PROBABILISTIC PHYSICS OF FAILURE ASSESSMENT OF THERMOMECHANICAL FATIGUE IN HIGH-I/O AREA-ARRAY INTERCONNECTS

Abstract

Thermal cycling durability of Plastic ball grid array (PBGA) interconnects is known to decrease as I/O count increases. This is due, in part, to mechanistic effects; such as increasing thermal expansion mismatches between component and PWB, due to increasing package sizes. Failure prediction due to these mechanistic effects is a deterministic process and is based on the load level found in the critical joint (joint with the most severe loading). However, due to probabilistic effects, for example manufacturing variabilities, premature failure may result in one of several joints in the neighborhood of the critical one. Failure probability increases as the number of joints in this critical region increases. Thus, observed failure rates are due to a convolution of deterministic and probabilistic effects. In effect, for large BGAs, deterministic predictions may overestimate interconnect durability. This thesis uses thermal cycling experiments and detailed mechanistic modeling to present a methodology for adjusting deterministic predictions of solder joint failure with a suitable probabilistic correction factor.