Dissolution Rate
Thermodynamic rate equations govern the mass transport of solid metal into liquid solder during thermal processing. The study of copper dissolution kinetics defines how fast surface copper features dissolve into molten tin-lead or lead-free alloys at specific wave soldering temperatures. Molten tin attacks copper land areas by forming intermetallic compounds while taking free copper into liquid solution.
Excess heat or prolonged contact time accelerates this rate dramatically.
Boundary Mechanism
Intermetallic compound growth at the liquid interface creates a thin layer of Cu6Sn5 that limits liquid access to parent copper. Although this barrier forms rapidly, local hydrodynamic flow sweeps dissolved copper away and maintains a steep concentration gradient across the fluid boundary layer. Higher solder temperatures lower fluid viscosity while raising the solubility limit of copper in molten tin alloys.
Under continuous wave contact, narrow conductor tracks reduce in cross-sectional area and thinned barrels fail mechanical pull tests.
Bath Control
Chemical analysis of wave solder pots monitors accumulated copper contamination to prevent solder joint brittleness and elevated melting points. As copper dissolution kinetics push total bath copper content past operating thresholds, liquidus temperatures rise and solder flow slows across dense board layouts. Operators adjust dwell times or lower pot temperatures to keep alloy purity within IPC specifications.
Unchecked dissolution leaves thin copper traces vulnerable to complete knee-cracking during thermal cycling tests.
