(357h) An Empirical Method for Assessing the Significance of Cure Shrinkage Stresses Generated in Pbga Underfills

In modern electronics packaging schemes, adhesive underfills are often used to improve the lifetime performance of components such as plastic ball grid arrays (PBGA’s) on printed circuit boards.  This underfill adhesive provides a stronger mechanical connection and distributes the stress generated from thermal expansion mismatches between the component and circuit board reducing stress concentrations in the solder joints.  Due to production demands and desires to speed up processing and throughput, the electronics industry has moved away from traditional encapsulants (e.g., epoxy-amine based systems) in favor of snap-cure chemistries.  These snap-cure materials are attractive due to their low flow times (low viscosity) and very short curing times (typically on the order of minutes).  However, questions remain about the cure shrinkage stresses generated by the snap cured material when cured in these highly confined environments.

Here we present a comparative analysis of a traditional alumina filled epoxy-amine encapsulant (EPON828 + Jeffamine D230 + alumina) and a commercially available snap cure encapsulant (Zymet X2321) for electronics underfill applications.  Due to the differences in chemistry involved, these materials exhibit drastic differences in terms of kinetics and especially in reaction exotherms.  Thermo-mechanical analyses of each fully-cured material, including modulus, thermal expansion, and relaxation behavior (via determination of a master curve), are measured and compared.  Further, a bimaterial strip test consisting of a film of encapsulant on an elastic substrate is used to make relative comparisons of the deformation states of these two materials both during and following the cure and cool down process.  This test provides the unique ability to separate the mechanical contributions to stress by comparing the substrate curvatures generated during cure to those arising from thermal strains during cooling.

Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.