The impact investigation of CSP IC packaging on Halogen-free board level performance

Abstract

In this paper, we are interested in the Halogen-free (HF) impact on the reliability performance of portable electronic devices. Due to the trend of environment protection, the Halogen-free materials of solder paste, molding compounds, PCB and etc. have been widely discussed. When the material changes happen, the transition of failure modes in the board level can be expected, and the challenges are how to identify the influences of various factors. The strategies of improving HF materials are how to decrease the coefficient of thermal expansion (CTE) and increase the glass transition temperature (Tg) to avoid the warpage issue in the lead-free assembly process. However, these strategies will cause higher stiffness in the integral structure of the board level. According to the preliminary study, the high elastic modulus of materials is the main factor which attributes to the brittle fracture mode in the high strain-rate test; therefore, our research will investigate the impact of Halogen-free materials on the performance through the high strain-rate bend test. In this study, various factors, i.e., package sizes, solder ball materials, HF PCB stiffness and its outer-later structure, are taken into different combinations to valuation the performance. The experiment is divided into three parts. Each part will only have one variation. In the first part, the variation is the package size. The solder ball material (SAC105) remains unchanged. The result shows that the smaller package has better performance than the larger one at the same strain. In the second part, HF PCB stiffness is the variation. The result shows that the HF PCB of higher flexure modulus has worse performance than HF PCB of lower flexure modulus at the same strain. The fracture position transfers from the intermetallic compound (IMC) crack of the component side to the IMC crack of the board side and PCB pad delamination with increase the strain range. In the last part, some package suppliers migrate their sphere alloys from higher Ag alloys (SAC405 or SAC305) to alloys with lower Ag contents. There are numerous perceived benefits to this transformation in terms of intrinsic characteristic and performance. In this paper, we will choose several low Ag alloys to compensate HF PCB stiffness to resist the brittle fracture. We find one of the phenomena is that the performance will be better at a high strain rate test if the solder has applicable yield strength and higher elongation properties