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Nickel-Palladium Bond Pads for Copper and Gold Wire Bonding
Keywords: wire bonding, nickel palladium, reliability
Nickel-based bond pads for wire bonding are emerging as an advanced alternative to the traditional aluminum bond pad with benefits for both gold and copper wire bonding, as well as reducing damage during wafer probe. These pads typically have a two-layer Ni-Pd or a three layer Ni-Pd-Au structure and can be very cost-effectively applied by well-developed electroless plating processes. The Ni-Pd-(Au) can either replace the Al pad or be directly plated onto existing Al pads without the need for any masks. Skyrocketing gold prices have recently accelerated the conversion of fine pitch devices from gold to copper wire bonding. Although copper wire has been well accepted in large wire diameter, low pin count applications, pad damage caused by the greater hardness of copper relative to gold can complicate the conversion in high I/O (> 200) devices. This pad damage may include Al splash, cratering or breaking of fragile dielectrics. Nickel pads are four times harder than aluminum and are shown to virtually eliminate the issues of pad splash and damage, while providing excellent bondability and reliability. In gold wire bonding, nickel-based pads provide a pathway to greatly enhanced reliability in fine and ultra-fine pitch gold wire bonding. Although great advances have been made in alloying gold wire to reduce Kirkendall voiding in gold-to-aluminum bonds, intermetallic growth is still a concern in the most stringent applications, such as under-the-hood automotive devices. Intermetallic growth is greatly reduced when bonding gold wire to Ni-Pd or Ni-Pd-Au bond pads. This paper presents the results of fine pitch gold and copper wire bonding on electrolessly plated Ni-Pd and Ni-Pd-Au wire bonds. Excellent bondability and bake-test reliability is demonstrated for both wire types No pull test failures were observed for a 4N-purity (>99.99% Au), 0.8mil gold wire up to 3000 hours at 175C.
Horst Clauberg, Principal Engineer
Kulicke & Soffa
Fort Washington, PA

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