Wafer-Level Microsphere Bumping (Flux)

In most cases, all that is needed is about a 25um layer of flux on the bonding surfaces and solder – usually 2-4% mass of the solder. Flux should almost never be of equal volume as your solder (except for some solder pastes). The perfect amount of flux will be enough to form a good solder joint, but will clean well (for water or solvent soluble fluxes) or appear clearer with less residue (no-clean fluxes). In extreme cases, lowering flux volume can improve reflow cycle time, because complete activation can occur sooner – and thermal inertia is decreased. More volume is also more expensive. Keep this in mind and dial in your process.