Degradation of Sn-Ag-Cu heat-sink attachment during thermal shock cycling

Purpose - To investigate the degradation of lead free solder heat-sink attachment by thermal shock. Samples with high voiding percentages were selected for the investigation in order to get information on the significance of voids on the reliability of Sn-Ag-Cu heat-sink attachment. Design/methodology/approach - Through the use of X-ray, C-mode scanning acoustic microscopy, dye penetration, cross section and scanning electron microscopy/energy-dispersive X-ray tests, the degradation of Sn-Ag-Cu heat-sink attachment during thermal shock cycling was evaluated. Findings - The results showed that the Sn-Ag-Cu heat-sink attachment where the area of voiding was 33-48 per cent survived 3,000 thermal shock cycles, although degraded. The main degradation mechanism for the solder attachment was not solder fatigue but interface delamination due to Kirkendall voids at the Cu/Cu3Sn interface. it was found that the large voids in the Sn-Ag-Cu heat-sink attachment were not significantly affecting the solder joint lifetime. Big differences of intermetallic compound growth behaviour and Kirkendall voids at device/solder and solder/Cu pad interfaces are found and the reasons for this are discussed. Originality/value - This work has clarified the general perception that large voids affect the thermo-mechanical lifetime of solder joint substantially and also provides further understanding of the Sn-Ag-Cu heat-sink attachment degradation mechanism.