INTERCONNECTMATERIALS
Where D is the diffusion rate and S is the solubility of the penetrating molecule. As mentioned, rubber like materials typically pass higher levels of moisture vapor because of the high free volume, or molecular space for diffusion. It would be expected that corrosion resistance of the rubber like ECA would be poor relative to the rigid ECA with much less free volume and slower diffusion rates. The novel approach to the rubber like ECA elasticity has shown solid progress towards a potential stable electrical resistance joint after exposure to damp heat conditioning.
Rubber like improved ECA
After examination of the rubber like formulation, some modifications were made to the rubber like ECA, and the damp heat testing was repeated. The improved version is labeled “rubber like improved”. As shown in Figure 8, not only is the rubber like ECA better, but the overall series resistance has been reduced by 60% after 1000 hours of damp heat conditioning.
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The test set up was repeated with both vendors and both conditions with the improved rubber like ECA, as well. Figure 10 below shows the typical improved performance of the rubber like improved version. When testing the improved version, MEK cleaning improved performance on ribbon from vendor 1, but performance was significantly compromised with cleaning with vendor 2 ribbons.
Figure 9. Rubber like ECA with two ribbon vendors and two cleaning conditions
Ribbon variables
When manufacturing with tin and tin silver ribbon, manufacturers want the liberty of using ribbons without any cleaning or special handling to ensure performance. Tin silver ribbons from two different vendors were tested for corrosion resistance under two different conditions. First, the ribbon was removed from the spool and used without cleaning, and was essentially used as received. The condition is labeled “as is”.
In addition, the ribbons from each vendor were subjected to a cleaning process with MEK and a lint free towel. Those samples are labeled “clean”. Both ribbon types had a 96.5/3.5 Sn/Ag composition and a 15-25 micron finish over a copper core. The damp heat conditioning test scheme was performed twice for each condition
After inspecting the samples, it was apparent that the large cross section of adhesive was out of position during the test affecting both “cleaned” trials from vendor 2.
Individual joints were tested and results were comparable to other well performing interfaces. Therefore, the higher resistance results are believed to be unrelated to interfacial performance changes on that cleaned ribbon from vendor 2.
Figure 10. “Rubber like improved” ECA with two ribbon vendors and two cleaning conditions
and the initial rubberlike ECA was used. The results are shown in Figure 9. A clear distinction between the “clean” and “as is” was observed on tin silver ribbon from vendor 1. Cleaning appeared to enhance damp heat performance with vendor 2, as well.
Figure 11. FTIR of residue from ribbon from vendor 1
www.solar-pv-management.com Issue III 2011
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