Reworkable Anisotropic Conductive Adhesive for Assembly of Medical Devices

Reworkable anisotropic conductive adhesives (ACAs) are of interest when the material is used for the assembly of electronic modules with high value, such as in medical ultrasound probes. Commercially available ACAs are generally difficult or even impossible to rework due to the common use of thermosetting adhesive matrix. ACA material with competitive performance compared to conventional ACAs and with reworkability at modest conditions is developed in this work. Adhesive matrices comprising a blend of a thermosetting epoxy and a thermoplastic polymer are selected becauseit has shown potential to ensure good electrical and mechanical integrity whilst still allowing reworkablity for ACA interconnects. This paper presents the findings of favourable mixing ratios between an epoxy system compatible with ACA applications and a high-performance thermoplastic polymer that offer good mechanical strength combined with reworkability. Die shear strength at varying temperatures relevant for production/storage (23°C), operation of medical ultrasound probes (50°C) and rework of ACA bonds (190°C) is studied. Complete removal of adhesive remaining on bonding surfaces, for successful rework, is verified The results show a high die shear strength at both 23°C and 50°C for adhesive formulations comprising up to 67 wt% of thermoplastic polymer, being comparable to the shear strength obtained for common lead-free solders and conventional ACAs. The die shear strength at 190°C drops dramatically, and agrees very well with the results from the rework evaluation.