Over the past 30 years, wire-bonding technology has been advancing using gold wire in the interconnection process. With the current market trend demanding lower packaging cost, copper wire bonding is rapidly picking up momentum as the semiconductor interconnection material versus gold wire. The price of copper material is up to 90% cheaper than gold. In addition to better electrical conductivity and stiffer mechanical properties, another advantage of copper is that its intermetallic growth at the Cu-Al interface is significantly slower than the Au-Al interface at comparable temperatures. This results in greater reliability at elevated temperatures. Copper is also known to have higher thermal conductivity than gold by 25% resulting in larger grain structures at the region adjacent to the free air ball after an electric flame-off (EFO) fire. The more efficient thermal conductivity of copper wire allows better heat dissipation and produces a shorter heat-affected zone (HAZ) in the wire during free air ball formation. The shorter HAZ, as well as the stiffer nature of copper wires, has many implications to its wire looping capability, especially in very low loop height formation. Today, market demand for copper wire bonding is not only for low pin count lead frame packages but has expanded to multi-tiers, very low loop height, stacked die, and die-to-die stand-off stitch bond (SSB) interconnect applications. Thus, development efforts in copper loop shape capability have been growing. Understanding copper wire performance in achieving different loop shapes will offer increased flexibility of its implementation in various packages. This paper will discuss the unique challenges in copper wire looping formation in meeting the same stringent loop height and wire consistency requirements as gold wire.
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