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White Plague and Purple Plague: Gold Intermetallics

August 6, 2014 08:26

Gold is commonly used in electronics because of its well-known conductivity. However, it needs to be applied carefully to prevent the development of intermetallics.

The wire bonding in some microelectronic applications can promote the formation of gold-aluminum intermetallics. The two most common forms of intermetallics go by the sinister-sounding names of white plague and purple plague. During the high-temperature environment of the wire bonding process, gold components and aluminum components can “fuse” (similar to an alloy) to create intermetallics.

White plague (described chemically as Au5Al2) has low electrical conductivity. If enough of it forms, the resulting electrical resistance can cause a total failure of the component. Purple plague (described chemically as AuAl2) is actually used in jewelry, but it’s problematic when it appears in electronics. As purple plague forms, it reduces in volume. This creates cavities in the metal surrounding the purple plague, which increases electrical resistance and structurally weakens the wire bonding.

To avoid introducing intermetallics into circuitry, gold and aluminum components must be bonded together without using heat. Ultrasonic welding (similar, but unrelated to ultrasonic assaying) is a common choice. Without it, circuitry must be designed using only aluminum-to-aluminum or gold-to-gold junctions.

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