Microstructure and mechanical properties of lead-free PV ribbon

Photovoltaic (PV) ribbon is one of the PV cell assembly components used to connect cells to form a PV array working at proper current and voltage. It is a copper ribbon coated with tin-lead or lead-free solders, which provide solderable areas to form solder joints and protect the copper ribbon surface from oxidization. Tin-lead solder has been widely used in PV ribbon manufacturing because of its satisfactory solderability and low price. However, lead is a hazard to human health since it can pollute the soils and ground water when disposed in landfills. Although lead-free alternatives to tin- lead solder have been extensively investigated by the printed-circuit-board industry rare literatures have been reported on the lead-free coating for PV industry. Lead-free PV assembly will be a future prospect without doubts. Thus, it is important for PV industry to minimize or avoid the use of lead in PV modules. In this work, PV ribbons coated with lead-free solders were examined through mechanical testing and microstructural characterization. Variation of mechanical properties and microstructural evolution of the solder coatings at different processing states were examined. When tensile rate is below 50 mm/min tensile strength increased with increase of the tensile rate but a higher tensile rate had no effect on enhancing the tensile strength. Heat-treatment at temperatures ranging from 180 °C to 220 °C was performed and we found an increase of grain size with increase of the temperature. It was found that the N₂ atmosphere with low O₂ concentration was beneficial for obtaining fine microstructure when the PV ribbons were reflowed at N₂ atmosphere.