Untangling the Mysteries of Plated Metal Finger Adhesion: Understanding the Contributions from Plating Rate, Chemistry, Grid Geometry, and Sintering

Historically, busbar pull tests have been used as a measure of metal-silicon adhesion for silicon solar cells; however, such measurements cannot be easily applied to evaluate finger adhesion and the propensity of metal fingers to peel. Finger adhesion will be increasingly important as the width of fingers decrease and busbars are effectively removed from the cell metallization. In this paper, we correlate metal-plated finger dislodgement measurements, which have been obtained using a stylus-based metallization testing tool, and busbar pull test forces with nanoindentation measurements of the Young's modulus in order to determine key determinants of strong finger adhesion. It is proposed that metal fingers with a higher Young's modulus dislodge at lower stylus impact forces because the energy associated with the impact is less easily dissipated along the fingers and consequently remains more focused on the impact location, causing not only finger dislodgement but more extensive finger peeling as well. It is shown how plating rate, chemistry, grid geometry, and postplating annealing can all contribute to plated metal finger adhesion, therefore necessitating an understanding of these factors for reliable plated metallization.