Bond Strengths of Inner and Outer Leads on TAB Devices

Chips with 43 leads mounted on three layer tapes were outer lead bonded to different substrates. The substrate materials were ceramics with Cu thick film, glass fibre reinforced epoxy, Cu‐lnvar‐Cu with polyimide coating and ceramics with polyimide coating. The strengths of the inner and outer leads were measured before and after storage at elevated temperature, vibration test and temperature cycling. The initial pull strength distributions of the outer lead bonds on the different substrates were analysed. The bond strength on ceramic substrates with Cu thick film was slightly lower than on the other substrates. TAB devices on all substrates endured 750 cycles or more when tested between −55°C and 125°C. Owing to the rather large difference in the coefficient of thermal expansion between the ceramics and the Cu leads, failures occurred first on the ceramic substrate. Storage at 200°C for 2000 h resulted in a somewhat reduced inner lead bond strength. Two mechanisms were responsible for this. The most severe process was interdiffusion between Al and Au across the barrier metal (Ti/W). The second process was diffusion between Cu and Au with the subsequent formation of Kirkendall pores in the Cu leads.