Lead-Free Soldering Technique by Using Medium-Frequency Electromagnetic Field

Area array soldering is one of the main interconnection technologies for the inter graded circuit (IC) packaging and assembly. The typical area array soldering processes include some conventional heating methods, such as forced convective hot-air reflow and infrared reflow, which have the merit of high throughput in mass industrial production and, however, sometimes may cause the warpage of components or printed circuit boards (PCBs) due to excessive heat supply and/or prolonged heating time. As such, lowering the temperatures of the component and the board during the reflow process for the lead-free solders will lead improvement in the device reliability. The localized heating methods have been developed and applied for electronic packaging to solve the aforementioned reliability problems. However, for most of the developed methods the equipment cost is tremendously high or the processes are rather difficult (if not impossible) to finish the board level soldering of area array packages. A novel lead- free interconnecting technique using medium-frequency (MF) electromagnetic field was investigated. The present study is a continuing effort to prove the feasibility of eddy current induced heating for the reflow soldering of area array packages. In this paper, experimental investigations and simulation on the heating effects of the MF electromagnetic field will be presented. The simulation was conducted to demonstrate the selective heating during the reflow process, which can also be proved by the experimental results. Comparison between the thermal characteristics and the interfacial microstructures of the lead-free solder joints on Ni/Au surfaces formed by a conventional thermal reflow process and MF inductive heating reflow process was conducted. The temperature profiles on solder balls and the PCB during the reflow were measured by infrared thermometer. The MF heating technique successfully created the lead-free solder joints through the intermetallic compounds (IMCs), while maintaining the substrate temperature as low as 180?C. This paper reports the soldering behaviors of the Sn-3.5 Ag joints on Cu and Sn surfaces by MF electromagnetic field. The solder joints formed by a conventional reflow process were compared with those soldered in the MF electromagnetic field, and their morphologies, the IMC formation and the joint formation were discussed.