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AUGUST 2021 I SMT007 MAGAZINE 73 Figure 1: Surface topology of MBB cells. It is reported that cells having a smooth sur- face on front as well as on the back side form reliable joints. [12] . Surface topology of cell met- allization is shown in Figure 1. As mentioned in the introduction, the technology used to make MBB cells can save silver paste. In our cells, the silver paste consumption of MBB cells was reduced by 20 mg (16.7%) compared to that of 4BB cells. As seen in Figure 1, only hexag- onal pads are printed on the front side of the MBB cells. Moreover, the height of the silver deposition is also about 8-11 micron, signifi- cantly lower than conventional 4BB solar cells. is also means that less surface area is avail- able and there is the probability of silver leach- ing during soldering. It has been shown that [12] the smoother the metallization, the better is the peel strength. is may happen because the difference between the highest (edge) and the lowest point in the middle of the busbar in some places more than >10 μm. e peaks at the edges will keep the ribbon off from the major part of the surface [12] . e consequence is an increase in thermal contact resistance between the busbar and the wire. In the case of "low quality" cells where the variations are high and paste is not printed smoothly, the impact is significant and this is the main rea- son for the failure mode. To evaluate the effect of metallization further, some cells were oxi- dized and soldered under similar conditions. In all those cases non-wetting or very low peel strength was observed. Solder wire Tin-lead (Sn-Pb) alloy is coated onto cop- per wire to form PV soldering wire. During the coating process, reactions occur between solder and copper wire and IMCs, which may nucleate and grow at the solder/copper inter- face, are formed (Figure 2). e presence of these IMCs is an indication of good metallurgi- cal bonding, and a thin and continuous layer is an essential requirement for good wetting and bonding [13] . As can be seen in Figure 2, our copper wire has uniform solder coating and continuous IMC layer is formed at the Cu-sol- der interphase. Raman shis are typically reported in wave- numbers, which have units of inverse length, as this value is directly related to energy. Raman spectral analysis is performed to determine the surface oxides. Results indicate the absence of tin oxide on the ribbon surface. e Tin oxide