![]() For this work we used a commercial unit from GP Solar (see Figure 1) which could record force vs distance across the busbar when pulling the wire up roughly vertical to the face of the cell. The conventional measurement used in the industry to examine the soldering process is a wire pull test whereby the force used to pull off the wires is measured and the failure interface examined. However, using crack data from the flexed modules was not an optimal tool to provide quick feedback during process and materials optimization. Since the cracks often intersected the soldered regions, we focused on improving the soldering operations. 2.2 Characterization methods As will be discussed below, we found that when shifting our wafer thickness below 200 microns, we observed cracked cells in our modules after bending load tests. Still lower-temperature and softer alloys may be of interest, but these also may have module reliability concerns. For this work we used our standard Sn 96.5 Ag 3.5 composition with some level of Cu contamination due to the hot dipping method employed by the wire vendors. However, Evergreen Solar has never used Pb containing solder in its interconnect wire and chose not to explore this possible solution due to the environmental concerns surrounding Pb. The effect of using a Pb containing solder can be seen in that the temperature differential over which the wire contraction can cause damage is lower due to the lower melting point. Due to the low yield strength of solder, it may accommodate some stress depending on its composition and the degree of brittle intermetallic formation. Solder composition is also an important variable. The literature mentions Cu-clad Invar wire as a material with excellent fatigue properties that may work well in this case as the Cu can provide the required conductivity while the Ni-Fe Invar core can restrain the contraction of the wire. A possible solution to minimize the stress is to use wire with a lower CTE value than Cu. ![]() In our model, this stress can cause the formation of microcracks in the Si and/or the propagation of existing microcracks. Potential sources of damage During the soldering operation, the cell and the wires heat up and expand and then later contract when the heat is removed Below the melting point of the solder, the differential contraction between the Cu and the Si, as shown by the CTE values in Table I, combined with thermal gradients, cause stress to build up in the system.
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