Ultrasonic Methods for Inline Solar Cell Interconnector Inspection: Method Overview, Applications and Limits

"Digitalization" of our industry in terms of materials, components and manufacturing as well as logistics and markets is one of the most promising concepts for achieving long-term overall cost reductions, high reliability and quality assurance within the "industry 4.0". To achieve this long-term goal, several challenges must he overcome: among others, reliable and inline-capable characterization methods are the key technology to deliver input and verification for the digital representative ("digital twin"). This paper gives an overview of three ultrasonic methods developed for inline characterization of copper ribbons used to interconnect solar cells in a solar module. Material parameters of these ribbons are crucial for the soldering process and the future reliability of solar modules. The three methods are based on different physical effects: 1) Determination of elastic constants by ultrasonic dispersion analysis; 2) Acoustoelastic microstructural analysis; 3) Determination of mean grain size by ultrasonic scattering analysis. Each method has its own specific range of application: Method I proved to be very stable and reliable for elastic constant evaluation (Young's modulus, Poison's ratio) in all areas of ribbon production and module manufacturing. Method 2 is most sensitive and suitable for detecting small changes in microstructure due to mechanical loading or manufacturing process variation. It is therefore well-suited to inline quality check in module production. Method 3 allows determination of the mean grain size, characterizing the annealing process. Mechanical loading must be avoided here, as generated dislocations also influence the attenuation. The newly developed methods and achieved results indicate enormous potential for ribbon characterization on an industrial scale and fit well in the "industry 4.0" concept.