ELECTRICALLY CONDUCTIVE ADHESIVES FOR SURFACE MOUNT SOLDER REPLACEMENT
Author: Michael Zwolinski Company: Ntn'l Cntr for Mnfctr Sciences Date Published: 9/10/1996
Surface Mount International
Abstract: Electrically conductive adhesives offer a lead free, relatively low temperature process for attachment of surface mount components to printed circuit boards. The National Center for Manufacturing Sciences Conductive Adhesives project has defined a requirement for an Isotropically Conductive Adhesive (ICA). In this study, 25 commercially available silver-filled epoxies were evaluated for joint resistance stability on copper and tin/lead surface finishes after 500 hours of 85°C/850ARH aging. Mechanical properties were evaluated through lap shear and impact testing. This testing lead to a set of requirements for a surface mount adhesive including a volume resistivity of 1 mOhm-cm, less than a 20% shift in resistance after aging, and ability to pass 6 drops with a PLCC 44 package from a height of 60”. Promising candidate adhesives identified in the screening study were evaluated on a circuit board test vehicle with a variety of components. Assembly revealed a narrow processing window when compared to solder. Impact testing performed by the consortium has shown that current materials have inadequate impact strength for many components and accelerated T/Ii aging reveals unstable electrical resistance with Sn/Pb finished components. Preliminary data is included on a new adhesive developed by the research group to address the shortcomings of currently available adhesives.