Growth of tin hillock and whisker under harmonic power conditions in tin-plated copper busbars for power system

Tin hillock and whisker growth pose a serious reliability concern in tin-coated copper busbars, particularly in lead-free systems subjected to dynamic electrical environments. In this study, the influence of harmonic current stress on hillock and whisker formation in tin-plated copper substrates was systematically investigated. The experimental parameters included tin layer thickness, conductor spacing, interfacial intermetallic compound (IMC) formation, and post-deposition annealing treatment. The results demonstrate that harmonic currents accelerate hillock formation, leading to larger size and higher density within the same stressing period due to significant surface resistive Joule heating accumulation at the tin/copper interface. Samples without post-annealing stress-relief exhibited the most severe hillock growth, while post-annealed samples showed suppressed hillock development due to IMC formation and enlarged grain size. Moreover, a thicker tin layer provided more free tin atoms to migrate under surface resistive Joule heating, further promoting hillock formation, whereas wider conductor spacing slightly mitigated this effect. These findings reveal the coupling effect between thermal, structural, and material factors on hillock behavior under harmonic stress and provide practical insights for enhancing the reliability of tin-coated copper busbars in power electronic systems.