Authors: Brian Roggeman and David Rae Company: Universal Instruments Corporation Date Published: 10/16/2011
Abstract: PCB pad craters are generally associated with single overstress events that cause immediate failure. However, significant reliability concerns are raised when partial non-catastrophic cracks are present under the pads of an assembly at the beginning of service life. This investigation empirically examines the effects of partial pad craters on board reliability. Partial pad craters were created through bending over-stress events which simulate damage modes experienced during assembly, handling, test, and shipment. Crack frequency and crack area distributions were measured for a variety of board flexure magnitudes. Final long-term reliability was then characterized for specific pre-damage levels in cyclic bending environments. The results revealed a dramatic degradation in reliability which was then correlated back to the initial crack distribution from the pre-reliability characterization damage event. Furthermore, the relative effect of pre-damage on reliability was markedly greater as the fatigue stress levels were reduced.