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Meeting the Challenges of High Reliability for ICs with Thin Pad Al
Keywords: wire bonding, bond pad, bond reliability
Traditional bond pads having Al – SiO2 interconnect structure and thin pad Al are seen to crack more easily when subjected to wafer probe and wirebonding stress. Increasingly robust pad designs help to prevent cracks, but the thin pad Al still presents challenges. Both thin top metal and circuit under pad (CUP) strategies are employed in ASIC designs to minimize die size. At the same time, higher reliability and improved thermal management are required in automotive applications. Thin pad Al makes the bond pads more susceptible to probe damage and cracking, while the under pad circuitry is more sensitive to cracks. Ball bond or wedge bond stresses couple the ultrasonic energy more directly into pad sub-layers, more easily causing cracks or craters. Probe tips gouge deep enough to cause bonding reliability concerns, especially on products receiving multiple probings. Au wirebond results in little or no percent metal displacement remaining (PMDR), and reliability issues of rapid intermetallic growth and the accompanying resistance increase. Cu wirebond imparts more stress to the pad structure, potentially leaving little or no PMDR Al beneath the bond, besides the increased risks of cracks and craters. Bonding recipes may have little or no “process window” when pad Al is too thin. Engineering tradeoffs in the above and some manufacturing success examples are discussed.
Stevan Hunter, Principal Engineer, Quality & Reliability
ON Semiconductor
Pocatello, ID

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