Long-term stable encapsulated solder joints on an Al/Ni:V/Ag metallization for silicon solar cells

Evaporated Al as metallization for silicon solar cells may be profitable as it requires less Al material than the common screen-printed Al/Ag-paste combinations and forms contacts with a lower contact resistivity. To achieve solderability a sputter deposited Ni:V and an Ag layer are deposited onto the evaporated Al layer. Tin-coated Cu-connectors with a lead-free and a lead-containing solder are employed in combination with two halogen-free no-clean fluxes with varying solid contents. The long-term solderability of the metallization stack is demonstrated by comparing 180¡ peel forces (DIN EN 50461) of Cu-connectors soldered onto the metallization stack directly after preparation, after accelerated storing following IPC J-STD-003B and after storing for half a year at room temperature. The long-term stability of the solder joints is verified by measuring the normalized contact resistivity and by a visual inspection of encapsulated samples during a damp heat test (DIN EN 61215). For encapsulation an EVA foil and a silicone elastomer are used as encapsulation materials. As the lead-containing solder does not adhere sufficiently after accelerated aging, the flux with high contend of solids may accelerate corrosion and the EVA laminates exhibit adhesion problems between the samples and the lamination foil. The lead-free solder and the flux with the lower solid content in a silicone elastomer based laminate emerge to be the most promising candidates for a long term stable interconnection of Al/Ni:V/Ag metallized solar cells.