Bonding wire is used to make the electrical connection or interconnection between the semiconductor component (die) and the package. Whilst some chip packages eliminate the need for bonding wire, evolution of bonding wire technology has maintained the position of bonding wire as the main interconnection technology.

Despite the cost of the raw material, gold is the most common bonding wire material because it is the easiest and fastest wire to use in production. According to SEMI¹ it represents almost 90% of the worldwide bonding wire by volume. 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. Solutions include reducing the overall length of gold wire through optimisation of loop height, reduction in wire diameter and design changes to minimise wire usage. See here for more information.

The price of gold has led to renewed interest in the deployment of copper bonding wire as a replacement for gold. Some issues to be considered in making changes to bonding wire material are highlighted below:

• Gold has an extensive track record of use for many decades and trillions of assembled chips have used gold.
• There is substantial industry know-how on gold bonding wire
• Stable wire bonding production is achievable with gold wire
• Gold is softer and has a lower flow stress than copper so bond pad cratering is rare and a robust 2nd bond is achieved
• Gold wire bonding has larger process windows than copper
• A high number of units per hour can be achieved with gold; allows productive, cost effective assembly operations
• The gold wire bonding process is stable in air (no protective gas infrastructure or consumable gases are required). 
• Gold provides high reliability for a vast array of packaging applications
• Gold is compatible with any substrate or lead materials (aluminium, gold, silver or copper)
• Although gold’s thermal and electrical conductivity are slightly lower than copper, they easily meet performance requirements.

Remember that for copper wire bonding the following issues need to be considered:

• Copper has to-date limited proven performance in-service
• There is substantially less industry knowledge of copper wire bonding
• There is limited industry experience of using copper for higher-end chips
• Challenges with chip packages facing lean, sway, or sweep issues in gold are un-tested in copper.
• Copper has a higher flow stress than gold, so is more prone to bond pad cratering
• Copper wire bonding has a smaller process window than for gold
• Copper wire bonding is slower due to longer bond formation; lower production rates (units per hour)
• The costs for infrastructure and consumable gases used to protect the free-air ball in copper bonding can negate some or even most of the gold metal cost.
• Copper reliability is still a significant issue for many applications; 2nd bond strength is often low in volume production and whilst 1st bond is often acceptable as bonded, it is prone to failure in heat and humidity tests
• As copper wire is harder than gold wire, bond pad modification is required for some applications
• 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.
• Copper has a limited shelf life on bonding machines of 3-4 days due to oxidation
• Copper may corrode more readily with epoxy moulding compounds commonly used with gold and some moulding compound suppliers may need to offer new formulations for use with copper wire

¹SEMI Global Packaging Materials Outlook 2009