This Small Business Innovation Research (SBIR) Phase II project will further the development of lead-free transient liquid phase sintering (TLPS) adhesives with very high thermal and electrical conductivity for packaging high-power semiconductor devices. Conductive adhesives are typically used for low?]cost assembly, but these materials represent the weakest point in the thermal path. To address this issue, TLPS conductive adhesives form metallurgical bonds with the adherent metallization and can provide an order of magnitude or more improvement in thermal performance versus existing adhesive technologies. These low?]cost, lead-free materials are designed as drop?]in replacements for existing manufacturing processes. During this Phase II project, the focus will be the formulation, characterization, and qualification of lead-free TLPS adhesives for high-volume semiconductor device manufacturing. The primary objective of this project will be the demonstration of an order of magnitude improvement in effective thermal conductivity compared to commercial conductive adhesives for electronics packaging. Advanced characterization techniques, along with durability studies, will be instrumental for bringing these materials to a readiness level suitable for market penetration.
The broader impact/commercial potential of this project is the development of new semiconductor die-attach materials suitable for the low-cost packaging of high-power semiconductor devices. A number of industries are aggressively developing innovative product lines centered on the concept of energy efficiency and higher performance; these include hybrid electric vehicles (HEVs) in the automotive sector, high-brightness light emitting diodes (HBLEDs) in commercial lighting, and concentrator photovoltaics (CPVs) for utility-scale electricity. Further, next-generation silicon devices, particularly those based on stacked-die architectures, also require improved conductive adhesives to fully enable their performance benefits. At present there exist no RoHS-compliant products that can satisfy all the needs identified by these markets in a cost-effective fashion. Yet these needs are becoming more urgent as a multitude of electronic devices reach the limits of today?fs heat dissipation technologies. The primary products which will result from this Phase II effort are advanced thermally and electrically conductive adhesives that can meet the thermal management requirements of advanced semiconductor packages while lowering their cost of manufacture.
In Phases I and II of this NSF SBIR program, Creative Electron demonstrated new solvent-free and lead-free transient liquid phase sintering adhesives that form metallurgical connections with adherent metallization upon cure, that exhibit order-of-magnitude increases in thermal conductivity and enhanced electrical transfer characteristics compared to polymeric conductive adhesives, and that exhibit excellent durability per preliminary accelerated aging. These materials were demonstrated to be suitable for application via needle dispensing equipment or for stencil application. In Phase II we productized these highly electrically and thermally conductive adhesive compositions for insertion into various commercial semiconductor die-attach applications. The Phase II effort yielded three commercial-ready variations of TLPS based technology. The immediate benefit of this technology stems from the fact that current silver-filled adhesive bonding materials are the major source of interfacial resistance between a device and a bond-pad. In Phase I, lead-free TLPS formulations were developed that had relatively optimal filler ratios to enable liquid phase sintering, and that also retained low viscosity for at least 24hrs at room temperature. In Phase II, the focus was to further improve the manufacturability and performance characteristics through optimization of the filler ratios and morphologies, with an aim on further increasing the thermal and electrical conductivity, as well as the thixotropy. Reducing the bondline thickness, particularly for 3D-IC applications, was also accomplished through optimization of the filler sizes and loading levels. Large device die-attach is also of interest, and approaches for reducing the modulus of the material was explored. For CPV applications, new embodiments of TLPS were developed aimed at increasing the scalability of the CPV module manufacturing process. Finally, we proved out the reliability of these formulations per thermal cycling and humidity testing. The thrust of this program was to investigate and resolve the following issues: * Optimization of the fluxing polymer binder formulation for increased shelf life, increased con-ductivity, improved process properties, dimensional stability over wide thermal cycling ranges, negligible outgassing, and B-stageability. * Metallurgical optimization, i.e., optimization of the best solder alloy types and high-melting point filler types, their relative distribution, morphology and purity. * Improvement in the thermal and electrical conductivity properties such that these properties are at least comparable with available lead-free solders. * Thermal cycling and humidity testing to demonstrate long-term reliability under high-stress conditions. * Integration of the material into the 3D-IC and CPV packages of interest. This program was concluded with the successful commercialization of a thermal adhesive product line.
