Validation of methodology to determine the contact resistivity of ECA-based bonds grounded on end–contact resistance measurements using redundant and modified TLM test structures

Abstract

Accurate determination of the contact resistivity between electrically conductive adhesives (ECAs) and solar metallization paste is a key component of optimizing the electrical performance of bonds based on ECAs, such as shingled joints. This work proposed and validates a methodology that accounts for non–negligible inhomogeneities of test structures based on ECAs and silver screen-printed metallization, such as changes in width of the ECA, to improve the accuracy in the calculation of the contact resistivity. The proposed validation uses statistical theory based on the central limit theorem and the law of large numbers showing that the contact resistivity of bonds based on two different commercially available ECAs can be clearly differentiated with a minimum confidence level of 95% and a maximum error margin of 5%. Moreover, the necessary sample size to achieve such requirements is calculated and shown to be equal to 24 test structures. The validation was done using an acryl-based adhesive loaded with silver flakes (∼10−4 Ω cm), which was compared to an epoxide-based ECA filled with silver–coated copper particles (∼10−3 Ω cm). The validated methodology indicates that the contact resistivity when using the acrylic conductive adhesive is 0.1791 ± 0.0766 mΩ cm2 while when using the epoxide-based adhesive is 0.5025 ± 0.1218 mΩ cm2. Additionally, it is shown that the standard deviation of the contact resistivity is highly dependent on the composition of the conductive adhesive. Hence, the sample size varies depending on the ECA. It is expected that the presented methodology will be a powerful instrument for developers and researchers in the improvement and optimization of ECAs and ECA bonding processes.