Micross

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The Growth of Copper Ball Bonding
Keywords: Wire Bonding, Copper Interconnection, Ball Bonding
The price of Gold is at historic highs (close to $1400/Tr-oz) driving the transition from Gold ball bonding to Copper ball bonding at an accelerated level. The electronics industry uses approximately ten million Tr-oz Gold annually ($14 Billion) of which gold bonding wire comprises approximately 55% ($7.7 Billion). Transitioning to copper wire bonding is potentially the most significant cost reduction available to microelectronic packaging. Low-cost, low lead count, high-power devices have already made the transition. As manufacturing becomes more accustomed to the changes required to mass produce these devices, more complex advanced devices will also be qualified. Bond pad metallization that has traditionally been used with gold ball bonding (Al-1%Si-0.5%Cu ) is not optimal for copper ball bonding. The bond pad is much softer than copper. Copper wire is intrinsically harder than gold and it strain hardens during deformation, increasing the hardness further. The result is thinning of the bond pad under the bond, cracking of inner layer dielectrics in multilayer bond pads, and displacement of pad metallization from under the bond, forming a ring of “splash” around the weld. New metallization recipes, using ENIPIG (Electroless Ni, Immersion Palladium, Immersion Gold) have been shown to eliminate the pad splash and produce high reliability welds. Heavy copper wedge bonding (8-15 mil wire diameter) has the potential to replace Al wedge bonding. Copper wire is a significantly better electrical and thermal conductor than Al and combined with the ENIPIG metallization has the potential to operate at higher temperature (2000C) and carry more current, a strong advantage in power hybrids.
Lee Levine, Consultant
Process Solutions Consulting, Inc.
New Tripoli, PA
USA


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