ACCELERATED LIFE TEST OF Z-AXIS CONDUCTIVE ADHESIVES

Conductive particle-filled adhesives have been widely used for flex-to-rigid board interconnections in many consumer electronics, such as calculators and palmcorders. Most of the applications were in coarse pitch interconnections where the adjacent conductor's distance is greater than 0.4 mm. 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. The accelerated life test conditions were 85-degrees-C/85%RH at three different voltages: 10 V, 50 V, and 100 V. The studies were focused on the samples' time-to-failure as well as the associated conduction and failure mechanisms. The test results showed significant metal migrations, and we are proposing enhanced electric field streSseS (10(2) to 10(4) V/MM) as the driving force for failures.