Understanding peel force variation during Ag bus bar adhesion measurements

Possessing adequate adhesion between front-side Ag bus bar and Si wafer is important to ensure as-manufactured crystalline Si (c-Si) solar module performance and the long-term operational stability of a c-Si module. Adhesion of screen printed Ag bus bar is commonly measured via a mechanical peel test, for which the figure of merit is the steady force needed to peel off a flat Cu ribbon that has been soldered to the bus bar of the solar cell. Very often, the peel force along the peel direction varies significantly, creating uncertainty in the characterization of the bus bar adhesion. To understand the origins of the variation and to improve the accuracy of the measurement, we use SEM to examine the microstructure of the conductor stack, which includes the ribbon (double sided solder coated Cu), Ag bus bar, and Si wafer. It is found that the thickness of coated solder on both sides of Cu can change significantly along the peel direction, causing strong variation in adhesion measurement. A mechanical model of this phenomenon is proposed and consistent with experimental results. In particular, a higher peel force is observed when solder thickness on the side of the ribbon contacting the Ag bus bar layer is reduced relative to the solder thickness on the opposite side of the ribbon.