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Too Much of a Good Thing



Notice this direction on the most archaic of fluxes’ “Apply a very little…”.  A source of quite a few beginner soldering mistakes is over-application of 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).






So why do I suggest optimizing flux volume?  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 is not always better.  In fact, more volume is also more expensive, since you could be using an order of magnitude more flux than you need to!  Keep this in mind and dial in your process.