Effects of thermal aging on plastic ball grid array microprocessor life under power and mechanical cycling

Abstract

Traditional finite element based predictions for solder joint reliability of electronic packages use the accelerated thermal cycling life testing method and are based on solder constitutive equations (e.g. Anand viscoplastic model) and failure models (e.g. energy dissipation per cycle model). The Accelerated thermal testing method has the drawback that it considers uniform temperature distribution in the entire package during operationwhich is not the actual case. Also, the constitutive equations and failure models are subject to change with thermal aging which is a common phenomenon in electronic assemblies that causes great changes in the microstructure, mechanical response, and failure behavior of Lead-Free solder joints, whichare not considered in the traditional finite element analysis of thermal cycling method.Besides, in commercially available solders, the percentage of silver is generally varied between 1 ~ 4%, which in turn affects the mechanical properties of solder joints. In this study, finite element analysis of a 3D plastic ball grid array (PBGA) package has been performed firstly for the power cycling simulations to observe the effects of variation of aging time (between 0 to 365 days) and specified range of Ag% of solder materials. The non uniform temperature distribution of the package is first obtained by a non-linear transient heat transfer analysis for this purpose. In next, another cycling named as mechanical bend cycling is considered in this study. Electronic packages may be subjected to mechanical cycling during manufacturing or assembly processes, in transportation, and in application stages that can pose a threat to the package life. Three different loading frequencies 1, 3, and 5 Hz with three different testing temperatures (25, 75, and 125 ℃) have been used in mechanical cycling simulations. Effects of solder aging (0 to 365 days) and variation of Ag (1 ~ 4 %) on the mechanical cycling life has also been investigated. Results show that solder aging enhances power cycling life of the PBGA packages while it becomes detrimental in case of mechanical bend cycling. Similarly, a boost up of the power cycling life has been observed with changing the Ag % from 1 to 4% while it has proven to deteriorate the mechanical cycling life of the PBGA package.