Accelerated life test of z-axis conductive adhesives

Conductive-particles filled adhesives have been widely used for flex-to-rigid board interconnections in many consumer electronics applications, such as calculators and palmcorders. Most of the applications were in coarse pitch interconnections where the adjacent conductor's distance is greater than 10 mil. The success of coarse pitch applications has increased the interest to use such adhesives in fine pitch applications, such as flip-chip on board interconnection. Since these materials contain metallic particles to conduct currents in the z-direction (i.e. perpendicular to the plane of circuit board), their propensity for metal migration is a concern. Therefore we have applied accelerated temperature, humidity and bias (THB) tests to a group of materials designed for fine pitch applications. Our studies include two phases. Phase-I focused on the high voltage (100 V) THB test and the associated conduction and failure mechanisms. Phase-II evaluates the time-to-fail under medium (50 V) and low voltage (10 V) THB conditions. The results of Phase-I studies showed significant metal migration in our tests, and we proposed enhanced electric field stresses (10/sup 4/ to 10/sup 5/ V/cm) as the driving force for failures.< >