Detection of Thermal Cycling-Induced Failures in RF/Microwave BGA Assemblies

The purpose of this paper was to investigate the effect of thermal cycling on the high-frequency behavior of ball grid array (BGA) interconnection structures. In order to characterize the applicability of RF measurements in predicting interconnection breakdown, a broadband BGA transition structure between a radio frequency printed wiring board (RF-PWB) and a ceramic module was fabricated. In addition to basic assemblies consisting of two BGA transitions between the module and substrate, the designed transition was applied in a passband filter module to demonstrate the effect of thermal cycling on the performance of a practical device, as well. The BGA test modules mounted on the PWBs were exposed to thermal cycling testing over a temperature range of -40degC to + 125degC. To detect interconnection failures induced by cyclic thermal stresses, both dc resistance and scattering parameter measurements were performed on the test assemblies at specific intervals. Parallel to the electrical measurements, crack propagation in the vicinity of the BGA transition structure was investigated using scanning acoustic microscopy (SAM). Moreover, scanning electron microscopy (SEM) was used to determine the failure mechanisms of the test assemblies. Degradation of the signal transmission characteristics of the basic assemblies was first observed at higher microwave frequencies as an increase in signal return loss (|S₁₁|) and/or a change in its phase. The effect of TCT on the filter assembly was more constant and clearer to observe in the phase than in the magnitude of S₁₁ in the passband. The dc resistance measurements showed no indication of degradation in any of the tested assemblies.