Contrastive Study on Multiple Reflowing in SnAg-Cu/Ni-P Joints by Laser and Reflow Soldering

The laser soldering process is considered to be an alternative to reflow soldering for industrial production due to its high heating speed and low thermal damage. In the electronic packaging technology, multiple reflow soldering is inevitable due to the difference in the melting point of the solder used to connect different components on the same substrate. In this study, initial Sn3.0Ag0.5Cu/Ni-P joints were prepared using laser soldering with three laser energy values (6.5 mJ, 7.5 mJ and 8.5 mJ) and reflow soldering, followed by multiple reflow soldering (one, three and five times) of the initial micro-solder joints. The microstructural variations at the Sn3.0Ag0.5Cu/Ni-P micro-solder joints were analyzed and ball shear tests were performed to investigate the variations in mechanical properties of the micro-solder joints after multiple reflows. The results shows that the morphology of (Cu,Ni)6Sn5 IMC in the the initial laser-soldered joints changed from the granular shape to the short rod-shaped after multiple reflowings, while that changed from the initial needle-like shape to irregular block-shaped in the initial reflow-soldered joints. Furthermore, the growth rate of IMCs layer after laser soldering was faster than that after reflow soldering after 1 time reflow soldering. The shear strength decreased with the increasing number of reflows during multiple reflows regardless of the soldering process. After 5 reflows, laser soldering with 7.5 mJ effectively reduced the thickness of the IMCs and ensured better mechanical properties of the joint than reflow soldering.