Influence of PCB Surface Finish and Thermal and Temperature/Humidity Aging on the Performance of a Novel Anisotropic Conductive Adhesive for Lead-free Surface Mount Assembly

Abstract

The electronics industry, in recent years, has been focusing primarily on product miniaturization and lead-free assembly. The need to eliminate lead-based materials as a means of interconnection has renewed the industry's interest in exploring other means of assembling surface mount devices reliably, especially using conductive adhesives. This paper will discuss the performance characteristics and research findings pertaining to a novel anisotropic conductive adhesive for lead-free electronics packaging applications. The applicability of the novel conductive adhesive in board level assembly has been demonstrated successfully. The IV characteristics and the breakdown current characteristics have shown the importance of achieving very low initial contact resistance after assembly, in order to enable longer life under thermal and temperature-humidity aging conditions. Thermal aging of the adhesive material has revealed improvement in contact resistance and temperature/humidity aging has shown deterioration in performance within the first 100 hours of aging. Area array packages with and without bumps have shown variations in performance and their has revealed the importance of placement pressure, placement speed and placement dwell in achieving low initial contact resistance. Preliminary experiment has shown that the HASL finish performs better than OSP. The experimental study presented in this paper examines the performance of the adhesive on various printed circuit board (PCB) surface finishes which include Sn/Pb (HASL), Organic Solderability Preservative (OSP), Immersion Silver (ImAg), Electroless Nickel Immersion Gold (ENIG) and Immersion Tin (ImSn). The findings and performances of the various finishes under thermal and temperature/humidity aging will be discussed in detail in this paper.