As the portable electronics industry continually drives electronic assemblies to higher functionality in smaller form factors, material compatibility and thermal dissipation issues are becoming considerably more acute. Most of the current approaches attempt to marry conventional materials technologies in order to achieve as much leverage as possible out of the established infrastructure. However, concurrent engineering of reliable, high-density electronic assemblies will require the introduction of a new material technology.
A novel base technology that is applicable to all of the major packaging and redistribution elements in an electronic module is presented. A single family of polymer/metal composite conductors can be used for chip packaging redistribution layers, multichip module or multilayer PWB interconnects, and SMT assembly. High-density multilayer circuits with landless blind and buried vias can be fabricated by filling the conductor paste into photoimaged dielectrics and thermal processing. Via layers are prepared directly on the inherently planarized circuit layer in an identical fashion. Building up layers sequentially in this manner results in multilayer circuits built on a single substrate layer and minimizes the number of interfaces between dissimilar materials. Because these composite materials are applied in an additive fabrication method, metal substrates can be employed for high thermal dissipation and excellent CTE control over a wide temperature range. Two variants of the composite conductor can successfully replace solder for surface mount and chip on board assembly. These reliable, highly-thermally and electrically conductive materials are compatible with the standard metal finishes of conventional technologies and can be adopted piecemeal as desired; however, the largest reliability and cost benefit is realized when all of the elements are used in conjunction with one another.
The conductor materials are based on interpenetrating polymer and metal networks that are formed in situ from metal particles and a thermosetting flux/binder. The metal network is formed when the alloy particles melt and react with adjacent high-melting point metal particles. Interaction also occurs between the alloy particles and pad, lead or previous trace metallizations provided they are solderable by alloys of tin. The new alloy composition created by the interdiffusion process within the bulk material has a higher melting point than the original alloy and thus solidifies immediately upon formation. This metallurgical reaction, known as transient liquid phase sintering, is facilitated by the polymer mixture. Integration of the polymer and metal networks is maintained by utilizing a thermosetting polymer system that cures simultaneously with the metallurgical reaction. Although similar in concept and performance to cermet inks, these compositions differ in that their process temperatures are compatible with conventional printed wiring board materials and that the polymeric binder remains to provide adhesion and fatigue resistance to the metallurgical network.