white plague
The white gold-aluminium intermetallic compound Au₅Al₂, which can be formed when gold and aluminium are heated in direct contact (for instance, at an electrical junction between gold and aluminium components).
white plague: gold-aluminium corrosion that kills connections
White plague is the brittle intermetallic compound Au₅Al₂ that forms when gold and aluminium are heated in direct contact. It appears as a white or grey crystalline deposit at the interface between the two metals, typically at electrical junctions, solder joints, or wire bonds in integrated circuits. The compound is electrically insulating and mechanically weak, making it a failure mechanism rather than a desirable phase.
The reaction occurs because gold and aluminium form a favourable phase diagram at elevated temperatures. Thermal cycling, resistive heating at poor contacts, or even normal operational temperatures in confined geometries can drive diffusion between the two metals. In hybrid integrated circuits and thick-film hybrids, Au₅Al₂ formation is particularly common where gold bond wires or gold plating meet aluminium pads, lead frames, or traces. The compound grows at the expense of both parent metals, consuming material and creating voids that weaken the joint mechanically.
The name derives from the appearance of the deposit and its destructive character. A plague suggests both the white discolouration and the contagious spread of failure across affected components. Once initiated at one junction, thermal stress and further heating can accelerate intermetallic growth, leading to open circuits and device failure. This is particularly problematic in power electronics and high-reliability applications where thermal excursions are common.
Mitigation and Industry Practice
Designers and manufacturers avoid white plague through barrier layers, material selection, and thermal management. A thin nickel or palladium barrier between gold and aluminium prevents direct contact and diffusion. Alternatively, using gold wire bonded to gold-plated copper pads instead of aluminium reduces the risk entirely, though at higher cost. Operating temperatures and thermal cycling limits are specified to keep junction temperatures below the regime where diffusion becomes significant.
White plague remains a persistent concern in legacy systems and cost-sensitive high-volume production where gold plating is applied over aluminium substrates. Modern hybrid and IC assembly tends to use gold-copper or gold-nickel combinations to sidestep the problem. Understanding the mechanism is essential for failure analysis: a white deposit at a failed Au-Al junction is diagnostic, not cosmetic.