Feature Interconnection Going for gold!
Throughout decades of innovation in the electronics industry, gold has remained the material of choice for manufacturers because of its contribution to quality and high performance. Here Richard Holliday, Director of the World Gold Council’s Technology Sector, discusses the precious nature of gold to electronics manufacturing and its composite assets
old's enduring track record in the electronics industry can be traced back to the early use of gold plating for contacts and connectors, followed by the use of gold bonding wire by the Bell Labs in the 1950s. Now, however, the viability of its continued use is being challenged due to its rise in price.
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A small minority of manufacturers are looking to cut costs by switching to cheaper materials like copper, particu- larly for high volume products with low margins, where reliability is far less of a concern.
In China, there has been a switch to copper for products with these charac- teristics, such as toys. The majority of manufacturers however, are not willing to compromise on reliability and qual- ity, and as a result demand for gold has not only remained strong but is growing. In 2010, 326.8 tonnes of gold was esti- mated to be used by the electronics industry in applications such as packag- ing, plating's and thick film pastes, a record amount at a time when the price of gold reached new highs.
Of this amount approximately 130 tonnes a year of gold bonding wire is used by the semiconductor industry. Each year, billions of wires are bonded and used for integrated circuits, transis- tors and LEDs.
Although these numbers re-affirm the position of gold wire as the metal of choice for the majority of manufactur- ers, it is important to acknowledge that an increasing number of companies are considering using copper for some new products, despite continued evidence that gold is more reliable.
A survey conducted last year by SEMI, the global semiconductor indus- try association, showed this to be the case, but also revealed that many within the semiconductor industry have seri- ous reservations about the reliability and yield of copper bonding wire. SEMI surveyed 46 leading semicon- ductor companies across the world to determine the extent of copper bonding wire programs in the industry and to identify the key issues and con- siderations related to decisions in
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selecting bonding wire material. Companies surveyed included both integrated device manufacturers (IDMs) and fabless semiconductor companies, with revenues totalling $137 billion in 2008, representing 55% of the global industry, and included responses from 14 of the top 20 supplier rankings for 2008. The results show that 59% of the companies surveyed do not use copper wire technology in their products, 41% use it in some products and none of the companies use it in the majority of products.
Of the companies surveyed, 72% are considering the switch to copper wire for some new products, 13% are con- sidering it for the majority of products and 15% are not considering switching. There were serious concerns about the move to copper usage, of which the main concerns were; in-service prod- uct reliability, process yield and unproven historical performance. According to some industry players, copper bonding wire can meet all the needs of the industry, yet the results of the survey suggest many people are not convinced that copper is the com- plete solution. There are serious con- cerns with the reliability and yield of copper wire, but why is gold the mate- rial of choice in the first place? Firstly, gold also has an extensive track record of use for many decades and each year millions of electronic devices trust in gold bonding wire to deliver reliable performance, so there is substantial industry know-how on gold bonding wire.
The unique properties of gold make it ideal for use in bonding wire technology
Image Copyright of World Gold Council
On a technical level, the unique properties of gold make it ideal for use in bonding wire technology. It has out- standing corrosion resistance, high electrical conductivity and its ability to be easily drawn into the thin diameter wire used in the microelectronics industry is a significant benefit. Gold is compatible with any sub- strate or lead materials (aluminium, gold, silver or copper) whereas copper may corrode more readily with epoxy moulding compounds commonly used with gold and some moulding com- pound suppliers may need to offer new formulations for use with copper wire. Gold also has a lower flow stress than copper and so is less prone to bond pad cratering than copper and a robust second bond is achieved. As copper wire is harder than gold wire, bond pad modification is required for some applications and stable wire bonding production is achievable with gold wire.
Gold wire bonding also has larger process windows than copper - a high number of units per hour can be achieved with gold, which allows higher productivity than copper and cost effective assembly operations. Reliability and package testing are also more difficult with copper. Normal decapping methods based on nitric or sulphuric acid to remove the thermoset plastic can be used with copper because unlike gold, it doesn't leave the wire intact. Alternative laser decapping methods are costly and time consuming.
Copper's inferior corrosion resist- ance means copper wire has a limited 'floor life' compared to gold, which can result in increased scrapping of mate- rial and associated costs. There is also the significant added cost of potential in-service chip failure.
Instead of switching to copper some industry players are seeking alterna- tive strategies for cost reduction. Reducing the amount of gold con- sumed, whilst maintaining the inher- ent reliability of a gold bonding wire solution, is a viable strategy to aid cost reduction efforts.
Solutions include reducing the overall length of gold wire through optimisation of loop height, optimis- ing bonded ball volume or design changes to minimise wire usage (for example, using thinner wires for signal wires and thicker wires for power wires).
It is also important to remember that when evaluating the costs associated with bonding wire selection the total cost, rather than just the basic wire cost, of a copper solution versus a gold solution is an important consideration. World Gold Council
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JULY 2011 Electronics
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