Journal of SMT Article

Gold Embrittlement in Leadfree Solder

Authors: Craig Hillman, Ph.D., Nathan Blattau, Joelle Arnold, Thomas Johnston, Stephani Gulbrandsen, Julie Silk, Alex Chiu, and Yew-Hoong Chan
Company: DfR Solutions and Agilent Technologies
Date Published: 10/31/2013   Volume: 26-4

Abstract: Gold embrittlement in SnPb solder is a well-known failure mechanism in electronic assembly. To avoid this issue, prior studies have indicated a maximum gold content of three weight percent. This study attempts to provide similar guidance for Pb-free (SAC305) solder. Standard surface mount devices were assembled with SnPb and SAC305 solder onto printed boards with various thicknesses of gold plating. The gold plating included electroless nickel immersion gold (ENIG) and electrolytic gold of 15, 25, 35, and 50 microinches over nickel. These gold thicknesses resulted in weight percentages between 0.4 to 7.0 weight percent.

Samples were aged for up to 1000 hours and then subjected to a range of environmental stresses, including thermal cycling, and both low and high speed shear testing. Results from thermal cycling indicated an elevated risk of early life failures in SnPb solder joints once gold weight percentage exceeds four percent, which is in line with the results from previous studies. SAC solder showed no indication of early life failures during temperature cycling, yet showed progressive degradation with increasing Au content.

The results from low and high speed shear testing seem to suggest that SAC305 solder is more capable of maintaining mechanical properties with increasing gold content. This improvement in gold embrittlement may be partially explained by the additional tin content. Fractography was performed to confirm transitions from ductile to brittle behavior. Metallographic inspections and x-ray mapping were performed to confirm calculated gold content and assess the influence of intermetallic morphology and location on mechanical performance.

Keywords: 

SAC solder, gold embrittlement, high speed shear testing, thermal cycling, intermetallic



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