Reflow looks like a solved problem until a board with a heavy ground plane meets a profile built for a lighter one. The oven does not heat components; it heats the assembly, and every assembly absorbs that heat differently.
Four zones do the work. Preheat brings the board up without thermally shocking ceramic parts. Soak lets the temperature even out across high and low thermal mass so that a large connector and an 0402 resistor arrive at the same place. Reflow takes the joint above the alloy’s liquidus long enough to wet properly but not long enough to grow brittle intermetallics. Cooling sets the grain structure.
Getting it wrong is rarely dramatic. Too little soak and you get cold joints under the largest components, which pass a continuity test and fail after a few thermal cycles. Too much time above liquidus and the joint looks perfect but is measurably weaker. Both defects survive visual inspection, which is why profiling is verified with thermocouples on the actual assembly rather than assumed from a recipe.
Profiles are developed per assembly and re-verified when a board revision changes copper weight or component mix. That discipline is unglamorous and it is the difference between a line that yields consistently and one that argues about intermittent failures.
