Solder binary alloy phase diagram

The microstructure of Pb-Sn solders can be described by the binary alloy phase diagram. The phase diagram is a two-dimensional construct that predicts phase development as a function of the temperature and material composition. An important caveat is that the phase diagram represents the material at equilibrium. Strictly speaking, equilibrium is achieved when the material has been cooled at an infinitely slow rate such that no further changes occur to the microstructure at the target temperature. 

The interface reaction product formed between Pb-Sn solders and most base metals is commonly referred to as an intermetallic compound layer. Intermetallic compounds exhibit highly directional bonding similar to ceramic materials and, as such, typically have a well-defined stoichiometry, high melting temperatures, high strength, and very low ductility. Unfortunately, the binary alloy phase diagrams provide only an approximate indication of intermetallic com-... 

The binary alloy phase diagram can provide some insight into potential intermetallic compound compositions [5], For example, only single-phase fields in the phase diagram can form at the solder/base metal interface. Dual- or higher-order phase fields would not appear, as they cannot develop the concentration gradients required to support diffusion rather, the latter would exist solely as interfaces. 

Massalski, T.B. Binary Alloy Phase Diagrams, 2ndEd. ASM International Materials Park, OH, 1990. Kim, H.K. Tu, K.N. Kinetic analysis of the soldering reaction between eutectic SnPb alloy and Cu accompanied by ripening. Phys. Rev. B 1996, 53, 16027. 

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