Electrical and reliability properties of isotropic conductive adhesives (ICAs) on immersion silver printed wiring boards (PWBs)

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). 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 isotropic conductive adhesives (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 degrees 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.