A study of moisture and thermally induced die stresses in plastic ball grid array packages

Abstract

Electronic packages absorb moisture when exposed to uncontrolled humid conditions during manufacturing processes and service life. At high temperatures, the effects of moisture absorption on electronic packages become even more significant. A number of failure modes are caused by moisture effects such as popcorn cracking, delamination, and electrochemical migration. In this study, the effects of moisture on die stresses in Plastic Ball Grid Array (PBGA) packages have been explored. The tested PBGAs were 27 × 27 mm in size, with 416 solder balls on a 1 mm pitch. They were assembled with silicon die of two different die sizes were used (5 × 5 and 10 × 10 mm). The complete state of stress at various points on the die surface was obtained using stress sensing test chip technology. The samples were exposed to a harsh high temperature and high humidity environment (MSL 1–85 °C, 85% RH) for various time durations, and allowed to adsorb moisture. The variations of the die stresses at several locations were characterized as a function of time during the hygrothermal exposure. The weight of each sample was also measured during the moisture exposure to quantify the uptake of water. After the moisture exposure, the samples were then baked in thermal chamber (85 °C) to check the reversibility of moisture absorption and die stress variation. In addition to the experiments at the package level, an investigation on the moisture properties of the BT substrate and mold compound in the PBGA was completed. The moisture properties (diffusivity D, saturated concentration Csat, and coefficient of moisture expansion β) of each material were experimentally obtained. Unlike the traditional method of measuring the out-of-plane coefficient of moisture expansion (CME) using a TMA instrument, a new approach was used in this work to characterize the in-plane CMEs using a nanoindentation system. Finally, a finite element numerical simulation was performed, and the predictions were correlated with the experimental results. The measured moisture properties obtained earlier were used in the model. Unlike conventional approaches using the moisture-thermal analogy, an advanced approach was implemented to perform coupled multi-physics simulations of the moisture diffusion process without the limitations that can be seen in conventional method. Good agreements between numerical predictions and experimental results were observed. Both the measurements and numerical simulations provided a valuable insight on moisture induced failure phenomena in Plastic Ball Grid Array Packages.