Mechanical reliability of CSP mounted on PWB

CSP (Chip Size Packages) are rapidly becoming more widespread with the popularity of portable electronic products. Portable products are subject to mechanical stress when dropped or pressed, so we performed a repetitive bending test on CSPs mounted on PWB (Printed Wiring Boards) in order to research and improve the mechanical reliability of CSPs. As a result, it was found that the defect modes transfer from solder cracks to via cracks as the strain range increases, and that the bending life of Sn-Ag-Cu lead-free solder was more sensitive to a change of span (the distance between fulcrums) than that of Sn-Pb eutectic solder. Therefore, we need to design in such a way that the strain range can be controlled sufficiently when using Sn-Ag-Cu lead-free solder. Furthermore, it was found that we could shorten the test period, because the bending life does not depend on the strain rate of the bending test.