Package structural integrity analysis considering moisture

There are three types of failures when an electronic package is exposed to a humidity environment: the popcorn failure at soldering reflow, the delamination and cracking at HAST without electrical bias, and the corrosion under biased HAST. Despite the difference in failure mechanisms, the common and most contributory factor to moisture induced failures is the degradation of adhesion strength in the presence of moisture. Therefore, understanding interface behavior with moisture becomes a key for package integrity and reliability analysis. In this paper, first, several methods to characterize the interfacial fracture toughness or adhesion strength at elevated temperature with moisture effect are presented. The interfaces between polyimide on silicon chip and underfill (PI/UF) are used as a carrier to investigate the influence of moisture. Both interface fracture mechanics based fracture toughness measurement techniques and the quick-turn method such as die shear test are applied to investigate the interface behaviors with moisture. Details of several sample preparation methods, by which the fracture can be made to stay along the desired interface, are illustrated. Key results on the influence of the moisture on fracture toughness are presented. Next, the hygroscopic swelling characterization techniques are reviewed. Due to the fact that the moisture diffusion is a slow process, specimens used in hygroscopic swelling measurement are often subjected to a non-uniform moisture distribution. This becomes a potential hidden error in obtaining the coefficient of hygroscopic swelling. Analytical solutions have been devised to predict the errors caused by the non-uniform moisture distribution. A simple procedure in obtaining the accurate swelling characteristics is proposed. Both TGA-TMA method and Moiré interferometry method are applied to measure the hygroscopic swelling behaviors of several underfills. A very good agreement between these two methods is achieved. Subsequent to hygroscopic swelling characterization, the paper presents a novel method to allow a time-dependent nonlinear finite element analysis for package deformations and stresses induced by hygroscopic as well as thermal mismatches. This has been a challenging problem since commonly used commercial finite element software such as ABAQUS and ANSYS do not explicitly allow the fully coupled nonlinear thermal and hygroscopic stress analysis. The existing linear superposition method, which couples hygroscopic stress with thermal stress analysis, can not apply to the problem with nonlinear materials such as polymers and solder materials. A fully integrated finite element stress modeling methodology is demonstrated through an example of flip chip package subjected to a multi-step humidity/temperature loading profile. The results of the effect of hygroscopic swelling on the inter-layer dielectric (ILD) and under bump metallurgy (UBM) structures reveals that the overall ILD stresses under HAST can be twice as high as those considered without the moisture effect. The contribution of these hygroswelling-induced tensile stress is very significant to lead to the interfacial damage and electrical failures during HAST.