Temperature of solder contact in back-contact Si solar cells and its effect on reliability of modules under localized shading environments

Reliability of Si solar modules during thermal cycling and shading conditions strongly depends on the maximum temperature of the solder contact between the solar cells and the cell to cell interconnects [1]. While the maximum temperature of the solder contact is known to be critical for the lifetime of a solder joint, typical reliability tests and simulations are conducted under the assumption that the solder contact is the same temperature as the cells. This test condition and assumption is true for typical front-contact Si cells. However, the actual temperature of solder contacts is cooler for back contact Si cells. The present study investigated the temperature of solder contacts in the SunPower cell with a high resolution IR camera and found a substantially lower temperature for the solder contacts of a hot cell neighboring a normal temperature cell. The pads on the hot cell are 0.34 degrees C cooler for each additional watt dissipated by the hot cell over its neighbor. A finite element analysis was conducted to validate the experimental results and to simulate the temperature of the solder contacts in real-world conditions. The simulation provides important insight on the temperature of the solder contacts under various mismatch conditions induced by localized shading. The good agreement between the simulation and IR experiments indicates that the interconnect design in SunPower´s back-contact cells reduces the temperature of the solder contacts under partial/full shading, providing an additional safety margin to the module reliability.