Meaning
Material degradation occurring when excessive amounts of gold dissolve into tin-based solder alloys creates brittle intermetallic compounds that compromise the structural integrity of a solder joint. The gold embrittlement phenomenon typically happens during the assembly of electronic components that feature gold-plated leads or pads. When the molten solder contacts the gold layer, the gold is rapidly absorbed into the liquid alloy and forms a specific chemical structure known as AuSn4.
This compound possesses low ductility and can act as a site for crack initiation and propagation under mechanical or thermal stress. Management of the gold concentration in the solder bath is critical for preventing the failure of critical electrical connections.
Chemical Reaction
Dissolution of the gold layer into the tin-rich solder occurs almost instantly during the reflow or wave soldering process. If the concentration of gold in the final joint exceeds a specific threshold, usually between three and five percent by weight, the intermetallic crystals become large and interconnected. These crystals of AuSn4 are naturally harder and more brittle than the surrounding tin-lead or lead-free matrix.
As the joint cools and solidifies, the brittle phases tend to concentrate at the interface between the solder and the substrate. This localization creates a weak plane that is susceptible to fracturing when subjected to vibration or temperature fluctuations. The resulting joint may appear physically sound but will lack the necessary toughness for long term durability.
Joint Failure
Structural weakness caused by gold embrittlement leads to the sudden separation of the component from the circuit board during handling or operation. Cracks typically develop along the line of the intermetallic compounds and can quickly spread across the entire width of the connection. Because the failure is often mechanical rather than electrical in the initial stage, it can be difficult to detect during standard functional testing.
Over time, the separation of the joint causes intermittent signals or a complete loss of electrical continuity in the circuit. This type of failure is particularly dangerous in high reliability applications like aerospace, medical devices or automotive electronics. The lack of ductility in the joint means that it cannot absorb the strains associated with the differing thermal expansion rates of the components.
Prevention Method
Reduction of the gold thickness on the component leads and the printed circuit board is the most effective way to avoid gold embrittlement in the assembly. Specifications for plating should limit the gold layer to a thickness that provides solderability and corrosion resistance without exceeding the safe weight percentage in the final joint. For components with thick gold plating, a process of double dipping or tinning can be used to remove the gold before the final assembly.
This involves dipping the leads into a separate solder bath to leach out the gold and then replacing it with a fresh layer of tin-lead or lead-free alloy. Frequent analysis of the solder bath in wave soldering machines helps to monitor the gold levels and ensures that the alloy remains within the safe operating limits. Using solder alloys with specific additives can also help to modify the intermetallic growth and improve the toughness of the joint.
