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Figure 9: Cross-section examples: (a) standard cross-section, (b) powder filling, (c) polarized light on right.
Figure 10: Examples of AE events not associated with pad cratering.
Table 2: Ranking of Laminate Materials
Conclusions
Pad cratering susceptibility of five Pb-free compatible laminates and one dicy-cure non-Pb-free-compatible laminate was evaluated using 4-point bend testing with acoustic emission. Cross-sectioning results confirm that the acoustic emission method is a viable tool to detect pad craters during the 4-point bend test. However, not all AE events are associated with pad cratering. Laminate cracks in locations away from the component or between layers are detected by the AE method.
Location analysis shows the majority of AE events concentrate in the largest BGA package in the test vehicle. It indicates that pad cratering is elevated with the larger size of BGA package, which can be explained by the higher stress concentration under the pads of large BGAs than under the pads of small BGAs.
Both the number of AE events and the cumulative energy of AE events at a given applied load indicates that Laminate F is prone to pad cratering, confirming our experience. However, there is no statistically significant difference in the lowest applied load to detectable AE among these six laminates. Thus, the lowest applied load to detectable AE may not be a good indicator for comparing laminates’ resistance to pad cratering. It is recommended to use the number of AE events and the cumulative energy of AE events at a given applied load to evaluate laminates’ resistance to pad cratering.
The ranking of the six laminate materials is different among the pad-solder level tests, the drop test, and the 4-point bend test with acoustic emission. The most effective test method for predicting pad cratering susceptibility is inconclusive from this study.
Acknowledgements
The authors would like to thank Dr. Jianbiao Pan of California Polytechnic State University, San Luis Obispo, for technical support; and Allen Green, formerly of Acoustic Technology Group, for AE expertise.
References
[1] Dongji Xie, Dongkai Shangguan, and Helmut Kroener, “Pad Cratering Evaluation of PCB,” Proceedings of IPC APEX Expo, 2010.
[2] Mudasir Ahmad, Jennifer Burlingame, and Cherif Guirguis, “Validated Test Method to Characterize and Quantify Pad Cratering Under BGA Pads on Printed Circuit Boards”, Proceedings of IPC APEX Expo, 2008.
[3] Mudasir Ahmad, Jennifer Burlingame, and Cherif Guirguis, “Comprehensive Methodology To Characterize And Mitigate BGA Pad Cratering In Printed Circuit Boards”, Journal of SMTA, January 2009.
[4] IPC 9708: Test Methods for Characterization of Printed Board Assembly Pad Cratering, 2010.
[5] Anurag Bansal, Gnyaneshwar Ramakrishna, and Kuo-Chuan Liu, “A New Approach for Early Detection of PCB Pad Cratering Failures”, Proceedings of IPC APEX Expo, 2011.
[6] Anurag Bansal, Cherif Guirguis, and Kuo-Chuan Liu, “Investigation of Pad Cratering in Large Flip-Chip BGA using Acoustic Emission”, Proceedings of IPC APEX Expo, 2012.
[7] IPC-9709 working draft, “Test Guidelines for Acoustic Emission Measurement during Mechanical Testing,” 2012.
[8] Wong Boon San and Julie Silk, “Pad Cratering Susceptibility,” Proceedings of SMTA International Symposium, 2012.
This paper was first published in the IPC APEX EXPO technical conference proceedings. Reprinted with kind permission from the authors Boon-San Wong and Julie Silk from Keysight Technologies (formerly Agilent Technologies’ Electronic Measurement Group) and Richard Nordstrom, Ph.D., Emerson Process Management (formerly of Acoustic Technology Group).
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