In the microelectronics industry, Sn is often electroplated as a protective layer on Cu conductors. Pure Sn layers on Cu develop whiskers that can cause component failures and have even been implicated in the loss of several satellites. Alloying Sn with Pb suppresses whisker formation, but the push towards Pb-free processing will make this unacceptable in the future. To understand the driving forces and mechanisms of whisker formation on pure Sn, we are measuring the kinetics of stress evolution and intermetallic formation in Sn/Cu layers. By using thin films of Sn and Cu, we can monitor the stress evolution in real time using wafer-curvature based techniques. Preliminary results of stress evolution in vapor-deposited films are presented showing the evolution of tensile stress in the Cu layers and compressive stress in the Sn layers.
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The authors appreciate helpful discussions with Gordon Barr and Ben Freund and gratefully acknowledge the support of Brown University’s Materials Research Science and Engineering Center (DMR0079964) for this research.
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Chason, E., Reinbold, L. & Kumar, S. Whisker Formation in Sn Coatings on Cu. MRS Online Proceedings Library 851, 316–321 (2004). https://doi.org/10.1557/PROC-851-NN5.17