Effect of electroless Ni plating bath conditions on the impact reliability of Sn-Ag-Cu solder joints

Abstract

Mobile devices can be damaged if dropped by the user. So, the impact reliability is one of the most important factors. Brittle fracture in solder joints when dropping mobile devices. Reducing brittle fracture can improve the lifetime of mobile devices. The brittle fracture behavior of solder joints strongly depends on the print circuit board (PCB) surface finish. Ni-based surface finishes, i.e., electroless nickel immersion gold (ENIG) and electroless nickel electroless palladium immersion gold (ENEPIG), are widely used on PCBs in mobile devices. The mobile device is excellent in reliability Ni-based surface finish is widely used. This study examines the effects of electroless Ni-P bath conditions (bath life, temperature, pH, and additive) and the microstructure of Ni-based surface finishes on the reliability of solder joints. Previous research on the lifetime of plating solutions has only reported the results of studies on impurities in plating solutions. Studies on the effect of solder joints have not been reported. In addition, the plating conditions so far have been focused on the pH of the plating solution. Therefore, in this study, the effects of solder joints depending on other plating conditions and Ni-P properties were analyzed. Firstly, the effect of electroless Ni-P (EN) bath life on the brittle-fracture behavior of ENIG/Sn-Ag-Cu (SAC) solder joints is evaluated. The bath life of the Ni-P for the ENIG surface finish in this study varied from 0 to 3 metal turnover (MTO). The shear strength of the 0 MTO sample was higher than that of the 3 MTO sample, and brittle fracture rate increased as the bath life of the Ni-P for ENIG increased. Cross-sectional transmission electron microscopy revealed that the number of nanoparticles in the Ni-Sn-P layer increased as the MTO of the Ni-P increased. The increase in nanoparticles with MTO was caused by an increase in the organic impurities in the Ni-P bath. The poor brittle-fracture behavior of the 3 MTO sample originated from the weak interfaces among the many nano-sized particles. The effect of the electroless Ni-P (EN) bath life on the brittle fracture behavior of ENEPIG/SAC solder joints was also evaluated. The 2nd point of the thesis is the temperature change of the plating solution. The effect of Ni plating temperature on the brittle fracture behavior of ENIG/SAC solder joints was observed. The Ni plating temperature was varied from 75, 80, and 85°C. The intermetallic compound (IMC) thickness after reflow and the nodule size of the Ni-P both decreased as the Ni-P plating temperature increased. The nodule size in the Ni-P layer affect the Cu diffusion through the Ni-P layer. The increased nodule size decreased the length of the nodule boundaries, which lowered the diffusion and the IMC growth rate. The sample prepared at 75°C had a higher IMC thickness than the 85°C sample. Therefore, brittle fracture decreased as Ni-P plating temperature increased. The 3rd point of the thesis is the change in the microstructure of Ni-P. Either structureless or columnar Ni-P layer can be fabricated by adding different additives in the Ni-P plating bath. IMC growth in the columnar Ni-P sample was fast since diffusion occurs rapidly through an amorphous boundaries. Due to the increased IMC thickness of the columnar Ni-P, the brittle fracture rate of the ENIG/SAC solder joint increased. The decrease in shear strength with aging time was lower structureless Ni-P sample for the than the columnar Ni-P sample. Finally, the effect of the pH change of the plating solution. The IMC thickness increased as the pH of the EN plating solution increased. In addition, as the pH of the EN plating solution increased, the shear strength of the solder joint decreased and the brittle fracture rate increased. In conclusion, this thesis studied that optimization of the EN bath conditions during the surface finish process is very important to ensure the reliability of the final solder joint. Optimizing the initial plating process conditions (MTO, pH, temperature and additives) can improve the reliability of the solder joint.