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.
Selective Soldering Erosion
Meaning
Material loss phenomena measured against the dissolution rates of copper pads in contact with flowing molten solder determine the integrity of a circuit board. Excessive selective soldering erosion leads to the thinning of the barrel walls in through-holes and endangers the electrical continuity of the assembly. This issue occurs when the high-temperature solder stream washes away the copper plating during the localized soldering of connectors or other through-hole components.
It is a major concern for manufacturers using lead-free alloys which have higher melting points and greater chemical activity.
Dissolution Rate
The speed at which the copper is removed depends on the temperature of the solder and the time the pad is exposed to the stream. During the occurrence of selective soldering erosion, the protective copper layer can be completely destroyed in just a few seconds.
Process Optimization
Engineers adjust the pump speed and the nozzle distance to minimize the turbulence of the molten metal.
Board Integrity
Verification of the remaining copper thickness is performed using cross-sectional analysis after the soldering is complete. If the thickness falls below the minimum required by the IPC standards, the board is rejected as a failure. Modern flux formulations are designed to reduce the surface tension and allow for faster soldering to limit the exposure time.
Monitoring the copper content in the solder pot is also necessary because high concentrations of dissolved copper can alter the properties of the alloy.