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|Effects of Bond Pad Thickness on Copper Wire Bond Reliability|
|Keywords: Copper Wire Bonding, IMC , Bond Pad|
|Copper wire bonding is now widely accepted as a replacement for gold, however, its’ use in high reliability requirement applications are limited due to early failures in high temperature and humid conditions. The switch in wire bond material is mainly driven by cost savings although other advantages such as, better electrical conductivity, thermal conductivity, slower IMC formation, reduced wire sweep during transfer molding, exists. Automotive, industrial and aerospace industries are still reluctant in adopting to this new, cost saving technology, primarily due to lack of sufficient understanding of the failure mechanisms and reliability data. Intermetallic compound formation, between copper wire bond and aluminum bond pad, is a dominant failure mechanism and is accelerated at high temperatures leading to the development of two phases, namely, CuAl2 and Cu9Al4. These IMCs are brittle and cracks under thermal cycling conditions, due to thermal expansion mismatch between the wire and mold compound. It is especially critical because the ‘green’ mold compound composition, in alignment with the recent RoHS guidelines results in a low CTE in the range of 10-12 ppm/°C, below Tg and around 40- 60 ppm/°C, above Tg. This gives rise to CTE mismatch between mold compound and copper wire bond (~16.5 ppm/°C), which causes stress concentration in the wire neck region and the ball bond-bond pad interface. It was observed that crack propagates above Cu9Al4 IMC causing an open circuit failure, thus making it the most detrimental IMC phase. Also, the resistance of Cu9Al4 is twice that of CuAl2, which could affect device performance. In an effort to retard IMC growth and improve copper wire bond reliability, palladium was used as a coat material on the wire bonds. Though it showed improved reliability, it came with an increased wire bonding cost, thus reducing the overall cost benefits of using copper wire bonds. This study aims at determining the effects of bond pad thickness on ball bond reliability. Thick bond pads allow more aluminum diffusion than thin bond pads, thus facilitating CuAl2 formation but postponing the growth of Cu9Al4. Copper wire bonds with 25.4 μm wire diameter are formed on bond pads of 0.5, 0.55, 0.675, 0.8, 1,1.5 and 3 μm, which are then subjected to temperature aging at 150°C, 175°C and 200°C. Devices are taken out at regular intervals for performing resistance measurements and cross sectional analysis to identify IMC phases and measure its’ thickness. Also, the growth rate and activation energy for each case is calculated for Cu-Al IMC in general and for each phase separately. An optimum bond pad thickness is also identified and prescribed for copper wire bond of 25.4 μm wire diameter.|
CALCE, University of Maryland
College park, MD