Compatibility of metals on MEMS and soldering for assembly

For assembly of MEMS by use of soldering, the metallization on the MEMS and the soldering process must be selected carefully to ensure long-term reliable joints. A range of combinations of adhesion metal layers and solders were tested in this work. The target was to find a metallization system for the MEMS without layers of pure Cu or Ni due to risks related to cross-contamination. All combinations were apparently strong and no degradation was measurable after thermal storage simulating more than 10 years of operation at 25 °C. However,the cross sections showed clear differences in microstructure as well as changes in morphology after thermal aging, which could potentially result in differences in reliability and joint quality. Both a first hypothesis related to NiCr outperforming TiW as adhesion layer, and a second hypothesis related to Au-richer systems to be less reliable than their counterparts with less Au, were confirmed from the fracture surfaces where differences were clear. A targeted cohesive fracture mode was achieved both in the as-bonded state and after thermal aging for a combination with limited amounts of Au and with NiCr used as adhesion layer on the MEMS.