Crack Development in a Low-Stress PBGA Package due to Continuous Recrystallization Leading to Formation of Orientations with [001] Parallel to the Interface

Abstract

Thermal cycling was imposed on plastic ball grid array (PBGA) packages with a small die, a package design that does not impose a large strain on solder joints. Less cracking was observed after 2500 cycles from 0°C to 100°C (with 10 min dwell times and 10 min ramps) than in a prior study with a higher-stress package design, so these samples were thermally cycled (TC) to 6400 cycles to investigate the relationship between cracks, microstructure, and grain crystal orientation. Cracked joint locations within the package were identified using the dye and pry method, indicating that cracks were most often found in joints near the perimeter of the die. Using orientation imaging microscopy (OIM), cracks were observed in many joints having a variety of dominant crystal orientations where the c-axis was between 0° and ∼50° from the package interface. Continuous recrystallization processes occurred and caused gradual rotations of initial orientations that reduced the angle between the c-axis and the package interface. While cracks were observed in joints with a variety of orientations, cracks were highly correlated with recrystallized grains having the [001] c-axis nearly parallel to the interface (“red” orientations) in those joints that did not initially have this orientation.