Nanomechanical Characterization of Intermetallic Compounds in Lead Free Solder Joints

Abstract

Intermetallics (IMCs) in the solder joints have received a great deal of attention due to their increasing volume fraction with continuously decreasing package size. They possess very different mechanical behavior than those of the Sn rich dendrites and Cu pads in the solder joint. Understanding the behavior of interfacial IMCs has become a major concern for the researchers as fracturing near the interfacial IMC layers is often found to be the primary reason for failures caused by drop impacts. While subjected to Isothermal aging, Pb-free Sn–Ag–Cu (SAC) solder joints lead to growth of intermetallic (IMC) particles in the solder bulk as well as growth of intermetallic layers at the joint interfaces with copper bond pads. The most common IMCs found in SAC joints are Ag3Sn and Cu6Sn5 binary compounds. Ternary IMCs like Cu-Ni-Sn can also form at the interface of Ni containing surface finish (i.e. ENIG) and SAC solder. The continuous performance demand and improvement in 3D packaging has led to increased joule heating and higher operating temperatures, which necessitates measuring the mechanical properties and time dependent deformation (creep) behaviors of these IMCs as a function of temperature. In this study, the mechanical behaviors of IMC particles and layers in SAC solder joints have been characterized using nanoindentation. SAC solder joints extracted from BGA packages were first aged for 6 months at T = 125 oC. Nanoindentation test samples were subsequently prepared by cross-sectioning the aged solder joints, and then mounting them in epoxy mold and polishing them to prepare the joint surfaces for nanoindentation. Intermetallics formed in the bulk solder region, copper pad to SAC solder and ENIG to SAC solder interfaces were observed and detected using SEM and energy-dispersive x-ray spectroscopy (EDS) technique. They were then indented to measure their mechanical properties which includes the elastic modulus and hardness at room temperature and at elevated temperatures (T = 50, 75, 100 and 125 oC) using a heating stage. Time dependent deformation (creep) behaviors were also evaluated at room temperature and at 100 oC to see the effect of high temperature on the creep rate of IMCs. The measured properties of the IMCs were found to be significantly higher than the Sn-matrix forming the solder joints. However, with the increased temperature, the elastic modulus and hardness value gradually decreased and significantly dropped at 125 oC compared to their room temperature measurements. The creep strain rate was found to increase as the temperature approached 100 oC for all of the IMC materials.