Reliability of laser soldering using low melting temperature eutectic Sn Bi solder and electroless Ni-electroless Pd-immersion Au-finished Cu pad

Abstract

The demand for flexible wearable devices/substrates with miniaturization and improved integration in microelectronic devices has intensified the research interest in low-temperature laser soldering processes as an alternative to conventional reflow soldering processes owing to their advantages, such as local heating, non-contact heating, and short bonding time. In this study, we compared and evaluated the reliability of laser soldered and conventional reflow soldered joints using representative low melting temperature eutectic Sn Bi solder and thin electroless Ni-electroless Pd-immersion Au (ENEPIG)-finished Cu pads. Laser soldering was performed using various laser powers (130, 150, and 170 W) and times (2 and 4 s). Furthermore, an aging test was performed at 110 °C for 2000 h to evaluate the long-term reliability of the soldered joints. The mechanical properties, including the top and cross-sectional views and fracture surfaces, of the soldered joints were analyzed by conducting shear tests after aging. During laser soldering, various intermetallic compounds (IMCs) were formed at the joints depending on the applied energy. The metallization layer and Cu reacted with Sn in the solder after different aging durations, and additional IMCs were formed and grown. After aging for 2000 h, the shear strength decreased, and the interfacial IMC thickness increased. As the aging time increased, the fracture mode changed from an initial ductile fracture to brittle fracture (between the solder and IMCs and/or between IMCs and the Cu pad). The reflow soldered joints exhibited stable shear strength, resulting in ductile fracture until aging for 500 h. However, the shear strength decreased sharply after aging for 1000 and 2000 h, and Bi-segregation was observed after aging for 1000 h, resulting in inferior long-term reliability. After laser soldering at 150 and 170 W for 4 s, the strength of the samples decreased sharply after aging for 1000 and 250 h, respectively, and Bi-segregation was observed after aging for 2000 h. The shear strength of the sample laser soldered at 170 W for 2 s gradually decreased with increasing aging time and maintained a stable shear strength until aging for 2000 h. Therefore, laser soldering at 170 W for 2 s was considered as the optimal condition for long-term reliability. • Soldering properties of laser soldered Sn-58Bi/Au/Pd/Ni-finished Cu joints evaluated. • Laser soldering results were compared with conventional reflow soldered joints. • Various IMCs were formed depending on laser power and irradiation time. • Laser conditions with low applied energy displayed superior long-term reliability. • Selection of the optimal laser conditions is crucial to ensure reliable solder joints.