Some fundamental issues in the use of Zn-containing lead-free solders for electronic packaging

A synergistic approach is applied to address the major concerns about the use of Zn-containing lead-free solders for electronic packaging. Using computational thermodynamics as a predictive tool, the phase stability of the Ag-Al-Cu-In-Sn-Zn system is examined to design a Zn-containing lead-free solder with melting characteristics similar to near-eutectic Pb-Sn solder. Theoretically, it is found that a Sn-0.3 mass%Al-4.2 mass%In-7.8 mass%Zn solder has a melting point (liquidus temperature) of 185°C and a solidification range of 10°C. It is demonstrated that environmentally benign fluxes containing tin-organometallics significantly improve the wetting behavior compared to rosin fluxes used for lead-tin solders. For the Sn-Zn eutectic solder on a Cu substrate at 260°C, it is found that the contact angle is reduced from 150° to about 25° when tin-organometallic fluxes are used instead of rosin flux. Severe accelerated tests (85% relative humidity at 85°C) for up to six weeks show that the mechanical properties of Sn-Zn eutectic solder interconnects are not affected adversely by the environment.