Longitudinal cracks along busbars

On the rear of the module, this signature shows as backsheet cracks that run lengthwise and follow the cell busbars underneath. It is one of two forms of thermo-mechanically induced backsheet cracking, also called type III. The other form is squared cracks under the gaps between cells. Two things drive it: mechanical stress at the busbar positions that is left over from lamination, and unbalanced thermo-mechanical properties of the backsheet, such as mismatched thermal expansion coefficients. The crack starts at the outer surface and grows through the core to the inner layer. Under a light microscope, the whole backsheet is broken through. Unlike squared cracks, these longitudinal cracks show no chemical degradation of the backsheet. The film is cracked through its full thickness, so it no longer meets insulation requirements. Insulation resistance can collapse, especially in wet conditions, which is a serious safety risk. The cracks appear as operating time increases and seem to depend on local weathering. When they occur, nearly all modules with the same bill of materials are usually affected. PVDF-based backsheets can also crack along the busbars through their entire thickness. Affected plants need to be either repowered or repaired with a suitable coating or film.

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• sentence 1: support check: partial - Passages say longitudinal cracks lie beneath busbars (rear/backsheet cracks), but "on the rear of the module" and "this signature" are only loosely supported; core is stated.
• sentence 8: support check: partial - PVDF busbar cracks through the full thickness are stated, but the repower-or-repair remedy is given for the general (PA) case with the same backsheet type, not specifically for PVDF. -->