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The thermodynamic driving forces that govern non-equilibrium solidification and subsequent microstructural evolution of solder joint microstructures are discussed in detail, including formation of intermetallic phases within, and at the interface that creates the bond between the solder, package, and board.

With the thermodynamic driving forces in mind, the observed microstructures and microstructural evolution that follows nonequilibrium solidification are discussed, with consideration of defect formation, intermetallic phases, and the influence of surface finish and microalloying on microstructure evolution.

The geometry of interconnections used in current electronic systems is described, and issues related to the reflow process that melts and re-solidifies the solder to make the interconnection are introduced.

Expanding use of electronic systems in close proximity to biological systems, marine environments, and polluted air introduce elements that attack solder joints. Mechanisms that cause damage and their effects on the lifetime of electronic system are discussed.

On-off operation of electronic systems leads to thermal cycling, which introduces thermal and strain energy into solder joints, resulting in microstructural evolution mechanisms of recovery, recrystallization and damage nucleation. The thermal strain history is very sensitive to the package design. The effects of microalloying on microstructure evolution bring both a challenge and an opportunity to improve the reliability of solder joints.

Understanding multifaceted microstructural evolution mechanisms is a key enabling foundation that will enable computational modeling and prediction of electronic system lifetimes before anything is built.