Microstructural evolution and mechanical properties of SAC305 with the intense pulsed light soldering process under high-temperature storage test

Abstract

Electronic packaging has been miniaturized for high density and performance. Therefore, the formation of reliable interconnections has become an important issue. However, the conventional reflow process depends on a high process temperature and long process time. Those process limitations of the reflow process cause problems with advanced package structures, such as chip delamination and warpage problems in soldering interconnections. Laser-assisted bonding (LAB) has been highlighted as an advanced soldering interconnection process because of an ultrafast bonding process and thermal selectivity. However, the use of a laser homogenizer is necessary to improve laser irradiation area accuracy and it reduces price competitiveness. To overcome the problems of the reflow and the LAB processes, the intense pulsed light (IPL) soldering process has been considered as the next-generation answer for soldering interconnections. The IPL soldering process could reduce thermal damage to polymer components with its extremely short bonding time, thus it could be a solution to the warpage problem. Furthermore, the grain size of solder alloys could be controlled, depending on IPL conditions. In addition, IPL could irradiate in a desired wavelength range using a filter because IPL includes light in a wide wavelength range from ultraviolet to infrared radiation. We investigated the microstructure evolutions and mechanical properties of the IPL soldering process using Sn-3.0Ag-0.5Cu (SAC305) solder. Test-kits for evaluating mechanical properties were designed according to the JEDEC-B117A standard. A copper (Cu) electrode was used with two kinds of surface finish, organic solderability preservative and electroless nickel electroless palladium immersion gold. The IPL soldering process was conducted using different pulse widths (2, 2.25, and 2.5 ms). Other IPL conditions were fixed, such as pulse frequency (3 Hz), pulse number (10), and pulse power (4 kW). The Reflow process was conducted at a peak temperature of 270°C for 5 min. After the soldering process, a high-temperature storage test was performed at 150°C to evaluate the mechanical reliability of the solder ball. The IPL soldering process generated a thinner and flatter-shaped intermetallic compounds layer than the reflow process at the interface between the solder and the Cu electrode. Moreover, small grain size and microstructure region aligned in the same orientation were shown in the solder alloy that was fabricated using the IPL soldering process. Shear strengths of IPL soldering are 6%-11% higher than the reflow process in all high-temperature storage test conditions. The novel soldering process using an IPL energy source is suggested as a solution to the warpage problem and to improve mechanical properties.