Deformation of Crystal Morphology in Tin Plated Contact Layer Caused by Loading

Terutaka TAMAI  Shigeru SAWADA  Yasuhiro HATTORI  

IEICE TRANSACTIONS on Electronics   Vol.E95-C   No.9   pp.1473-1480
Publication Date: 2012/09/01
Online ISSN: 1745-1353
DOI: 10.1587/transele.E95.C.1473
Print ISSN: 0916-8516
Type of Manuscript: Special Section PAPER (Special Section on Recent Development of Electro-Mechanical Devices - Papers selected from International Session on Electro-Mechanical Devices 2011 (IS-EMD2011) and other recent research results -)
tin plated contacts,  contact resistance,  contact load,  contact trace,  connector,  FEM,  

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Tin (Sn) contacts are widely applied to connector contacts. Surfaces of plated tin layer are covered with an oxide film that results in high contact resistance. However, it is possible to obtain low contact resistance by using high contact load. Current downsizing trends often make it difficult to obtain high contact loads. Therefore, it is important to conduct basic studies of the contacts resistance characteristics under low contact load conditions. In this study, relationships between contact resistance and the changes of contact traces were examined. When a platinum (Pt) hemisphere contacted to tin plated flat coupon, it was found that the hemisphere surface sank into the softer tin plated flat surface during loading resulting in a piling up tin crystal grains along the periphery of the contact trace. During this process, sudden decrease in contact resistance was observed. To clarify the phenomenon, morphology changes of contact traces were observed by AFM, SEM and EBSD. FEM analysis was also used to analyze the mechanical stress distribution in the tin plated layer. Due to the peculiar distribution of stress, the crystal grains are separated and push out the contact area. This phenomenon is very different from commonly observed decrease in contact resistance due to elastic and plastic deformation inducing mechanical fracture of the surface oxide film.