Electrical and reliability properties of isotropic conductive adhesives on immersion silver printed-circuit boards

Abstract

Electrically conductive adhesive (ECAs) have been used for many years in both the anisotropic conductive adhesive (ACA) and isotropic conductive adhesive (ICA) forms for microelectronic packaging applications and as an alternative to lead (Pb) soldering technology. Especially, environmentally friendly ICAs offer many advantages over solder, such as simple and low temperature process conditions and better thermo-mechanical performance. However, a number of reliability questions linger, and continue to be studied. In particular, the ICA-to-pad contact resistance can increase with time due to the galvanic corrosion of dissimilar contact and ICA-filler metals (Lu et al., IEEE Trans Electron Packag Manuf 22:228–232, 1999). ICAs are usually silver (Ag) epoxy composites, and the corrosion potential should be completely eliminated by the use of Ag contact pads. To completely eliminate lead (Pb) from electronics, the printed wiring board (PWB) must also change from hot-air-leveled solder (SnPb) to alternative metallic finishes, such as immersion-silver (Ag), immersion-tin (Sn), electroless nickel (Ni)/immersion-gold (Au) and organic solderability preservative (OSP) (Pas, in European institute of printed circuits summer conference, 2005). Especially, immersion-Ag is one of the leading Pb-free final finish choices for many OEMs in the telecommunications, computer, automotive and consumer electronics industries, because of its excellent properties and reasonable cost. This paper presents the electrical properties of Ag epoxy composite ICAs materials on Cu-finished and immersion-Ag finished PWBs, as solder replacements for SMT or flip chip technologies. All PWBs were subjected to 85°C/85% relative humidity (RH) aging testing, with junction resistance monitored for comparison of the immersion-Ag board to the Cu-finished board as a control. We expected that the corrosion potential, which is one of main causes to degrade conductivity between ICAs and PWB, should be eliminated by the use of Ag contact pads with the Ag epoxy composite ICAs materials. Not only is the junction resistance of immersion-Ag finished boards lower than that of Cu finished boards, but its junction resistance changes are smaller than those of Cu-finished boards, as expected.