$210^{\circ}\mathrm{C}$ reflow technology study in 3D Packaging

Low temperature reflow is promising to relieve the warpage issue caused by the mismatch of thermal expansion coefficients (CTE) of PCB and BGA components during assembly. While Sn58Bi with a melting point of 138°C has a low reflow temperature, its brittleness can lead to solder joint reliability problems. SAC305 solder alloy has excellent soldering yield and reliability performance, however, the reflow temperature is difficult to be reduced further by element addition or modification. In this paper, the mixed solder of SAC305 and Sn58Bi was prepared by solid-liquid low temperature soldering. low temperature reflow is promising to relieve the warpage issue caused by the mismatch of thermal expansion coefficients (CTE) of PCB and BGA components during assembly. We found that after reflowing at 210°C for 5 min, the solder ball and solder paste were completely mixed. This temperature is much lower than that of SAC305, which can effectively reduce the thermal warpage and thermal budget. In addition, according to the shear test data of solder joints under each reflowing condition, the shear strength increases obviously with the increase of mixing degree. The shear strength of samples refluxed at 210°C for 5 min (63.68 MPa) is about 10 MPa higher than that refluxed for 1 min (52.42 MPa). The results show the mitigation of brittleness in Bi contained solder joints. Our research results provide a promising solution for low-temperature soldering technology in 3D package.