High performance Cyanide-free Immersion Gold

Printed circuit board requires surface finishes to be applied to functional areas prior to assembly operations. There are a number of final finishing technologies available in the market. Electroless Nickel Immersion Gold (ENIG) is a widely used surface finish which is suitable for use as a solderable and aluminum wire-bondable finish, and also a contact surface for metal contact switches. It is because of its excellent solderability and ability to resist corrosion. Conventionally, cyanide containing gold salt for example potassium gold cyanide, is used as gold source in the immersion gold bath. It is well known to have excellent bath stability, easy bath control and high repeatability performance. However, environmental, health and safety (EHS) concens arose on this process. Potassium gold cyanide is well-known as a highly toxic substance. It may be fatal if swallowed, contacted or inhaled. It is also highly toxic to environment especially to aquatie lite. Mishandling of the gold salt or the gold bath may lead to release of cyanide gas which causes environmental issues. As green material and process safety are the future trend in the industry, development of cyanide free immersion gold process is one of remarkable research topics. However, up to now, there is only limited amount of cyanide free immersion gold bath available in the market. The challenge is to maintain good bath stability during continuous operation when getting rid of cyanide to complex the gold ions. In this paper, a novel cyanide free immersion gold bath will be described which can be applied in ENIG process for continuous operation. Beside free of cyanide, the bath can be operated at about 45 degrees C and neutral pH. This operating temperature is much lower than traditional cyanide containing immersion gold bath which operated at about 80 degrees C. This bath can solve the EHS concerns and serve for energy saving as well. The bath was stable with no significant change in plating rate after long time idling. Most importantly, the plating performance is equivalent to conventional immersion gold bath.