Most of the groups were then soldered under different conditions, followed by wire pull tests for 3 cells (2 wires/cell) and breakage strength tests for the remaining cells (2 tests/cell). 2.3 Experiment and Results In one experiment we randomized a large group of cells made from 190-micron thick String Ribbon Si wafers into 11 groups of 26 cells each. Also, we found that cells were weaker when tested sunnyside-up vs sunnyside-down, and so conducted most tests in sunnyside-up orientation. Thus, we used it to improve our understanding of the processes and to optimize the processes and materials. We found that this data correlated well with the mechanical yield of the process and in many cases correlated to the number of cracks seen in modules. In particular we concentrated on the metric of the % of weak cells from the testing where the cell broke with minimal force (gauge reading <75g). The maximum downward force achieved prior to cell breakage is recorded. Figure 1 shows a breakage strength tester which clamps the cell behind the busbar region and then applies a downward force to the region of unsupported cell.
TABBER SERIES CRACK
Since it was not obvious to us or others that higher pull strengths were better for either crack minimization or long term module reliability, and since microscopy also did not lend itself toward quick or easily quantifiable feedback, we developed a new measurement which would better correlate with the soldering damage. Also, the desired interface was not obvious.
TABBER SERIES CRACKED
We found weak correlation between the pull strength values and the number of cracked cells found in the modules. We used the area under the curve instead.
![tabber series tabber series](https://i.ytimg.com/vi/uiKaai4ka7s/maxresdefault.jpg)
The peak force can be misleading since the average value can be much lower. Figure 2 shows an example of pull strength data. 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.