Growth kinetics of intermetallic compound layers at the interface during laser-assisted bonding depending on surface finish

Abstract

Mass reflow (MR) has been widely used in the interconnecting process of electronic packaging. However, use of conventional MR shows a limitation in the miniaturization trend of electronic devices because of thermal damage to heat- sensitive polymer components from the high process temperature and long bonding time. Furthermore, MR cannot avoid warpage issues because it cannot provide thermal selectivity to the bonding area. These representative problems of MR indicate the need for an advanced bonding process with selectivity and mass product possibility. To meet the requirements of electronic packaging, advanced bonding processes were needed. Laser-assisted bonding (LAB) has been spotlighted as a next-generation interconnection technology to overcome the problems of MR. LAB has extremely fast ramping up speed with accurately controlled wavelength and excellent thermal selectivity. The LAB process could be applied in bonding of flexible, foldable, and even stretchable applications involving a polymer component with a low glass transition temperature. Furthermore, the short process time of LAB could provide not only enhanced productivity, but improvement in mechanical properties as a result of finer grain size and reduction in intermetallic compound (IMC) formation.SAC305 solder paste was printed on two kinds (OSP and ENEPIG) of surface-finished Cu electrode, and MR and LAB processes were performed to create solder joints. The mechanical reliability of solder joint was evaluated with shear test according to the JEDEC standard. Formation and growth of an IMC layer at the interface between SAC solder and Cu after soldering were identified with FESEM. A high- temperature storage test was performed to evaluate the longterm reliability of LAB compared to that of MR. Voids in solder joints were identified by x-ray scanner to compare the soldering processes and surface finish depending on aging time. Smaller IMC layer between solder joint and electrode thickness and fewer voids in solder joints were produced with LAB compared to MR.