by Rick Lathrop, Karl Pfluke
For crystalline silicon solar cell front contact metallizations, silver thick film formulations are ubiquitous. For backside contact pads, either silver or silver/ aluminum formulations are common. The trend for back contact metallizations is towards low lay down formulations, resulting in thin fired films. Although there are many different reflow methods used to “string” cells together, the need for fast wetting, leach resistant and well adhered front and rear contact metallizations are common to all methods. In order to accurately predict a material's compatibility with the module assembly process, quantitative tests needed to be developed due to an absence of industry standard tests. Classic thick film solder pot leaching and wire peel adhesion tests do not emulate the solar module assembly process well. Although more similar in process, SMT solderability tests also lack close correlation. To fill this gap, several solderability tests specifically designed for module assembly and cell metallizations have been developed and will be discussed in detail. These tests are, in fact, a hybrid of SMT and thick film tests but tailored, for the solar module assembly process. Wetting assessment is accomplished by measuring the reflowed area and height of a precise volume of solder using a confocal measuring system. For ribbon adhesion, manual and automated methods are compared, as well as various peel angles. From these studies, a ribbon attach method and adhesion test emerges suitable for benchmarking contact metallization formulations. Recommendations on how to recognize and prevent silver leaching are also discussed.
tabbing ribbon, metallization, silver leaching, adhesion, wetting
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Posted on 20 Oct 2011
