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Physics of Failure of Ribbon Bonds
Keywords: Ribbon Bonding, Interconnects, Reliability Testing
The expansion of hybrid electric vehicles in the automotive market has led to increased performance demands of power electronics modules. The trend towards higher power densities, current levels, and operating temperatures has shown that traditional packaging designs using wire bonding technology cannot meet the industry’s performance needs. Temperature variations during operation and coefficient of thermal expansion (CTE) mismatches between the wire material and silicon dies cause failure in the heel of the wire through flexure fatigue as well as bond wire liftoff. For high-current power modules, the limit in diameter of a bond wire to 500 µm has required multiple wires to be bonded in parallel, increasing the time and cost of bonding. These limitations have generated interest in replacing wire bonds with ribbon bonding technology. Lower loop height requirements for ribbon bonds help to reduce heel stress and ultimately flexure fatigue. However, due to the ribbon’s larger geometry, higher bonding energies and forces are required and bond pad contact areas become larger. Damage initiated during the bonding process could become more likely under ribbon bonding, and CTE mismatches between the interconnect material and silicon dies could cause failure under thermal cycling conditions. While the performance and reliability of conventional wire bonding is well understood, lifetime uncertainties of ribbon bonding form a barrier for industry acceptance. To provide the automotive industry with a comprehensive durability assessment, ribbon bonds will be attached to a substrate under a variety of materials and geometries, and then subjected to accelerated thermal, power, and humidity test conditions that will highlight the same failure mechanisms found under normal operating conditions. Analytical models previously developed for lifetime estimations of wire bond failure mechanisms will be updated appropriately to reflect the geometry of ribbon bonds.
Douglas DeVoto,
National Renewable Energy Laboratory
Golden, CO

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