Authored By:
Anurag Bansal, Gnyaneshwar Ramakrishna and Kuo-Chuan Liu
Cisco Systems, Inc.
San Jose, CA USA
Summary
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.
Conclusions
A board level acoustic emission monitoring method has been developed as a means for detecting the onset of pad cratering failures in PCB bending tests. Using this approach, the board level strain limits for PCB bending in assembly, test, and handling operations can be determined at different strain rates. Based on the acoustic method, the PCB strains at the onset of pad cratering were found to be significantly lower than prior expectations, which were based on electrical failure of daisy chain circuits. In comparison with a larger and stiffer 1.0 mm pitch HSBGA package, a smaller and more flexible 0.8 mm pitch CABGA package was found to have a significantly lower strain limit.
This implies that the effect of a pad size is a significant variable and finer pitch BGA components should be considered at high risk for pad cratering. Pad cratering failures were found to be relatively independent of the strain rate in HSBGA packages, but the smaller CABGA packages had a lower pad cratering strain limit at high strain rates.
Initially Published in the IPC Proceedings
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