Wire bonding change from Au to Cu and Current Flaws

Cost saving has reached gold wire bonding, there is a global trend on reduce IC’s cost by the conversion of gold wire to copper wire, this started five to six years ago as the most important cost cut on IC’s manufacturing.  In general suppliers have been called this change a “Simple One”, on paper it may be but it has turned not to be a “Simple One”.

Gold ball wire bonding in being a reliable IC interconnection for the last 40 years, all this years has turned on a lot of global knowledge on how to produce reliable bonds and how to control the process, however precious metals are lately in the turmoil of cost increase, global economy become volatile turning gold as an alternative investment when currency exchange experience ups and downs, all this made IC’s suppliers accelerate decision of wire conversion to copper due to precious metals prices are not stable.

Copper is a very obvious option because is cheaper than gold and has superior electrical properties, copper wire is about 10% to 20% of the gold wire cost. However copper is very prone to oxidation, since ball bonds surface is usually heated up to reduce ultrasonic energy to produce reliable bonds without causing cratering then the problem is even worse. The introduction of inert gas atmosphere during ball formation and during wire bonding came up as solution against  copper oxidation phenomenon.

Suppliers are selling the change to customers by converting their machines from gold ball bonders to copper ball bonders with a conversion kit consist in a nozzle that injects inert gas prevents oxidation during ball formation (EFO). Almost every ball bonder supplier has conversion kits then bonds can be safely produced on an inert atmosphere preventing oxidation and allowing reliable bonds. So then what is the flaw?

The flaw may be on the strategy to introduce the change, a flawless change introduction may include but no limited to the following steps:

1)      Parameters optimization: Copper wire is harder than gold, harder material requires more ultrasonic energy to make the wire plastic and produce reliable bonding, then the thermo-sonic system requires a thorough process optimization that most follow a structured statistical analysis of the results, DOE is the most common tool.

2)      Product validation. Parameters optimization has to produce quality bonds that will pass thermal and vibratory profiles checks to have a higher likelihood interconnection will survive customer requirements.

3)      Process validation. Finally bonding process must be kept on a tight process control to ensure long time product reliability.

Gold wire bonding is being on the marked since 70’s, then there is more that forty years of known good process and materials for a reliable Gold ball bond interconnection, meaning copper ball bonding starts from scratch resulting in a lot of unknowns engineers must figure out. The suggested items are minimum requirements for a flawless launch, not follow a similar approach is asking for troubles, because the change was not a “piece of cake” as we were told at the end.

Are you aware of any other key element on wire bonding conversion to copper wire? Are you planning to change soon? Let us know your experiences.

References:

1. Siliconfareast.com “Copper Wire bonding” http://www.siliconfareast.com/wirebond-copper.htm USA 2005. Recovered Sempetember 2012.
2. Cupido, Luis “Wire Bonding (For Microwave and Millimeter-Wave, on a low budget)”  http://www.qsl.net/ct1dmk/articles.html Recovered September 2012.
3. Harman, George “Wirebonding on Microelectronics” International Sociaty for Hybrid Mecroelectronics. United States 1991.