Mechanisms for the formation of conduction paths in a solderable epoxy composite with a mixed low- and high-melting-point solder filler

A novel bonding system for a solderable epoxy composite (SEC) containing a mixed low-melting-point solder (LMS) and high-melting-point solder (HMS) filler (LH-SEC) was proposed to overcome the thermal and mechanical limitations of SECs with LMS fillers. To evaluate the influence of the LMS/HMS mixing ratio on the wetting properties (i.e., the wettability and selective patterning) of the resulting LH-SEC, six types of LH-SEC with different LMS/HMS mixing ratios (100:0, 80:20, 50:50, 20:80, 10:90, and 0:100) were synthesized, and planar and line pattern wetting tests were conducted. The LH-SEC with only the HMS filler exhibited very weak wetting properties because of the excessive curing of the epoxy composite before the HMS melted. In contrast, two wetting modes were observed for the LH-SECs that contained both LMS and HMS. In the LH-SEC with less HMS than LMS, conduction paths were established via the internal flow, integration, and wetting behaviors of the molten LMS containing solid-state HMS within the melting temperature (T-m) range of the LMS filler. For the LH-SECs with more HMS than LMS, conduction paths were formed via the wetting behavior of the molten HMS filler at a temperature range lower than the typical T-m range of the HMS filler because of the decreased T-m of HMS due to the wetting of the molten LMS to the HMS and the diffusion of Bi atoms into the interior of the HMS filler.