Inhibiting CoSn3 growth at the Sn/Co system by minor Zn addition

• Liquid-state and solid-state Sn–Zn/Co interfacial reactions were examined.
• CoSn3 growth is greatly suppressed with 0.1 wt%Zn addition in solder.
• Inhibition of CoSn3 growth is more significant in solid-state interfacial reactions.
• Special linear growth of CoSn3 was found.
• Growth kinetics of CoSn3 was investigated with various Zn additions.

CoSn3 is the major reaction product in Sn/Co interfacial reactions. Compared with other substrate materials, e.g., Cu and Ni, the CoSn3 growth is much faster during reflow soldering and thermal aging. However, excessive formation of intermetallic compound (IMC) layer might deteriorate the reliability of solder joints. This study demonstrated that the CoSn3 growth was effectively slowed down by adding minor amounts of Zn in Sn solder. The solder reactions between Co and Sn with addition of various Zn contents (0.05 wt% ∼ 2 wt%) were systematically examined in liquid-state and solid-state reactions. For liquid-state reactions at 240 °C–260 °C, the CoSn3 growth rate was decreased by 50% with 0.1 wt%Zn addition, and further decreased by 75% with 0.5 wt%Zn. Moreover, the growth kinetics of CoSn3 was investigated with various Zn additions. The CoSn3 growth was found to be proportional with reaction time, and their apparent activation energies were very close (between 105 kJ/mol ∼ 110 kJ/mol). For solid-state reactions, the effect of Zn addition on the inhibition of CoSn3 growth was more significant. The growth rate was greatly reduced by 75% with only 0.05 wt%Zn addition. The possible inhibition mechanism was discussed.