Finnish researchers have tried a new eddy current-based soldering method for solar cell striping and found that it significantly reduces cracks and damage caused by thermomechanical stress in the process. They claim that the new technology can already compete with conventional soldering methods in commercial production.
Relieving thermomechanical stress in solder joints is challenging because it occurs between two types of material at high process temperatures, the researchers said. “The risk of fatigue damage during use increases, leading to increased series resistance losses, which can pose a major challenge in optimizing the interconnect soldering process,” they explained.
The newly developed technique consists of an eddy current soldering method, which must be applied through a layer of glass. An eddy current that is arises from anything that causes a change in the strength or direction of the magnetic field in the conductor, is a non-destructive method that can be performed on conductive materials.
The Finnish team tested this process in a polycrystalline silicon solar cell measuring 52 mm x 19 mm x 0.21 mm with a single silver 1.8 mm wide power rail on the front and a 1.8 mm continuous solder pad on the back. Soldering was performed using a microscopic cover glass plate of size 76 mm x 26 mm x 1 mm and soldering with coated copper strips made of lead-based solar alloy Sn63Pb37 at a melting temperature of 183-190C.
“Small weights (34.4 g each) were placed on the end of the glass plate to ensure proper contact between the strip and the silver rail,” the researchers said, noting that the heat source used was a Power Cube 32/900 high-frequency. A generator supplied by the US CEIA, whose heating coil maintains a distance of about 2.5 mm.
The scientists claim that this setup and the induced eddy currents produce localized heat without direct contact from the heating coil.
Through several tests and microstructural analyses, they have been able to ensure that no other cracks or damage were observed in the soldered cells, except for a few samples. They also confirmed that a longer exposure time is better to avoid microcracks in the solder joint assembly.
Photo: University of Oulu, solar energy materials and solar cells, Creative Commons license CC BY 4.0
They concluded that the new method is technically and commercially feasible and competitive with traditional contact soldering techniques. “The proposed technique has been developed to be used in addition to strip soldering to repair and refurbish a solar panel used in future research after failures or increased series resistance due to the passage of time in the connections of wires or busbars without disassembling the solar panel. modular panel,” they also stated.
The soldering process is described in the study “Eddy current soldering of solar cell strips under a glass layer”, was published Solar energy materials and solar cells.
