Thermal cycling reliability of Sn-Zn lead-free solders in sensor application

There is a demand for low melting temperature solder in some applications (e.g., sensor attachment) where the components being soldered are temperature-sensitive. However, the same solder needs to meet the life cycle reliability requirements for the entire product. Among low temperature lead-free solders, eutectic Sn-9%Zn (wt.%) lead-free solder offers good mechanical reliability and low melting temperature. However, the presence of Zn makes it susceptible to oxidation especially at elevated temperatures. In this paper, the reliability of sensor attachments using Sn-9%Zn solder and capillary underfills was studied under thermal shock. Three different underfill materials were used with two of them containing fillers. The thermal shock test results showed that the underfills substantially improved the lifetime of the solder joints, and the underfills wth fillers provided the best mechanical support to the solder joints. The reliability of the Sn-9%Zn solder joints with underfills was found to be comparable to that of the Sn-Pb-2%Ag solder joints. Failure analysis revealed that a uniform distribution of the underfill was critical to achieve a reliable sensor attachment.