Active solid-state devices (e.g. – transistors – solid-state diode – Encapsulated – With specified encapsulant
Reexamination Certificate
1997-09-30
2001-07-03
Meier, Stephen D. (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Encapsulated
With specified encapsulant
C257S787000, C257S789000, C257S791000, C257S795000
Reexamination Certificate
active
06255738
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to filled curable polysiloxane encapsulant compositions having bi-modal and tri-modal particle packing distributions of filler particles, in which the filler particles are individually covalently bonded to one or more polysiloxane chains. In particular, the present invention relates to filled curable encapsulant compositions having bi-modal and tri-modal filler particle packing distributions with filler loading levels that maximize the relative bulk volume of the filler particle packing.
In the construction of semiconductor chip package assemblies, it has been found desirable to interpose encapsulating material between or around elements of the semiconductor packages in an effort to reduce or redistribute the strain and stress on the connections between the semiconductor chip and a supporting circuitized substrate during operation of the chip, and to seal the elements against corrosion, as well as to insure intimate contact between the encapsulant, the semiconductor die and the other elements of the chip package.
In the field of elastomeric materials for encapsulating semiconductor chips, most elastomers are poor conductors of heat. This creates difficulties in removing waste heat from semiconductor chips that run hot, such as high-end microprocessors. It also makes it difficult for heat to pass through the package for soldering and de-soldering.
Most elastomers have a high coefficient of thermal expansion (CTE), so that undesirable levels of stress are produced as the various materials used in an semiconductor chip package expand and contract at different rates in response to temperature changes.
Encapsulated metallic conductors such as wire bonds are particularly susceptible to fatigue failures. Typical wire bonds incorporate sharp corners and sharp changes in cross-sectional area at junctions between the fine bonding wire and the connected parts. For example, in a “ball-bond,” each fine wire joins a relatively massive ball of wire material at one end. These features tend to create stress concentrations at the junctures. If the wire is flexed repeatedly during service, it can fail at such stress concentrations. When the wire is encapsulated, differential CTE of the chip and encapsulant can cause repeated flexure of the wire and fatigue failure. Attempts have been made to avoid such fatigue failures by using very soft encapsulants such as soft elastomers or gel, at a sacrifice of tensile strength and solvent resistance. Fatigue failure of encapsulated wire bonds remains a significant problem.
A few elastomers have poor solvent resistance. When exposed to certain solvents such as cleaning agents, they swell as they absorb the solvent. Like the stress produced by differences in CTE, the stress produced by solvent swell can also cause package failure. For semiconductor chip packaging, it is important to have good resistance to the cleaning solvents commonly used in the electronics industry, such as esters and terpenes.
U.S. Pat. Nos. 3,649,320; 4,946,878 and 5,001,011 disclose the use of silane coupling agents to improve the mechanical properties of filled thermosetting and thermoplastic resins. A means by which the mechanical properties of curable polysiloxane encapsulant compositions such as Young's modulus and tensile strength may be improved without a sacrifice of thermal conductivity or CTE would be highly desirable.
SUMMARY OF THE INVENTION
The present invention addresses these needs. Filler particles may be incorporated into curable polysiloxane encapsulant compositions to increase the tensile strength and solvent resistance of the encapsulant composition but at the same time reduce CTE and increase the thermal conductivity. Preferred embodiments of the present invention provide curable polysiloxane encapsulant compositions with heretofore unknown combinations of high tensile strength, solvent resistance and thermal conductivity, and low CTE.
In methods according to one aspect of the present invention, filler particles are employed having surfaces modified with coupling agents that covalently bond individual filler particles to one or more polymer chains of the polysiloxane. The polysiloxane is formed by reacting a siloxane and a hardener compound, wherein the siloxane has functional groups reactive with functional groups on the hardener compound. The siloxane and hardener compound are both difunctional so that adjacent siloxanes are linked by the hardener compound. Coupling agents are selected that react with the siloxane compound so that some of the hardener compound covalently bonds individual filler particles to one or more polymer chains of the polysiloxane upon curing, instead of linking together two siloxanes.
Most preferably, the filler has a bi-modal or tri-modal particle size distribution. Such a particle size distribution provides an even greater increase in tensile strength and solvent resistance in the cured encapsulant composition and even greater improvements in thermal conductivity and CTE reduction. The curable encapsulant compositions of the present invention thus possess a particularly advantageous combination of thermal and mechanical properties for use as encapsulant compositions for semiconductor chip packages.
Therefore, according to another aspect of the present invention, a filled curable siloxane encapsulant composition is provided containing a siloxane base resin with functional groups reactive with functional groups of a hardener compound to form a polysiloxane, and filler particles with surface functional groups reactive with the hardener compound functional groups, so that upon curing, the filler particles individually covalently bond to one or more polymer chains of the polysiloxane, wherein the filler particles have least a bi-modal particle packing distribution in the polysiloxane of first filler particles having a first diameter and second filler particles having a second diameter smaller than the first diameter, and the first and second filler particles are present in amounts effective to provide the particle packing distribution with a relative bulk volume of at least about 90 percent.
The term “relative bulk volume” is a measure of the packing density of a particular filler. As further discussed below, in a filler with a bi-modal particle size distribution, the smaller particles can fill empty spaces between larger particles, so that a filler with a bi-modal particle size distribution typically will provide a relative bulk volume higher than an equivalent filler containing only the larger or smaller particles.
The improvement in thermal and mechanical properties increases as the packing density of the filler particles, expressed as relative bulk volume, increases. Thus, even greater improvement in thermal and mechanical properties are obtained when a tri-modal particle packing distribution is employed.
Therefore, according to a preferred embodiment of this aspect of the present invention, the filler particles of the curable polymer encapsulant composition further include third filler particles having a third diameter less than the second diameter so that the encapsulant has a tri-modal particle packing distribution with a relative bulk volume of at least about 95 percent.
The curable encapsulant compositions of the present invention may be employed as part of a two-component encapsulant system in combination with a separately packaged hardener compound for the curable siloxane, wherein the siloxane and hardener are combined immediately prior to use of the encapsulant. In the alternative, the hardener and associated catalysts may be compounded with the curable siloxane and filler materials, provided that prior to use the curable encapsulant composition is maintained at a temperature below that at which the curing reaction takes place. The encapsulant is then cured by heating the curable composition to a temperature at which the curable siloxane and hardener compound react to form a fully cured polysiloxane.
In the alternative, the curable encapsulant compositions of the pre
DiStefano Thomas H.
Kovac Zlata
Mitchell Craig
Thorson Mark
Guerrero Maria
Lerner David Littenberg Krumholz & Mentlik LLP
Meier Stephen D.
Tessera Inc.
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