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60 SMT007 MAGAZINE I JULY 2022 lem in the semiconductor industry. At reflow, different sizes of spherical solder particles are formed. ese solder balls lead to short circuits and leakage current in circuitry 4 . Although there are many potential causes for this defect, stencil aperture, metallization quality, fast ramp rate, wetting of a paste, and reflow profile characteristic are the dominant ones. e sil- ver leaching and solder ball formation defects were minimized in our study by doing the printing and reflowing optimization. From 300–900-micron range tested, 600 micron or 1:0.6 aperture yielded sufficient sol- der volume and spread (Figure 4a). Solder paste spread well across the busbar and there was no slump formation. Wetting was also acceptable under optimized conditions. We observed that 3 mil of roughly dispensed sol- der paste at a height of ~65 micron reduces to around 55 microns aer the reflow (Figure 4a) at 600-micron aperture. is is just enough paste on the busbar to avoid any stress or crack- ing during IBC assembly. Unlike ECA, sol- der paste has better wetting and spread. Paste coalesces aer reflowing and yields volume transfer efficiency of over 80% (not shown) at varying printing speed. is proves that print- work. To strengthen and maintain the elec- trical and mechanical properties of the inter- connections, including the bulk of the mate- rial and the solder-pad interfaces, solder paste is best suited. Low temperature solder joints, especially with this alloy, can withstand cyclic contractions and expansions that can deterio- rate and weaken the solder joints. As a first part, 3- and 6-mil stencils were fabricated for the experiments. ese are through-cut stencils made with stainless steel. A DEK03Xi printing machine was used to do automated printing. Similarly, paste could also be dispensed on the cell just before the assem- bly. e paste volume was optimized by print- ing/dispensing paste pads from 300 to 900 microns on the 1-mm and 300-micron wide busbar (Figure 3a). Solder paste, if not pro- cessed properly, can give several problems 4 : • Solder ball formation • Silver metallization leaching • Solder diffusion through the metallization paste • Non-wetting on the pads Figure 2 shows typical solder ball forma- tion aer reflowing. is is a common prob- Figure 3: a) Paste printed through 300–900 micron pads on the busbar; b) Solder paste spread well across the busbar and there was no slump formation. a b