The implementation of RoHS compliant materials and RoHS compatible soldering processes has driven the PCB laminate suppliers to provide laminates that are very robust to thermal processes. Common qualification tests include 6X reflows at 260C and time to delaminate at 260C and 288C. A common criterion in IPC-4101 is 30 minutes at 260C with no delamination. An unforeseen effect of increasing the thermal robustness is that the PCB laminates have become more brittle. A major contributing cause is the use of phenolic cured PCB materials compared to the pre-RoHS dicy cured materials. This brittleness results in an increasing incidence of pad cratering and component breakage from the PCB pads. Pad cratering occurs when PCBs assemblies with BGAs are flexed. This could occur at in-circuit test or during process such as depanelization. Although this problem has been observed in the industry before 2005 (1), the PCB laminate suppliers have still not addressed this issue by improving the mechanical robustness of laminates. It does appear that the laminate suppliers are dragging their feet on this issue. Currently the RoHS compatible laminates are over designed for thermal robustness and under designed for mechanical strength. One would think that after 8 years the laminate suppliers would now offer new material solutions to this problem.
It would be desirable to define an industry standard test that the suppliers could use to provide laminate strength data to customers. There are some industry standard test methods such as the ball pull test (IPC-9708) and the spherical bend test (IPC-9707). There is no test acceptance criterion within these procedures and the laminate industry does not publish data from these tests on data sheets or test reports.
References
1. F. Joyce, Transient Bend Board Flexure Initiative, IPC Presentation, June 2005
2. M. Ahmad, Comprehensive Methodology to Characterize and Mitigate BGA Pad Cratering in Printed Circuit Boards, SMTA Journal, Vol 22 Issue 1, 2009
3. C. Tulkoff, SMTA Pad Cratering Webinar, April 10, 2012
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