Meaning
Chemical reactions and phase transformations occurring between molten solder and metallic substrate metallizations establish metallurgical bonds during thermal reflow. Studies of reflow metallurgy analyze the diffusion of elements between liquid tin alloys and copper contact pads to form continuous intermetallic layers. Peak reflow temperature and liquidus dwell time determine the phase structure and thickness of resulting interfacial compounds.
A well-formed intermetallic layer secures mechanical attachment, whereas excessive layer growth creates brittle fracture paths under mechanical stress.
Intermetallic Formation
Molten tin reacts rapidly with copper surfaces during peak reflow temperatures, forming copper-tin intermetallic compounds. In reflow metallurgy, initial formation of scallops evolves into a continuous layer of Cu6Sn5 during liquid phase exposure. Subsequent solid-state aging converts a portion of this layer into a planar Cu3Sn phase adjacent to the copper substrate.
Metal Dissolution
Substrate surface finishes like electroless nickel immersion gold alter reaction kinetics by introducing diffusion barrier layers. Nickel plating slows copper dissolution into liquid solder, yielding nickel-tin intermetallic compounds that exhibit superior thermal aging stability.
Interfacial Degradation
Excessive peak reflow temperatures accelerate substrate metal dissolution, depleting protective barrier layers and forming Kirkendall voids. Void accumulation along interface boundaries degrades long-term mechanical shock survival in portable electronic devices.
