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58 SMT007 MAGAZINE I JULY 2022 We present solder paste as an alternative material for interconnecting back contact cells. Solder paste offers many advantages over ECA and traditional tabbing methods, such as 3 : • Formation of reliable joints • Can be dispensed or printed • Stable viscosity and higher shelf life compared to ECA • Low voiding • Better thermal and electrical conductivity compared to ECA • Resistant to moisture • Lower cost than ECA In this work, we investigated application of low temperature lead-free solder paste in printing and optimizing interconnection joints in IBC cells. A design of experiment (DOE) was performed to evaluate effect of reflow con- ditions in attaining complete wetting and reli- able bond strength on metallization paste and ribbon or wire. We also studied the role of the solder paste on the electrical and mechanical properties of soldered cells. Study included evaluation of printing, voiding, microstruc- ture, reflowing properties of solder paste for making it suitable for interconnecting IBC cells in existing automated machines or modi- fied tabbing machines. ing and is down to around 300 to 500 microns. e interconnection can be carried out by either ECA (electrically conductive adhesives) or by using direct ribbon/wire. Both materials have created some challenges. e poor peel strength is oen the major issue. e uniform IMC (intermetallic layer), which is character- istic of reliable bond strength, is absent with ECA. e metallization paste used in IBC is low-temperature-curing silver paste. e paste is fired at lower temperature around 500°C or less and deposited on silicon cell. In addition to that, the height of the paste is only about 8-12 micron. Because of this, silver leaching during interconnection is commonly observed (Figure 1). e metallization just comes off during interconnection at high temperature. If used, solder wire, cold solder joints, and solder diffusion through the cell are the major issues. Poor adhesion between cell and ECA, and interconnecting wire, high contact resistance are other common problems. Accordingly, when a circuit or conductive layer or intercon- nection is formed on a substrate using such conventional pastes, damage to the substrate or failure in reliability of the device may occur. Further, when ECA is used, silver in the ECA is expensive and appears on various restricted chemicals lists due its short supply. Figure 1: Silver busbar leaching from the cell. Figure 2: Solder ball formation in IBC interconnection when processing parameters are not optimized.