Evaluation and optimization of the mechanical strength of bonds between metal foil and aluminium pads on thin ASICs using gold ball studs as micro-rivets

We are developing an active implant for epidural spinal cord stimulation. A thin application specific integrated circuit (ASIC) (~80 μm) is to be embedded within it. The laser patterned tracks are electrically and mechanically thermosonically bonded on the ASIC pads using gold ball studs, forming micro-rivets through holes in the foil of the tracks, an interconnection method called electrical rivet bonding, or microflex [1]. In this work, we sought to characterize and optimize the technique, with respect to its bonding strength. The technique is relatively new and, so far, the mechanical strength of the bonds has only been investigated for interconnection on gold tracks. Standard ASICs however, normally come with aluminium pads. We ran a series of pull tests on the bonds between the metal tracks and aluminium ASIC pads. In these tests, we were concerned with the effect of the different parameters on the bond strength, and more specifically the size of the gold balls and the size of the holes in the foil. We recorded the maximum force (stress) before bond failure for different combinations of parameters. Our results indicate that average stress values can vary between 9.6 and 60 cN, depending on the process parameters. Different failure mechanisms have been identified and these are discussed. Overall, we conclude that larger holes provide larger contact areas with the substrate and generally result in stronger bonds, but the right combination of ball and hole sizes, could lead to strong bonds even with smaller holes.