Materials for High-Density Electronic Packaging and Interconnection (1990) / Chapter Skim
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4 MATERIALS ISSUES
4 MATERIALS ISSUES
Pages 59-70
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From page 59... ...
Operating environment is a design factor best considered on a system level, but materials susceptibility to outside influence is often given too little emphasis. Design trends associated with increasing density of electronic circuitry include the following items: Power dissipation per unit area or volume is increasing.
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From page 60... ...
In general, however, there is an empirical inverse relationship between thermal conductivity and dielectric constant of substrate materials, as shown in Figure 4-1; some typical values are given in Appendix E This suggests that a physical design that separates heat flow paths from signal propagation media will provide the most effective strategy in regard to dielectric materials.
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From page 61... ...
The very low thermal conductivity of the plastic makes attachment of a finned heat sink relatively unattractive. The silica filler used in epoxy encapsulants is also a poor thermal conductor (see the relevant section in Chapter 5~.
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From page 62... ...
Dielectric breakdown normally is not a problem in interconnections. In printed circuit boards, hybrid circuits, and multichip modules, strip lines of various description occur.
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From page 63... ...
Even so, foamed ceramics deserve exploration. New epoxy resins based on more highly functionalized materials now are being introduced for high-performance circuits when their dielectric constant is not the driving property.
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From page 64... ...
Thus, a silicon chip with an attached metal alloy lead frame which is transfer-molded in a silica-filled epoxy, will develop stresses as the assembly is cooled from the molding temperature to room temperature. In some cases, the stresses can be sustained by the material system, but in other cases the stresses cause the package to warp, twist, or deform.
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From page 65... ...
Although computer design aids and greater understanding of adhesion will improve design effectiveness, it is doubtful whether new materials or processes can be introduced without an extensive testing program. As an example, epoxies bond well to many different surfaces, but moisture ingress, poor design, or surface changes can weaken its adhesive strength.
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From page 66... ...
Suitably prepared ceramics and metals are impermeable to gases and can produce effective hermetic packages. Typically, the chip is bonded inside a ceramic body cavity, and a lid (metal or ceramic)
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From page 67... ...
Specifically, the copper commonly employed in lead frames, printed circuit boards, connectors, wire, and cable is subject to corrosion. In general, liquid water must be avoided.
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From page 68... ...
Because of the desire for maximum packaging density, there is a tendency to use flat packs rather than dual- incline packaging. Similarly, in recent years, when the push for closer lead spacing and higher lead counts has put pressure on military packaging, leaded and leadless chip carriers, surfacemounted to high-density printed circuit boards and to ceramic modules, have become dominant in military systems.
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From page 69... ...
The replacement of wire bonds by techniques that apply many bonds simultaneously beg., tape automated bonding, flip chips, or other techniques) will permit significant improvements in module reliability, since the wire bonds are still a relatively fragile interconnection methodology.
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