Elimination of tin whisker growth by indium addition to electroplated tin in electronic packages

Abstract

Whisker growth from Sn coatings is a serious reliability concern in electronic packages particularly when the required service life is over a decade e.g in aerospace and defense applications. Until recently whisker growth from tin coatings was mitigated by the addition of a small amount of Pb. However current requirements for lead-free electronics preclude using Pb as an additive requiring an alternate approach to whisker mitigation. Here we present the results of In-alloying on whisker growth from 1 pm and 3 pm thick electroplated Sn films and demonstrate that when 5–10% indium is incorporated throughout the thickness of the Sn films whisker growth is completely eliminated. The reasons behind this mitigation are experimentally explored using X-ray diffraction (XRD) and finite element analysis (FEA). XRD is used to elucidate the role of Cu-In intermetallic formation. FEA is used to investigate the role of In-incorporation in the surface-oxide as revealed by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). Based on the analyses it is inferred that the observed mitigation due to In addition occurs due to a multitude of factors including reduction of the driving force for whisker growth due to incorporation of In in the native Sn-oxide film and the formation of Cu-In intermetallics at the substrate-coating interface.