microsphere spacers

Info

Publication number: WO1998005067A1
Application number: PCT/US1997/013324
Authority: WO
Inventors: Richard J. Ross; Troy Alering
Original assignee: RJR Polymers Inc
Current assignee: RJR Polymers Inc
Priority date: 1996-07-29
Filing date: 1997-07-25
Publication date: 1998-02-05
Anticipated expiration: 1999-01-29
Also published as: AU3742097A

Abstract

In the assembly of electronic packages, lead frames are bonded to heat dissipative base blocks by a bonding resin containing microspheres to prevent direct contact between the parts. A dense monolayer of microspheres within the resin without stacking of the microspheres in the resin is achieved by first applying the resin without the microspheres, then applying a monolayer of the microspheres to the tacky resin surface, then joining the parts with pressure to cause the microspheres to penetrate the resin while retaining the monolayer configuration.

Description

ELECTRONIC PACKAGES CONTAINING MICROSPHERE SPACERS

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of co-pending United States provisional patent application, serial no. 60/022,723, filed July 29, 1996, the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention resides in the field of integrated circuit packages, and addresses in particular the bonding of a lead frame to a heat-dissipating base block in a secure yet electrically insulating manner.

2. Description of the Prior Art

Chips containing integrated circuits, or "dies" as the chips are commonly referred to in the electronics industry, are frequently incorporated into packages that render them ready for use. The typical package includes the die enclosed in a protective casing, means for dissipating heat generated by the circuit in the die, a lead frame that permits joining of the die circuitry to external circuitry in which the die may be inserted such as a printed circuit board, and wire leads joining the die circuitry to the lead frame. In one type of construction, the floor of the protective casing is a backing or base block that dissipates the heat while serving as a support for both the die and the lead frame.

Since the base block is heat conductive, it is electrically conductive as well, and care must be taken in the securement of the lead frame to the base block to keep these parts electrically insulated from each other once they are bonded together. In fact, circuit packages in the electronics industry often contain multiple metallic layers that must be bonded together but kept out of direct contact so that electrical short circuits or electronic wave interference can be prevented.

In some instances these metallic layers include copper lead frames such as those stamped from copper sheet, or lead frames of other conductive metals that are similarly stamped. Unfortunately, stamped frames generally have a slight burr of metal at the stamp cleavage point. The typical specification for burrs is a maximum of 0.001 inch (0.0025 cm) in height. When stamped pieces are used as successive layers in a package, the separation between the pieces must be maintained at greater than 0.002 inch (0.0051 cm) to assure that any burrs that are present will not cause short circuits. This is done by joining the two parts with a layer of dielectric adhesive of a thickness sufficient to keep the parts out of direct contact. One means of achieving this is by incorporating microspheres into the adhesive.

The use of microspheres as fillers for plastic resins is well known. Microspheres serve a variety of functions, including cost reduction and enhancement or modification of the physical properties of the resin, in addition to maintaining separation between elements of assembled parts, as they do in the packages discussed above. When an adhesive containing dispersed microspheres is placed between two planar or complementary surfaces and a force is applied to flow the adhesive and spread it evenly along the surfaces, the microspheres act as mechanical stops to prevent the surfaces from attaining closer proximity than the diameter of an individual sphere.

The use of microspheres presents certain difficulties, however. The lead frames in many electronic packages contain leads that are very closely spaced. With microspheres of at least 0.002 inch (0.0051 cm) diameter, lead widths of 0.005 inch (0.013 cm), and spacings between the leads or 0.005 inch as well, for example, a resin containing microspheres in excess of 10% of the resin volume is needed to assure that each lead will be separated from the base block by at least one microsphere. Adding such a large amount of microspheres to liquid resin increases the viscosity of the resin so much that the resin becomes difficult to apply to the surface of the base block, particularly when the method of application of the resin is simple dispensing, screen printing or roller coating. In addition, the microspheres when present in such a high amount have a risk of stacking when the resin is compressed rather than forming a layer one microsphere in thickness. This is detrimental to uniformity of the microsphere dispersion and thickness control of the resin layer. SUMMARY OF THE INVENTION

It has now been discovered that microspheres can be incorporated into bonding layers in electronic packages in a manner that will form a microsphere monolayer in a more efficient and more reliable way. This is achieved by the use of a procedure that avoids the incorporation of the microspheres in the bonding composition before the composition is applied. The procedure includes first placing a layer of curable bonding composition (resin) without microspheres on the surface of one of the two parts to be bonded, adjusting the conditions of the layer to provide it with a tacky surface unless the layer is already tacky upon application, applying the microspheres as a monolayer to the tacky surface, pressing the second of the two parts to be bonded against the microsphere monolayer to force the microspheres to penetrate the resin, and curing the resin. The resulting bonding layer has a uniform and dense monolayer distribution of microspheres, and the process provides improved control over the location and distribution of the monospheres in the bonding layer, thereby reducing if not eliminating the risk of direct contact between the bonded parts.

The term "tacky" is used herein in its conventional sense, i.e. , referring to a surface having a sticky or adhesive quality that permits adhesion of the microspheres upon contact but does not permit penetration of the microspheres into the bulk of the bonding composition until a force greater than the force of gravity is applied to the microsphere layer. A layer of bonding composition with a tacky surface can thus be either a layer of highly viscous liquid, or a layer of composition with a gel-like consistency.

In addition to contributing to the integrity and uniformity of the bonding layer, the microspheres can be constructed to serve additional functions, such as modifying the electrical characteristics of the bonding layer to render the layer even less electrically conductive than it would otherwise be. The invention is particularly useful for bonding lead frames to heat dissipative base blocks.

The process of this invention lends itself to a range of variations in both the materials used and the methods by which they can be applied, and detailed explanations of these and of the basic steps of the process as listed above will be evident from the descriptions that follow.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

Microspheres useful in the practice of this invention can be formed of any material that is solid and can be formed into substantially spherical particles or beads of substantially uniform size. The term "microspheres" is used primarily for convenience, since the spherical shape, although preferred, is not absolutely essential. It is preferred that the diameter of the microspheres in a single lot vary by no more than about 10%, and preferably by a maximum of about 5%. The average diameter can vary considerably, depending on the needs of the particular system in which the microspheres are used. As examples, lead frames 0.008 inch (0.020 cm) in thickness have been satisfactorily assembled to base blocks with the use of adhesives and microspheres by the method of this invention with a space of 0.008 inch (0.020 cm) between the lead frames and the base blocks. In general, however, the most useful microspheres for electronic packages are those having an average (i.e. , a particle average) diameter of from about 0.001 inch

(0.0025 cm) to about 0.005 inch (0.013 cm). A preferred range is from about 0.002 inch (0.0058 cm) to about 0.004 inch (0.010 cm). In certain specific applications, microspheres averaging either 0.0026 inch (0.0066 cm) or 0.004 inch (0.010 cm) are particularly useful. If the metal parts to be joined by the bonding composition are processed to remove burrs prior to being joined, the average sphere diameter can be reduced to 0.001 inch (0.0025 cm) and still be effective.

Preferred microspheres are those that have dielectric constants below about 10.0, and further preferred are those that a have dielectric constant low enough to reduce the dielectric constant of the bonding resin. Hollow, gas-filled microspheres are particularly useful in this regard. Glass or plastic are among the materials that can be used effectively. The preferred microsphere is air-filled glass. Hollow microspheres are commonly called "microballoons" in the trade.

The resin must be one that bonds both to the parts to be bonded and to the microspheres, and is preferably one that has a heat distortion point or glass transition temperature T_g that is about 150°C or higher, and most preferably within the range of about 160°C to about 190°C This will help insure that the bond once formed will have an integrity that will be maintained through the subsequent steps of the typical package assembly procedure. In general, the bond must be rigid enough to withstand the high processing temperatures encountered during wire bonding of the leads and the soldering temperatures during surface mount applications, without becoming dimensionally distorted or breaking the bond. The bond must nevertheless be flexible enough to withstand the mechanical stresses that are often encountered during manufacturing of the package and installing the package in a printed circuit board or other application frame or system. A wide variety of resin systems that meet these criteria can be used. One class of resin systems is epoxy-based thermosetting resins. Preferred among these are blends of resins including an epoxy resin. Epoxy systems can be formulated to achieve a tacky surface at a selected temperature that can vary over a wide range, extending from 25 °C to 125 °C. An example of a useful blend is the combination of structures including a novolac structure, a glycidyl ether structure, and a modified bisphenol A structure, such as that of a carboxy-terminated butyl nitrile (CTBN) graft copolymer. In certain applications, an amine-based resin such as those based on 4,4'-methylene dianiline can also be included, to enhance adhesion of both metal and glass surfaces by increased hydrogen bonding. An example of an epoxy-based blended formulation is shown in Table I:

TABLE I Epoxy Formulation

Trade or Brand Name Chemical Composition CAS Number

QUARTEX^® 1410 Polymers of Epoxy 025036-25-3 Resin and Bisphenol A

ARALDITE^® 0510 Multifunctional Epoxy 5026-74-4 Amine

ARALDITE^® ECN1273 Epoxy Cresol Novolac 2969-82-2 Resin

EPON^® Resin 58005 Modified Bisphenol A 25058-38-6

Epoxy Resin (CTBN 68610-41-3

Rubber Epoxies)

(crystalline silica) SiO₂ 14808-60-7 ANCAMINE^® 2014AS Modified Aliphatic Polyamine

AMICURE^® 1400 Cyanoguanidine AMICURE^® UR Dimethyl Urea 101-42-8-3 (Curing Agent)

A further class of resin systems is high-temperature thermoplastic adhesives. Examples are the group of products bearing the name STAY SΗCK, available from Alpha Metals, Inc., of Jersey City, New Jersey, particularly STAY SΗCK 301. This type of adhesive can be applied as a solution or as a preform.

A third class of resin systems is phenoxy-based hybrid thermoplastic/thermosetting resins. Examples are phenoxy resins crosslinked with phenolic resins, melamines, isocyanates or ureas. An example of a resin formulation of this type is shown in Table II: TABLE II Phenoxy-Based Formulation

Trade or Brand Name Chemical Composition

Paphen FE Phenoxy Resin

[poly(hydroxyether)]

GPRI 7550 Phenolic Resin Solution

(Butylated Phenol Formaldehyde Resin)

Solvent Methyl Cellosolve

Like other resin systems used as binder in electronic packages, the resin systems used in this invention can contain solid inorganic paniculate filler materials to modify or enhance the properties of the resin. Properties that can be modified by the inclusion of fillers are hardness, stiffness, and heat conductivity. The most useful filler materials are those which have a high thermal conductivity and a low electrical conductivity. In terms of the resin in which the material is dispersed, the most useftil materials are those which are chemically inert relative to the resin and wettable by the resin. Examples of materials useful in general as the filler material are alumina, aluminum nitride, silicon nitride, boron nitride, silicon carbide, and combinations of these materials. Particular materials in this group may be optimal for particular resins, as will be understood by those of skill in the art. Preferred among these materials are alumina and aluminum nitride, and mixtures of the two. Flaked or powdered metals such as aluminum, copper and silver can also be used, provided that they do not conduct substantial electricity from the die to the lid. Flaked or powdered metals are best used in combination with metal compounds such as oxides, carbides or nitrides. In many applications, however, the preferred resin will contain no solids at all.

While typical binder resin formulations contain 40% or more of filler particles by weight, this amount is preferably reduced in resins used in the present invention, with the microspheres substituting for the filler material, even though the microspheres are added only after the resin is applied to the part to be bonded.

The resin is applied to the surface of the part to be bonded by any conventional method. Examples are dispensing (as for example a bead from a liquid or gel dispenser), screen printing, stamping and roller coating. The resin can be applied either as a single layer to one of both of the parts to be bonded or as multiple layers.

In preferred embodiments, the area of the part surface to which the resin is to be applied is a raised flat surface bounded by sharp edges such as right angle corners. Raised bonding surfaces contribute to the ease of application of the resin by sharply defining the area to be covered by the resin and retaining the resin in that area by surface tension. For a raised surface that is 0.025 inch (0.064 cm) in width, the layer applied over the raised surface will typically have a height within the range of from about 0.002 inch (0.0051 cm) to about 0.007 inch (0.018 cm), tapering to zero height at the edges. The surface area of the convex surface for a layer of these dimensions is approximately 25 % greater than the surface area of the raised edge itself. A bead with a convex surface can also be formed along an edge that is not raised.

The convex bead can be formed in a single layer or in multiple layers. When multiple layers are used, the microsphere monolayer can be applied over any of the individual layers, either an internal layer (i.e. , one of the layers preceding the last layer to be applied) or the external layer (the last layer to be applied).

The manner in which the surface is made tacky will vary with the choice of the resin and the resin formulation. Certain resins will be made tacky by the application of heat; others will be formulated as a viscous liquid with a viscosity of at least about 100 poise, whose surface will be inherently tacky immediately upon application. The condition of the resin and whether or not heating is required will be readily apparent to those skilled in the art. For epoxy resins, the tacky condition is generally achieved by heating the applied resin to a temperature in the range of 65° C to 100°C. For a high- temperature thermoplastic adhesive such as STAY SΗCK, the adhesive is applied as a liquid solution, which is then heated to remove the solvent, and heated further (to a temperature between 265 °C and 325 °C) to achieve the tacking condition.

The quantity of microspheres applied over the tacky surface of the resin will generally be as many as can be applied while still retaining the monolayer arrangement, both at the time of application and after the parts are pressed together. When the tacky resin surface is convex, as described above, the microsphere monolayer layer as first applied will be convex as well, conforming to the curvature of the resin surface. When the parts are pressed together, the curved microsphere monolayer will flatten out, often without expanding, thereby causing the monolayer to become more dense. With these considerations in mind, the quantity of microspheres applied to the surface of the resin can vary to some degree. In most applications, best results will be obtained when the microsphere monolayer as first applied covers at least about 75% of the tacky surface.

Application of the microspheres is achieved by spraying, brushing or any conventional method that will cover the surface of the resin. Excess microspheres are then removed by vacuum, by brushing or by any means that assures that only a monolayer remains.

Once the monolayer is formed, the two parts are joined using heat and pressure to even out the resin between the parts and cure the resin. The layer is reduced to the point where the microspheres serve as a stop to further compression, the resulting thickness of the layer being approximately or substantially equal to the microsphere diameter. The temperature and time required for curing will vary with the choice of resin and will be readily apparent to those skilled in the art. With epoxy resins, the temperature is within the range of 125 °C to 225 °C; with high-temperature thermoplastic adhesives, the temperature is within the range of 300°C to 375°C, and with phenoxy-based thermoplastic/thermosetting hybrids, the temperature is within the region of 175 °C to

225 °C.

The present invention is applicable to the assembly of electronic packages varying widely in structure, configuration and use. Examples are those known in the trade as SOIC, SOP, and SSOP packages.

The foregoing is offered primarily for purposes of illustration. It will be readily apparent to those skilled in the art that the materials, dimensions, shapes, assembly procedures, curing conditions and other parameters of the method and package described herein may be further modified or substituted in various ways without departing from the spirit and scope of the invention.

Claims

WE CLAIM:

1. A method of securing a lead frame to a heat-dissipating base member in an integrated circuit package to achieve a substantially dielectric bond, said method comprising: (a) forming a layer of curable liquid adhesive with a tacky exposed surface on one of said lead frame and said base member; (b) contacting said tacky exposed surface with electrically nonconductive microspheres to cause a layer of microspheres no more than one microsphere thick to adhere to said tacky exposed surface; (c) pressing the other of said lead frame and said base member against said layer of microspheres to cause said microspheres to penetrate said adhesive and contact both said lead frame and said base member; and (d) curing said adhesive to join said lead frame to said base member while maintaining a separation therebetween whose thickness is substantially equal to the diameter of one of said microspheres.

2. A method in accordance with claim 1 in which step (a) comprises forming said layer of curable liquid adhesive on said base member, and step (c) comprises pressing said lead frame against said layer of microspheres.

3. A method in accordance with claim 1 in which said microspheres are hollow, gas-filled microspheres.

4. A method in accordance with claim 1 in which said microspheres are hollow, air-filled glass microspheres.

5. A method in accordance with claim 1 in which said microspheres are of substantially uniform diameter, ranging from about 0.001 inch to about 0.005 inch.

6. A method in accordance with claim 1 in which said microspheres are of substantially uniform diameter, ranging from about 0.002 inch to about 0.004 inch.

7. A method in accordance with claim 1 in which said microspheres are of substantially uniform diameter, averaging either 0.0026 inch or 0.004 inch.

8. A method in accordance with claim 1 in which said base member is shaped to include a flat raised area, step (a) comprises forming said layer of curable liquid adhesive on said flat raised area, and step (c) comprises pressing said lead frame against said layer of microspheres.

9. A method in accordance with claim 8 in which said layer of curable liquid adhesive is convex in cross section and fully covers said flat raised area, said tacky exposed surface thereby being a curved surface spanning said flat raised area and said layer of microspheres being curved to conform to said curved surface.

10. A method in accordance with claim 1 further comprising applying a plurality of layers of said curable liquid adhesive in succession, step (b) comprising causing said layer of microspheres to adhere to one of said layers prior to application of any successive layers.

11. A method in accordance with claim 10 in which said plurality of layers consists of an external layer defined as the last layer to be applied and one or more internal layers defined as layers applied prior to said external layer, and step (b) comprises causing said layer of microspheres to adhere to one of said one or more internal layers.

12. A method in accordance with claim 10 in which said plurality of layers consists of an external layer defined as the last layer to be applied and one or more internal layers defined as layers applied prior to said external layer, and step (b) comprises causing said layer of microspheres to adhere to said external layer.

13. A method in accordance with claim 1 in which said liquid adhesive of step (a) contains no solids dispersed therein.

14. A method in accordance with claim 1 in which said liquid adhesive of step (a) is a resin that has a glass transition temperature of about 150°C or higher.

15. A method in accordance with claim 1 in which said liquid adhesive of step (a) is a member selected from the group consisting of epoxy-based thermosetting resins, thermoplastic adhesives, and phenoxy-based thermoplastic and thermosetting resin combinations.

16. A method in accordance with claim 1 in which said liquid adhesive of step (a) is an epoxy-based resin comprising a combination of novolac resin, glycidyl ether resin and modified bisphenol A resin.

17. A method in accordance with claim 1 in which said liquid adhesive of step (a) is an epoxy-based resin and step (a) comprises heating said resin to a tacky state after said layer has been formed.

18. A method in accordance with claim 1 in which said liquid adhesive of step (a) is a thermoplastic adhesive applied as a solution in a solvent, and step (a) comprises forming a film of said solution, evaporating said solvent, and heating said thermoplastic adhestive thus remaining to a tacky state.

19. A method in accordance with claim 1 in which said liquid adhesive of step (a) is a combination of phenoxy resin and a crosslinker selected from the group consisting of a phenolic resin, a melamine, an isocyanate, and a urea.