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
by Karl Pfluke
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Photovoltaic Module Assembly Using SMT Assembly Materials and Processes
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EMS providers specializing in SMT are seeking to diversify and fill capacity. Photovoltaic module assembly is a popular choice. PV cell stringing in solar module assembly is achieved using many common SMT materials and processes. Solders, fluxes, and reflow technologies produce electrical interconnects in a-Si and c-Si photovoltaic assembly technology.
flux, bus ribbon, solar, tabbing ribbon, CIG, Copper Indium Gallium, photovoltaic
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Posted on 1 Jul 2009
by Jim Hisert
The interconnection of solar cells is a technology that has been around for hundreds of years, but is a relatively new application of the soldering process. By combining the metallurgical knowledge of solder joints (which has been developed through other applications) and new materials designed specifically for solar manufacturing, solar cells and cell strings can be effectively connected with high throughput, conductivity, and reliability.
Stringing of solar cells is used across the solar industry, and is a process that newcomers to the solar industry should be familiar with. However, it is a process that even experts still need to optimize. The top layer of a solar cell is a transparent conductive oxide (TCO) to which solder will not adhere. Therefore, a metallization paste is used to bond to the TCO and provide a solderable surface for strips of solder-coated copper called tabbing or stringing ribbon. These ribbons are commonly applied as parallel strips that weave from the top of one cell to the bottom of the next to connect the positive and negative sides of the cells in series. Once connected, the tabbing ribbon channels electrical current to larger solder-coated copper strips, known as bus ribbon. Bus ribbon serves as an input/output for the entire solar array to the module junction box.
bus ribbon, flux, solar, tabbing ribbon
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Posted on 15 Oct 2009
