A New Approach for Early Detection of PCB Pad Cratering Failures

Pad cratering refers to the initiation and propagation of fine cracks beneath BGA pads in organic substrates or printed circuit boards. These cracks, which usually initiate under the application of excessive mechanical loads, represent a serious reliability concern for the industry. In typical board level reliability tests, solder joint failures are detected by an increase in electrical resistance of a daisy chain circuit followed by failure analysis. However, board level testing to determine the onset of BGA pad cratering has been problematic because the early stage of this failure mode is not associated with an electrical signature. Based on the mechanism of pad cratering, it is known that the cracks initiate beneath BGA pads and grow under continually increasing stresses until the pad completely separates from the substrate and a pad “crater” is formed. The catastrophic fracture of an interconnect, which causes an electrical “open”, is in fact the final and most catastrophic stage of the failure. At present the higher strain levels based on electrical resistance monitoring are being reported and used in design practices. In this study, a new monitoring approach based on acoustic emission has been introduced for early detection of pad cratering failure. Two different lead-free daisy chain test vehicles were used with 1.0 mm pitch HSBGA-1096 and 0.8 mm pitch CABGA-160 packages, and four-point bend tests were performed to induce pad cratering. Acoustic emission activity from the test vehicles was monitored along with the electrical resistance of the daisy chain circuit. The bend test results, in conjunction with failure analysis, have shown that acoustic emission monitoring is indeed an effective methodology to detect the onset of the pad cratering. In contrast, the electrical daisy chain failure was detected at significantly higher strain. Using the acoustic emission approach, it has been found that PCB pad cratering failures can initiate at strain levels significantly lower than previously reported. This board level test methodology may now be used to evaluate the propensity of different materials and packages to pad cratering, and also to improve back-end manufacturing processes without using daisy chain test vehicles. Introduction The term “pad cratering” refers to the formation of fine cracks underneath BGA pads either in the PCB laminate material or in organic package substrates. Figure 1 schematically depicts the possible location of these cracks. Due to external mechanical load, the cracks initiate due to stress concentrations arising from the corner of the copper pad and the material property mismatch between copper and the laminate resin. The cracks propagate within the resin, up to the point where the underlying glass fiber bundles are encountered. Under subsequent loads, the cracks continue to grow along the interface between the resin and glass fibers, and also beyond the glass fiber bundle. Prior studies have shown that the extent of crack growth under the BGA pads is highly dependent on the location and orientation of glass fiber bundles with respect to the BGA pad [1]. Figure 1 Schematic Depiction of Pad Cratering Pad cratering is a serious concern because depending on the loading conditions, the cracks can act either as a source for instantaneous catastrophic failure or as latent defects that can affect long term reliability. The ongoing industry transition to lead-free assemblies has further magnified this concern due to several factors. The higher reflow temperature involved in As originally published in the IPC APEX EXPO Proceedings.