Miniaturization for Implants and Other Medical Products Employing Ultra-Fine Pitch Flip Chip TechnologyAuthors: Rainer Dohle, Ph.D.
Company: Micro Systems Engineering GmbH
Date Published: 9/16/2015 Conference: Medical Electronics Symposium
The accentuated trend to smaller and lighter implants and other medical products has spurred many efforts towards size reduction and increased performance or functionality in high-density medical electronics. One of the interconnection technologies for these applications with the potential for very high integration and cost savings is flip chip technology. For all experiments, specially designed flip chips with a pitch of 100 µm and solder bump diameters of 60 µm, 50 µm, 40 µm, or 30 µm have been used employing SAC305 solder alloy. The UBM of the chips has been realized in an ENIG process. The flip chips with 60 µm or 50 µm solder bumps have been reflow soldered onto BT based PCBs with ENIG surface finish. The chips with 40 µm and 30 µm solder bumps have been flip chip mounted onto thin film ceramic substrates, overcoming the scaling limitations of PCBs caused by the solder mask. High performance underfill has been qualified beforehand. Due to reduction of solder bump diameter, along with accordingly narrower solder pads and the scaling down of line/space geometries, thermal diffusion has a more pronounced impact on reliability and lifetime of the solder connections. In addition, current densities within single solder bumps increase, so that electromigration in flip chip interconnects arises as a reliability concern. This study presents results of our comprehensive reliability tests with emphasis on electromigration performance. Multiple combinations of individual current densities and temperatures were adapted to the respective solder bump diameters, proofing excellent electromigration resistance of the flip chip assemblies employed in the study.
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