Conductive adhesive-an alternative to solder in SMT

Isotropic conductive adhesives (ICAs) provide an environmentally friendly alternative to solders for interconnections in electronic applications with advantages of low processing temperature and an improved fine pitch capability. However, unstable electrical conductivity under elevated temperature and humidity conditions, and low interconnect impact resistance were major obstacles preventing ICAs from becoming a general replacement for solders in SMT until now. Recently developed ICAs promise improved electrical and mechanical reliability on nonnoble metallizations as conventionally used in SMT and a complete curing during a typical reflow soldering cycle. In our study, selected new generation ICAs from different manufacturers are evaluated combined with the four most common finishes: electroless deposited Sn, Ag, NiAu and an organic solderability preservative (ENTEK Plus). The curing cycles were optimized by monitoring bond electrical conductivity during the curing process. These tests showed onset of conductivity immediately after placing the SMT. Long-term behavior is evaluated by observing the contact resistances during and after exposure of the samples to an elevated temperature/humidity environment (85°C/85% RH). Moreover, the results of mechanical strength tests before and after the accelerated aging process are discussed to argue the mechanical long-term behavior. The goal of this study is to discuss advantages and drawbacks of ICAs as a general replacement for solders in SMT and to present some recent progress in ICA technology development