Conductive Adhesives containing Ag-Sn Alloys as Conductive Filler

Abstract

Ag filled isotropic conductive adhesives (ICA) have been investigated as promising altenatives for lead containing solders in surface mount technology (SMT) applications; however, one serious concern is the ionic-migration of the silver filled in the ICA, especially when used in high density interconnection assemblies. In his study, ionic-migration resistance and the contact resistance of the ICA containing Ag-Sn alloy conductive fillers were evaluated. It was found that ionic-migration resistance depended on the Ag-Sn ratio and that Ag-Sn alloys containing 25 to 75 mol% of Sn had excellent ionic-migration resistance though their contact resistance was rather high, compared with silver. For the compatibility of the ionic-migration resistance with lower contact resistance, and to understand the effect of a Sn-Bi alloy powder mixture, several additives and polymer matrixes were also studied. New ICAs including these additives and a conventional ICA (as reference) were evaluated for SMT. Chip components with Sn plated terminations for stability in both contact resistance and adhesion strength were monitored during reliability tests. From the test results, it can be concluded that the new ICA is a potential material for SMT applications. Moreover, if mount loading was not sufficient and/or mount alignment was not suitable, initial contact resistance was unstable. Therefore, we selected materials with reduction effect and we dereased the Sn ratio as low as we could. The filler content rate was also increased. As a result the loading dependence was decreased. We were sucessful in keeping contact resistance more stable while maintaining good ionic-migration resistance.