Sure Connect™
Controlling cost in gold plating
As a coating material gold has only one major drawback – raw material cost. Gold is the most expensive contact finish, with tin alloys the least expensive. Gold-flash-palladium-nickel lies somewhere between the two, although probably closer to tin. However, for the engineer who understands the performance benefits of gold and has a preference to specify the material, raw material cost can be significantly controlled and reduced by use of appropriate design and processing techniques.
As an example, the evolution of selective plating techniques have resulted in significant cost reduction and control in the use of gold for plating finishes. In many instances gold has been chosen whereby thicker levels of gold have been used in the so-called major zone and gold flash used for the balance of the contact.
Remember - the least expensive connector plating is the one that functions properly for its expected service life
In evaluating the costs associated with bonding wire selection it is recommended that the total cost (rather than just the basic wire cost) of a copper solution versus a gold solution is considered. There is much more to the story than just the cost per meter of gold versus copper bonding wire.
While it is a challenge to make a comprehensive cost model that is close to any one end-users situation, below we highlight some of the issues that will help in the development a total solution cost:
- Copper wire bonding is slower due to longer bond formation resulting in lower units per hour production rates.
- The costs for infrastructure and consumable gases used to protect the free-air ball in copper bonding need to be accounted for during the costing process.
- Remember to account for costs associated with higher bonding temperatures and bonding forces.
- Reliability and package testing are more difficult with copper; using normal decapping methods based on nitric or sulphuric acid to remove the thermoset plastic leaving the gold wire intact can’t be used with copper. Alternative laser decapping methods are costly and time consuming.
- Coppers inferior corrosion resistance means copper wire has a limited ‘floor life’ compared to gold. This may result in increased scrapping of material and associated costs.
- The significant unknown in costing copper and gold bonding wire technologies is the potential cost of in-service chip failure.
Reducing the amount of gold consumed, whilst maintaining the inherent reliability of a gold bonding wire solution, is a viable strategy for cost reduction. Solutions include:
- Reducing the overall length of gold wire through optimisation of loop height
- Optimising bonded ball volume
- Reduction in gold wire diameter
- Design changes to minimise wire usage (for example, using thinner wires for signal wires and thicker wires for power wires.)
The table below highlights the relative cost savings in switching to a smaller diameter gold wire and it is clear that transitioning to smaller diameters can yield substantial cost savings, whilst maintaining the inherent reliability of gold.
| Existing Diameter |
New diameter (μm) | ||||||
|---|---|---|---|---|---|---|---|
| 28 | 25 | 23 | 20 | 18 | 15 | ||
| 30μm | 13% | 30% | 40% | ||||
| 28μm | 20% | 31% | 46% | ||||
| 25μm | 14% | 33% | 44% | ||||
| 23μm | 22% | 34% | 49% | ||||
| 20μm | 16% | 35% | |||||
| 18μm | 29% | ||||||
Relative Percent Cost Savings (at $900 per trz)
K. Dittmer and J. Yeung, Heraeus Materials Singapore, ‘Cost Savings Potential for Bonding Wire’ SEMICON Singapore 2009
With the rising gold price it is important that wire suppliers assist their customers to reduce the impact of the metal price on the overall cost of gold bonding wire technology. Ask your gold bonding wire supplier how they can help you or contact us for further information.