Lead Free Solder Interface Intermetallic Kinetics Basics
Managing lead-free solder intermetallic growth requires strict reflow thermal limits to prevent brittle phase formation and latent interfacial field failures.
Strain Rate Fracture
Meaning
Structural failure modes measured against the deformation speed of a solder joint determine the reliability of electronic devices under rapid mechanical loading. When a circuit board is bent quickly during a drop or a handling event, strain rate fracture occurs because the material cannot redistribute the stress through plastic deformation. This failure is often located at the interface between the solder ball and the pad on the printed circuit board.
It is more prevalent in lead-free alloys which are inherently more brittle than the traditional tin-lead compositions.
Velocity Dependency
The strength of the joint is not a fixed number but changes based on how fast the load is applied. Under the conditions of strain rate fracture, a joint that appears strong in a slow-pull test can fail instantly when hit with a high-velocity impact.
Energy Absorption
Brittle materials have a low capacity to absorb energy before they snap.
Fatigue Evaluation
Testing equipment for this failure mode uses specialized sensors to capture the force and the displacement at millisecond intervals. Engineers use this data to select materials that can withstand the sudden shocks of daily use. Improving the ductility of the alloy through micro-alloying is a common strategy to mitigate this risk.
The analysis of the broken surfaces under a microscope reveals a characteristic smooth fracture that indicates a brittle break.