Effects of solder alloy compositions on microstructure and reliability of die-attach solder joints for automotive applications

Abstract

For harsh electronics environments, where some applications (e.g., automotive and defense) require operating temperatures at 150 °C or higher, the Pb-free SnAgCu (SAC) solders such as Sn3.0Ag0.5Cu (SAC305) and Sn3.8Ag0.7Cu (SAC387) alloys are not reliable enough to replace the high-Pb, high melting temperature solders. Harsh environment electronic industries that are currently exempted from RoHS regulations are actively searching for a suitable replacement solder. In this study, new Pb-free solder alloys based on SnAgCuSbBi with variations of Sb, Bi, and Cu alloying contents and Ni/Co dopants were tested against the conventional high-Pb solder (92.5Pb5Sn2.5Ag) in an effort to meet the requirements for the harsh environment electronics applications. Evaluations were conducted on Si die-attach assemblies on Ni-plated Cu lead-frame made with solder preforms in thermal shock cycling tests (-40°C to 150°C with a dwell time of 20 minutes). Crack length measurements in the solder joints and die shear strength tests were performed after various cycles of the thermal shock testing. The solder joint microstructures and effects of alloy compositions were also investigated by means of optical and scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (EDS). The results show that the solder alloys compositions significantly influence the microstructure and reliability of die-attach solder joints. The novel Pb-free solder alloys based on SnAgCuSbBi outperform the high-Pb high-temperature solder in the reliability testing.