Thermal Aging Reliability of Socketable BGA Packages With Ni–Au-Coated SAC305 Spheres

Abstract

Land grid arrays (LGAs) and ball grid arrays (BGAs) constitute the board-level interconnect technologies for a large number of packaged ICs. These technologies are used in socketing and surface mount (SMT) applications, respectively. The concept of socketable BGA packages, with solder spheres coated with a barrier layer/noble metal coating, was introduced to develop a single package design for processor ICs that are used in both socketing and SMT applications. While this concept has previously been demonstrated, the feasibility of these packages in socketing conditions has not yet been studied. This article assesses the applicability of these packages in socketing by subjecting the packages to thermal aging at a temperature of 120 °C. The change in microstructure with thermal aging along with the consumption of the barrier layer was understood. The joint shear strength evolution with thermal aging and the contamination of the top surface of the spheres was studied by the X-ray photoelectron spectroscopy (XPS) analysis. It was found that the trend for experimental consumption of the barrier layer closely follows the theoretical model predictions. Solid-state solder wicking of the Sn57.6Bi0.4Ag (SBA) solder joint due to creep was confirmed at high homologous temperature for the solder. This, along with grain coarsening over time, led to a decrease in the joint shear strength from 33.5 to 16 MPa. Polymer collars are suggested as a potential solution to prevent the solder wicking and make the package feasible for socketing applications.