Improved quality test method for solder ribbon interconnects on silicon solar cells

The field of photovoltaic (silicon solar cells) is an important driver for regenerative energy techniques. The technology and efforts regarding efficiency factor, quality, and costs are still under development. Currently, typical silicon solar cells are connected to so called strings by two or three solder coated copper ribbons. The common interconnection technology of silicon solar cells is soldering of solder coated copper-ribbons. The advantages of this joining are well known: low costs and high reliability. Thus the solderability and reliability of the solar cell interconnection zone is an important criterion which has to be ensured. A popular method for quality control is the so called "Ribbon-Pull-Test". Pull tests are well known and standardized in the semiconductor and microelectronic packaging industries, but the method is not simply transferable for soldered solar cells with the available standard test equipment. Firstly, the pull angle and velocity have to be defined. Secondly, the differences in the coefficient of thermal expansion between silicon and copper lead to thermo mechanical stress during the soldering process. Consequently in some cases, the solar cells were predamaged. Pulling of ribbons from pre-damaged cells leads to large silicon disruptions. Therefore, instead of testing the solder interconnection, the stability of the silicon wafer is measured. The paper focuses on a development of an innovative "Solar-Cell Pull-Tester", evaluation of the test method and first results for the interconnection reliability. It will present how important a reproducible test procedure is to provide a clear benchmark for solderability and quality of interconnects. In addition, it will demonstrate an effective in-time analysis and reporting tool, using camera control and failure-mode to load correlation. Ultimately the presented test method and equipment will provide the first available basis for creating a mandatory uniform test standard for soldered ribbon int- - erconnects on silicon solar cells.