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Critical Barriers of Copper Bonding Wire
Keywords: alternative bonding wires, reliability, process comparisons
Critical barriers associated with copper wire Wire bonding is widely used in the industrial and automotive industry to assemble the vast majority of packages, electrically connecting Aluminum (Al) bond pads on the IC chips to the corresponding bond pads on the plastic or lead-frame chip carriers. The Industry has short listed several common Bond wires which usually consist of good conductor metals such as Gold (Au), Aluminum (Al), or Copper (Cu). Gold has been the most widely used material because of its long successful history and extensive reliability data, resistance to surface corrosion and wire ductility during the bonding process. Many companies looked at bare Copper wire as a viable replacement option for Gold wire however many shortcomings of conventional Copper wire use in advanced packaging and assembly were encountered. Comparisons of bare Copper wire versus Gold wire will be presented. A breakthrough in the wire industry was the introduction of Palladium Coated Copper (PCC) Wire in 2008. The control of surface oxidation and stability in high volume manufacturing enabled PCC wire as the preferred alternative interconnect material of choice to Gold and conventional Copper wire. Comparisons of PCC wire versus conventional Copper wire will be presented. The critical barriers in fine pad pitch applications associated with bare copper wire such as 2nd bond instability (NSOL), short tails, limited shelf/bonder life (wire oxidation) and high temperature/humidity reliability have been solved with the development of a Palladium coated copper bonding wire. As a result of the maturity of Palladium copper wire, some process, reliability and design limitations have been confirmed such as harder FAB (free air ball), fine pitch process issues, capillary life and wire oxidation at extended high temperature storage. An improvement and solution is a copper alloy bonding wire which has been introduced as an alternative wire to overcome the limitations of bare copper wire and PCC wire. Wire comparisons will be presented. The advantage of switching from bare copper wire and PCC wire to copper alloy wire is that copper alloy wire meets ball bonding performance requirements, soft FAB and comparable loop formations while maintaining productivity requirements. Copper alloy wire shows good potential as it offers several advantages compared to bare copper and palladium coated copper wires; mainly, acceptable bonding performance and reliability. Summary of bonding wires will be presented. Copper alloy wire may not work for all applications but it can certainly help support many application areas where cost and performance define a product. Copper alloy wire usage is expected to increase in the next few years in high reliability applications.
William Crockett, Business Development/Technical Specialist
Tanaka Kikinzoku International
Morgan Hill, Ca

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