Design of BGA Assemblies with Enhanced Thermal Cycle Capability Using Solder Coated Polymer Balls

Abstract Solder coated polymer balls have been successfully employed for attaching packages to circuit boards with minimum standoff height, while accommodating large mismatches in thermal expansion coefficients. Dramatic improvements in temperature cycling performance are often realized by using them in place of solid solder balls, with five-fold increases in mean cycles to failure reported by a number of investigators. The sales literature, provided by suppliers of solder coated solder balls, attribute this superior temperature cycling performance to the soft, compliant polymer core of the product. Our study of the mechanics of solder coated polymer balls has revealed that their stiffness is in fact comparable to that of solid solder balls. Their rigidity results from a composite construction in which a nearly incompressible polymer material is surrounded by a copper shell that is not easily deformed from its spherical shape. We have employed finite element analysis and mechanical measurements to obtain ...