Solder paste wicking in socketable BGAs

Abstract

The trend of the electronics industry to miniaturize package design has caused the need to adopt BGA packages for a variety of applications. OEM microprocessors have conventionally used LGA designs press fitted into sockets for ease of reworkability. However, BGA packages, which have been widely used for surface mount (SMT) applications, face challenges when they are used in sockets. One of the major challenges faced is the formation of intermetallics between the bare solder ball and the gold paddle of the socket. To address these challenges, Georgia Tech is developing a universal solution for socketing and SMT applications by surface modification of solder spheres. In this new approach, the solder spheres have an outermost noble metal layer which prevents any damage to the BGA and doesn't react with the gold paddle, when placed in a socket. The surface coating would also collapse with the solder ball under specific conditions during reflow when used in SMT applications. This paper focuses on the process of attaching these modified solder spheres on the package and subsequent assembly of the package on the board. We evaluated high and low melting solder pastes to attach the coated spheres to the package. A major challenge observed was the wicking of the solder paste to the entire surface of the solder sphere during reflow. This was addressed by studying the wetting characteristics of the solder paste on different metal surfaces and controlling the volume of solder paste required. Optimization of solder paste volume was done to control wicking and at the same time, achieve a strong and stable joint. Microstructure of the solder joint was analyzed to determine its effect on the joint stability. It was found that the amount of wicking is a strong function of the composition of the solder paste, the reflow time, and the material in surface with which the solder paste is in contact with. This work advances the assembly requirements of socketable BGAs.