Effects of isothermal aging and in-situ current stress on the reliability of lead-free solder joints

The microstructure and mechanical behavior of Sn-Ag-Cu (SAC) soldering alloys can change significantly over time when exposed to isothermal aging and in-situ current stress. Electronics assemblies built with SAC solder joints are exposed to elevated ambient temperatures and relatively high current densities for prolonged periods of time. Therefore it is important to understand the impact of these stresses on the long term reliability. In this study, board level thermal cycle reliability tests have been performed on daisy chain lead-free 0.5 mm pitch CTBGA, 1.0 mm pitch PBGA, and 0.5 mm pitch MLF packages. The tests were performed with and without isothermal pre-aging. During the thermal cycling tests, the daisy chained solder joints were subjected to different levels of constant current density. The thermal cycle profiles were tailored in order to evaluate the effect of current density on solder joint fatigue, while minimizing the secondary effects of joule heating. The results show that the effects of both thermal aging and current stress are consistent for a given package type, but the trends can be different for different package designs. In 0.5 mm pitch CTBGA, isothermal aging significantly reduced the solder joint life, while current stress caused the solder joint life to slightly increase at moderate current densities, and eventually degrade at higher current densities. In PBGAs, the effect of thermal aging was insignificant but current stress caused a slight degradation in life. In MLF packages isothermal aging caused a serious degradation in solder fatigue life, while the effect of current stress was not significant.

Introduction

The present industry migration of networking and server products to lead-free soldering technology poses several challenges from a reliability standpoint. Unlike eutectic tin-lead solder, the microstructure and consequently the mechanical behavior of tin-based Sn-Ag-Cu alloys changes over time at room temperature. At elevated temperatures the changes occur even more rapidly [1]. Due to this complex material behavior, the impact of various environmental stresses on the microstructure and thermo-mechanical reliability of lead-free solders is not fully understood. BGA components are often stored at room temperature for prolonged periods of time, and once assembled they often operate under elevated ambient temperatures. Hence the second level solder joints are constantly being aged. Furthermore, in fine pitch components for high power applications, the relatively high current and reduced cross-sectional area results in prolonged exposure of certain BGA interconnects to relatively high current density. The increasing use of lead-free flip-chip bumping and lead-free surface mount power conversion devices will further exacerbate this concern. Figure 1 shows the expected relationship between current and nominal current density of Ball Grid Array (BGA) and Micro Lead Frame (MLF) solder joints with varying interconnect dimensions. Figure 1 Nominal current density versus current for typical interconnect dimensions