Effect of reflow time on wetting kinetics, microstructure and joint strength of Sn-Cu and Sn-Ag-Cu solders

Abstract

In the present study, the effect of reflow time on wetting behaviour, microstructure and shear strength of the Sn–0.7Cu, Sn-0.3Ag-0.7Cu and Sn-2.5Ag-0.5Cu lead-free solders on bare and nickel coated copper substrates was investigated. The solder alloys were reflowed at 270 °C for various reflow times of 10-10000 s. Furnace and quench cooling methods were adopted to assess the effect of varying cooling environment on solder/substrate systems. Final contact angle (θf) decreased with increase in reflow time. The spreading behaviour of the solder was categorized into capillary, gravity (diffusion), and viscous zones. Intermetallic compound (IMC) thickness initially increased with increase in reflow time up to 500 s in all solder/substrate systems. Furnace cooled solder/Cu substrate systems showed a drop in IMC thickness for the reflow time above 500 s due to spalling effect. IMC thickness increased with further increase in reflow time above 1000 s. Similarly, Sn- 0.3Ag-07Cu and Sn-2.5Ag-0.5Cu solder alloys reflowed for 500 s on Ni coated Cu substrates showed a drop in IMC thickness. IMC thickness was fitted to a growth model to study the growth kinetics. The period corresponding to the end of gravity zone (Tgz) was measured from the relaxation curve obtained from wetting studies. Tgz was found to be 25 s for Sn–0.7Cu and 40 s for Sn-0.3Ag-0.7Cu and Sn-2.5Ag-0.5Cu solder alloy spreading on bare Cu substrate. The corresponding values were 50, 70 and 80 s respectively for the same alloys on Ni coated Cu substrate. Solder alloys were once again reflowed for the period corresponding to the end of gravity zone. Ball and single lap shear tests were performed to assess the joint strength as a function of reflow time. The joint strengths were found to be maximum at reflow times corresponding to the end of gravity regime for all solder/substrate systems. Ball and single lap shear tests were also carried out for eutectic Sn-Pb solder alloy on bare and Ni coated Cu substrate to compare the results obtained with those of lead free alloys. Joint and yield strengths of all lead based samples were also found to be maximum at Tgz. Ball shear strength of Sn-Pb was found to be lower than lead free alloys. Yield strength of Sn-Pb solidified on bare Cu plate was comparable with Sn-0.7Cu solidified on Ni coated Cu plate. A dynamic contact angle at the end of gravity zone (θgz) was found to be a better wetting parameter compared to the final contact angle (θf) and the time to reach this value was used to assess the effect of wettability of liquid solder on microstructure and joint strength. The present investigation revealed the significance of gravity zone in the assessment of optimum reflow time for lead free solder alloys.