Issue link: https://iconnect007.uberflip.com/i/1398328
76 SMT007 MAGAZINE I AUGUST 2021 ufacturing process. Undetected micro-cracks could result in a less than expected field lifes- pan [18] . e effect of soldering time and tem- perature is investigated (Figure 5). In this study, soldering temperature was varied from 230° to 270 o C and soldering time was varied in between 1 and 3 seconds. As can be seen from Figure 5, the higher peel strength was obtained at 250-260 o C. It was also observed that higher temperature or longer soldering time does not necessarily yield higher peel strength. In fact, silver leaching and crystal damage, and elimination of the silver bus bar from the sur- face of the silicon, may be observed in the case of excessive soldering time and tempera- ture [20] . Precise control of time and tempera- ture reduce the possibility of forming micro- cracks in the substrate is necessary. Solder joint thickness should have adequate capacity to transfer gen- erated current with min- imal residual stresses in the joint. In addition, the solder joint should have enough thickness to prop- erly function as a mechan- ical support as well as a thermal conduit. Proper soldering will result in an intermetallic layer that is within 0.5-1.5 μm. Sol- dered cells were cross- sectioned and analyzed with FESEM for solder joint adhesion and IMC formation at the intercon- necting area. For prop- erly processed samples, SEM analysis indicates that there is excellent sol- der adhesion with distinct continuous IMC layer for- mation in the intercon- necting area (Figure 6). Intermetallic compounds (alloys) form when- ever two different metals are soldered together Figure 4: Wetting curve of PV-21 flux. Figure 5: Effect of soldering temperature and time on peel strength. Average of 10 strings each. Figure 6: IMC formation in MBB solder joint.