Effect of blind hole depth and shape of solder joint on the reliability of through silicon via (TSV)

TSV is a very hot topic today. It enables higher pin-out and finer pitch. However, the thermal-mechanical reliability is a big issue. The coefficient of thermal expansion(CTE) of copper, silicon and siliconoxide is about 17.5 × 10^-6/°C 2.5×10^-6/°C, 0.6×10^-6/°C respectively. So when the TSV is subjected to thermal cycling load, large stress and strain would be created at the interface of the materials. In this work, Finite Element(FE) method was used to investigate the thermal-mechanical reliability of TSV. It is found the critical location is the junction between the overlaying flange Cu pad and the SIO2. And as the height of overlaying flange Cu pad decreases from 6μm to 2μm, the equivalent plastic strain decreases from 0.0195 to 0.0056. So the TSV with lower overlaying flange Cu pad would be more reliable. we also investigated reliability of blind hole. The bottom of the via is another critical location due to the mismatch of CTE. But the reliability is not sensitive to the depth of the blind hole. Finally, we investigated the effect of the geometry of the solder on the reliability. When the solder is hourglass joint and its width increases, the shear plastic strain is not sensitive to the width of solder. However, when the solder is spherical joint and its width increases, the shear plastic strain at the critical location increases significantly. Therefore, the reliability of spherical solder joint is much sensitive to its shape. And when the solder is cylinder column, the shear plastic strain is minimal.