Electroplating Bath Organic Impurity Degradation and Intermetallic Interface Voiding Dynamics
Organic bath impurities codeposit in plated copper, accelerating Kirkendall voiding at intermetallic boundaries during reflow and degrading joint strength.
Copper Intermetallic Compounds
Meaning
Crystalline phases formed by the chemical reaction between copper and tin represent copper intermetallic compounds. These structures develop at the interface when molten or solid solder reacts with a copper substrate. Their presence is necessary for a metallurgical bond but excessive growth leads to brittleness in the joint.
Thermal Evolution
Growth rates for the copper intermetallic compounds depend heavily on the temperature and duration of the soldering process. Solid state diffusion continues long after the initial reflow, causing the layers to thicken during the operational life of the electronic device. High temperatures accelerate the migration of copper atoms into the tin matrix.
Molecular Composition
Chemical analysis identifies two primary species known as Cu6Sn5 and Cu3Sn. The Cu6Sn5 phase usually forms first and appears as a scalloped layer next to the solder bulk. Heat treatment causes the copper intermetallic compounds to transform, with the Cu3Sn layer appearing between the copper base and the Cu6Sn5 layer.
This second phase is often thinner but more prone to defect formation.
Mechanical Property
Fracture toughness within these layers is lower than in the surrounding bulk metals. The copper intermetallic compounds create a path for crack propagation when the assembly experiences thermal cycling or mechanical shock. Large grains or thick layers increase the risk of interfacial failure during handling or shipping.
Reliability depends on controlling the total thickness of these brittle regions through careful material selection.