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.

Conductive adhesives have been used in the electronics industry for several years to attach chips to package lead frames in the semiconductor industry and for general interconnection of components to flexible circuits for various consumer products. Generally, these materials conduct equally in all directions. To obtain pad isolation, the adhesives are screen printed to the pattern of the circuit pads. In the last few years, a new class of adhesives that are conductive in a single direction have been developed. These are referred to as Anisotropic Conductive Adhesive Films (ACAF). These anisotropically conductive adhesives provide electrical as well as mechanical interconnections for fine pitch applications. The conductivity of ACAF materials is only in the Z-direction (perpendicular to the plane of the board) while electrical isolation is maintained in the X-Y plane. Currently, at least 15 ACAF materials are commercially available. We have developed a methodology for evaluating these materials for their mechanical and electrical properties and interconnection use in the 8 to 15 mil pitch range. In addition, we characterized the materials as to their physical properties and cure characteristics. This paper details our findings with a comparison of physical form to assembly/cure and final electrical properties. We include in this study data from scanning electron microscopy, thermal analysis of the ACAFs, and cure and assembly studies on mixed substrate test vehicles. Information on initial electrical testing and long term reliability testing is also given.< >