Non-Eutectic Low Melting Temperature Alloys for Standard c-Si Interconnection
11 Pages Posted: 29 Mar 2018 Last revised: 20 Apr 2018
Date Written: April 15, 2018
The critical component for transitioning to lead-free solders is the development of cost-effective, high-performance solders that mirror the properties of current leaded solders in use. New non-eutectic low melting temperature tin-bismuth (Sn-Bi) based alloys are reported for coating PV (photovoltaic) ribbon and thus interconnecting standard c-Si (crystalline-Silicon) solar cells. These alloys have Bismuth (Bi) content in the range of 35 and 40 wt.% and special additives were selected and added to improve their mechanical and thermal reliability. Alloys and tinned ribbons were tested and benchmarked against standard leaded alloys (tin-lead) for thermal, mechanical and functional properties. New alloys have liquidus temperature of 180oC ± 2oC and exhibit equivalent thermal and mechanical properties to that of standard tin-lead alloys. These alloys also have uniform morphology, low resistance to silver and copper erosion and fast wetting time. Ribbons could be easily tinned to get uniform coating of alloy. It was found that ribbons make reliable joints with metallized silver, obtained peel strength was >2 N/mm on front and >3 N/mm on the back side of the cell. Unlike conventional eutectic Sn-Bi alloy, new alloys do not experience secondary reflow during high temperature laminating processes (145oC) due to their higher liquidus temperature. New alloys are compatible with EVA (ethylene vinyl acetate) and most PV (photovoltaic) fluxes. Modules prepared for damp heat and thermal cycling pass the testing requirement as per IEC 61215. All tests and properties suggest that these new low temperature non-eutectic alloys can be “dropped in” to existing tabbing and stringing lines.
Keywords: Lead-free, tin-bismuth, PV ribbon, tabbing and stringing, reliability, peel strength
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