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 (10⁴ to 10⁵ V/cm) as the driving force for failures