Coating Can Improve Copper Bond Wires
John Baliga, Contributing Editor -- Semiconductor International, 7/1/2006
Copper bonding wire has been under investigation as an alternative for gold wire for several years. Cost is the main reason, especially now with gold prices at higher levels. One of the main problems with copper bonding wire is the strength of the second bond. Researchers with Sumitomo Electric (Osaka, Japan) have identified an electroplated coating for copper wire that improves second bond strength while maintaining the quality of the first bond. The results of their work were recently reported in the IEEE Transactions on Advanced Packaging.
The reason for lower bond strength in second (stitch) bonds is oxidation of the copper wire, which is not a concern for gold wire. Electroplating the wire with an oxidation-resistant metal can improve second bond strength. Copper wire electroplated with gold, silver, palladium and nickel all showed an expected increase in second bond strength, but in most cases, the first (ball) bond was affected.
Unlike gold wire, copper wire requires some help with ball formation. In this work, the bonders used an electrode torch, which provides a spark that melts the end of the wire to form the ball. An inert gas jet was added to help reduce oxidation. When coated with a second metal, surface wettability and diffusion mechanics come into play.
Copper wires coated with either gold or silver had elongated "spearhead" shapes. The elongated ball tended to move sideways during the bond process, giving an asymmetric bond that was likely to short with an adjacent bond. Wires coated with nickel tended to form a spherical ball, but the ball tended to deviate from the center of the bond capillary. This also led to asymmetric bonds that shorted.
Copper wires electroplated with palladium had ball formation properties that matched uncoated copper wire. Detailed analyses showed that the palladium tended to stay on the outer part of the copper ball, diffusing slowly and uniformly into the copper. With nickel coatings, diffusion was more chaotic, leading to deviated ball formation. For gold and silver coatings, the elongated shape was attributed to a difference in melting point from that of copper.
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