RELIABILITY ASSESSMENT OF ISOTROPICALLY CONDUCTIVE ADHESIVE JOINTS IN SURFACE-MOUNT APPLICATIONS

A study of conductive adhesive joining technology on printed circuit boards (PCB) was conducted. Six different isotropically conductive, silver-loaded epoxies were evaluated for the surface mount process. These adhesives were used to attach electrically and mechanically 160-lead, 25-mil pitch, quad hat package (QFP) and 0805 chip components on PCB's with hash gold, passivated copper, and tin/lead metallization. The reliability of the conductive adhesive joints was evaluated in two types of environmental test: Constant humidity at 60 degrees C and 90% RH for 1000 h, and temperature cycling for 1000 h from -40 degrees C to 85 degrees C. Under these conditions, both the electrical performance, in terms of contact resistance and surface insulation resistance (SIR), and the mechanical strength of the adhesive bonds were evaluated. The results show that reliable conductive adhesive joining can be achieved for both chip components and QFP components on PCB's with all three metallizations tested. Under well-controlled conditions, and with the right choice of adhesive for the application in mind, no significant increase or variation in electrical resistance during the temperature cycling test was encountered. The humidity exposure test was found to have a minor impact on both contact resistance and adhesion strength of most adhesives tested. In general, there was little difference between the different PCB metallizations used (except for SIR measurements). No evidence of silver migration could be observed after the humidity exposure for any of the adhesives tested.