Characterization study of lead-free Sn-Cu plated packages

Secondary alloying element addition in Sn deposits, like Pb, Cu, Ag or Bi, to reduce the melting point of Sn and improve the plated substrate wettability, and even interrupt the grain growth boundary to inhibit whisker formation, has been explored for many years. Various alloying elements co-deposited with Sn give rise to specific impacts on the plating process, cost and physical properties of the deposit. Co-deposited Pb has been known as the most efficient method (Kakeshita et al., J. Mat. Sci. vol. 17, p. 2560, 1982), but based on the environmental trend to eliminate Pb, finding drop-in alternatives becomes both necessary and urgent. The Pb replacement must possess compatibility with existing plating equipment, stable electrolyte, easy process control, good mechanical properties, acceptable solderability with lead free solder paste, whisker free behaviour and low chemical costs. In this report, a Sn-Cu plated package is selected as test vehicle to describe solderability, whisker growth mechanism and solder joint reliability. It includes solder dipping, wetting balance Cu composition measurement, deposit microstructure, and whisker observation under HAST (130°/85%RH, 33.5 psi) for gull-wing and J-leaded packages. In addition, lead pull tests and electrical O/S tests under TCT (-40°C/125°C) with solder pastes such as Sn-Pb and Sn-Ag-Cu are also explored, and the failure mechanism is scrutinized by SEM/EDX. The results show that the 2% Cu co-deposit in the Sn-Cu plating is free of whisker growth and solder cracking, and provides reliable solderability and robust solder joints. Cu migration during HAST is also addressed.