Analysis of Grain-Size Distribution and Yield Strength of Interconnector Ribbons and Wires at Different Streching Conditions Using Color Etching

In this paper we analyze the microstructure of solder coated copper ribbon and wire interconnectors for silicon solar cells from different manufacturers at different stretch levels in longitudinal and cross microsections by color etching and microscopy. The used etchant colors each grain according to its crystallographic orientation. This enables the extraction of the grain size and frequency information by image and data processing tools. Furthermore we measure the yield strength of the solar cell interconnectors for strain levels of 0.5 %, 5 % and 10 % and evaluate the impact on the microstructure of the interconnector. We find a large variation in the copper microstructure, especially for wires and observe an inverse relation between yield strength and grain sizes corresponding to the Hall-Petch relation. The lowest measured yield strength for a wire interconnector is about 82 MPa (avg. grain size: 237 mm²), which is about 20 MPa higher compared to the lowest yield strength measured for ribbons (avg. grain size: 247 mm²). The wire with the highest yield strength of 148 MPa shows fine grains (avg. grain size: 29 mm²). In the ribbon analysis we find the same overall correlation between grain size and yield strength with some exceptions. This underlines that grain size distribution is not the only attribute which affects the yield strength. The analysis of copper ribbons at different stretch levels discloses a deformation or refinement of the copper grains associated with rising yield strength. In general the results show that the approach of a color etching, optimized for solar cell interconnector cross and longitudinal sections, is a suitable, fast and cost-effective solution to quantify the grain size distribution and evaluate mechanical impacts like stretching or bending on the copper microstructure.