The intention of the continuous development of automobile is to provide people with high safety advantages while traveling, and now many cars have high-tech driver assistance and electronic control systems to improve the human safety. Hence, we must rely more on the reliable functions of those controllers, the degree of probability of errors in the electronic circuit of driver assistance can lead to a wide range of safety challenges. As a result, the automotive industry has placed great emphasis on improving the reliability of manufacturing processes in electronic developments.

In this project, a very prevalent manufacturing method in electronic, the reflow soldering process in surface mount technology (SMT), was experimented to determine the most reliable process parameters. The aim of this experiment was to determine the optimum levels of thermal characteristics of SMT reflow soldering process. Soldering material SAC305/OM-338 was used for the experiment, and 32 SMT PCBs were subjected with unique trailing parameters of reflow soldering process zones. The preheating ramp, ramp up, time duration of temperatures above liquidus, reflow peak temperature, and cooling ramp were the parameters that were deeply investigated. After the reflow soldering process, all PCBs were investigated by using an automatic optical inspection (AOI) for solder crack initiations. A thermal shock experiment was then performed for evaluation of crack propagation trends, and finally all cracks were sent for microscopic examination to obtain the microstructural evidence for further crack initiation and crack propagation tendencies. According to the results of investigation, only 3 PCBs (out of 32 PCBs) were initially identified as having Body Soldering Cracks (BSC) inside the solder filet after the reflow process, when the Preheating and Cooling Ramps of reflow were at the high ends, at 1.2oC/S and 1.1oC/S respectively. Then, after the thermal shock process later, other 8 PCBs were identified as having Edge Soldering Cracks (ESC) at the edges of solder fillets when the Time Duration of Liquid (TDL) was at the low end, at 80S and the Temperature Above Liquidus (TAL) was at the low end, 0.7oC/S. In this rate, based on the evidence of microscopic inspection, the side cracks at the edges of solder fillets have been recognized to be an effect of the different thermal expansion coefficients (α) of the Cu PCB base material and the solder material during the thermal shock process, it results a significant thermal stress (δ) concentration at the intermetallic layer, which tends to cause cracks due to the low strength of intermetallic bonding. In conclusion, high-end temperature ramps have made BSCs, and low-end TDL and TAL have made a low-strength intermetallic bond layer between the base material Cu and the solder material to initiate ESCs.