The Effects of Thermal Aging on the Mechanical Behavior of Fine Pitch Electronics Packages

Abstract

The direction of Electronics packaging industry is to design smaller packages with higher complexity while focusing on Sn-Ag-Cu as the alloy of choice for lead free electronics assembly. While performance capabilities have increased, thermo-mechanical reliability is a significant concern for harsh environment applications. The assemblies may be subjected to shock and vibration during day to day usage or while shipping and handling. The Inter-Metallic-Thickness which is formed during the reflow soldering process dictates the performance characteristics of the solder joint. In case of most electronics which are heavily subjected to thermal cycling due to frequent use and idleness causing fatigue, solder joint and interface failure of the component, where as portable electronics and critical equipments on Air craft’s and land vehicles are subjected to repeated vibration and drop causing fatigue. The solder joint failure can be attributed to the structural dynamics of the product. It is necessary to understand the effect of time and temperature on the SAC solders joints. This research uses a collection of fine pitch electronics packages to match a wide variety of packages available and test vehicles were built. These Test vehicles were subjected to Isothermal aging at 55°C over a period of 6, 12 and 24 months and then subjected to vibration and shock to understand the rate of reliability deterioration. The test was conducted in a step stress manner with the primary objective being the study of rate of deterioration of the two commonly used SAC alloys over time. ii This dissertation deals with the challenges posed in the manufacturing of the fine pitch electronic parts due to bridging and solder paste printing process as well as the challenges posed by the solderability issue of ImSn boards at Continental Electronics, Huntsville, Alabama. It also deals with the issues caused by non standard test boards for the respective tests. The effect of temperature over time on the mechanical behavior was compared between two SAC alloys 305 and 105 and with the recent thermal cycling data to identify and the differences in the behavior of the SAC alloys. The results shed light on the serious problems with SAC alloy reliability. This research shows clear reliability issues with using SAC alloy solder for harsh environment applicants. In addition it provides necessary data for the modeling of solder joint reliability to gain a deeper understanding of their behavior and approaches to address the issue.