The performance matches SAC, but with shinier joints.

• Copper leaching is a critical issue with SAC 305. After multiple soldering processes, the copper pad may have disappeared when it comes time to rework.
• Reliability of SAC 305 is acceptable for component leads; it can accommodate strain – e.g., from SOICs and QFPs – but for leadless components, where the solder has to absorb all the strain (BGAs, LCCCs), SnCuNi is more reliable.¹
• SnCuNi shows less voiding in the majority of applications thus far.
• SAC 305 joints are dull and grainy, with the surface showing a microstructure of coarse intermetallic crystals. While simple SnCu eutectic exhibits a dull cracked surface when cooled slowly, SnCu with the nickel addition solidifies with a generally smooth bright finish under the same conditions.²

• No significant difference is seen between the 3 min. linear and 5 min. ramp-soak spike heating profile.
• One heating cycle before reflow soldering resulted in a slightly thinner intermetallic layer (5% on average). This may be caused by more oxidation of the copper surface.
• The average IMC thickness of SnCuNi is more or less equal to that of SAC 305, although time above liquidus and superheat are less.

• No significant differences between SnCuNi and SAC alloy when reflowed under the same conditions. However, the SnCuNi solder is shinier, much like SnPb.
• A minimum peak temperature of 240ºC is required for SnCuNi to achieve good solder coalescence.
• Although time above liquidus is less for SnCuNi (higher melting point), the initial intermetallic thickness is not different from that of SAC alloy. Previous investigations showed that on account of nickel, the intermetallic does not increase as much as that of the SAC alloy after aging.

1. Thomas Kolossa, Balver Zinn presentation on SN100C reliability.
2. Frédéric Martinez, Syed Asif Arnaud, Keith Sweatman and Tetsuro Nishimura, “Improving the Performance of the Tin-Copper Eutectic as a Pb-free Solder.”