Tin-silver-copper

Tin-silver-copper (Sn-Ag-Cu, also known as SAC), is a lead-free (Pb-free) alloy commonly used for electronic solder. It is the main choice for lead-free surface-mount technology (SMT) assembly in the industry, as it is near eutectic, with adequate thermal fatigue properties, strength, and wettability. Lead-free solder is gaining much attention as the environmental effects of lead in industrial products is recognized, and as a result of Europe's RoHS legislation to remove lead and other hazardous materials from electronics. Japanese electronics companies have also looked at Pb-free solder for its industrial advantages.

Typical alloys are 3–4% silver, 0.5–0.7% copper, and the balance (95%+) tin. For example, the common "SAC305" solder is 3.0% silver and 0.5% copper. Cheaper alternatives with less silver are used in some applications, such as SAC105 and SAC0307 (0.3% silver, 0.7% copper), at the expense of a somewhat higher melting point.

In addition to traditional silver-containing SAC alloys, silver-free tin-copper based alloys with minor additions of nickel and germanium have been developed to enhance mechanical and reliability properties while reducing material costs. One such alloy, SN100CV (Sn-1.5Bi-0.7Cu-Ni-Ge), is designed as a drop-in replacement for SAC305 solder in surface-mount assembly processes. According to standardized IPC testing, SN100CV solder paste demonstrates excellent flux activity, wetting, and slump stability suitable for reflow soldering. It exhibits strong corrosion resistance evidenced by high surface insulation resistance and electrochemical migration resistance after prolonged environmental stress. These properties suggest improved long-term joint reliability and mechanical stability, making SN100CV a viable alternative to traditional SAC alloys in various electronic assembly applications.

Despite widespread regulatory encouragement, the transition to lead-free solders such as SAC alloys faced significant technical challenges. Misconceptions about melting points, solder joint reliability, and equipment compatibility initially complicated manufacturing adoption. Practical industry experience has shown that selecting eutectic or near-eutectic alloys and carefully adapting reflow processes and flux chemistry are critical to achieving reliable solder joints. Challenges including fillet lifting, corrosion potential, and changes in mechanical stress response required focused research and process optimization. Continued development and understanding of these phenomena have enabled SAC solders to achieve performance levels comparable to traditional tin-lead solders in many applications.

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