Moisture-sensitive devices, and why floor life decides the reflow
A perfectly designed board, placed perfectly, can still fail at reflow because of something that happened to a component weeks earlier in a stockroom. Moisture is the reason.
What moisture actually does
Plastic-encapsulated devices absorb moisture from the air. On its own that is harmless. At reflow it is not: the assembly goes through a peak well above the boiling point of water, the absorbed moisture turns to steam inside the package, and the pressure has to go somewhere. It delaminates the die from the encapsulant, cracks the package, or lifts the internal bond wires. The failure mode that gets talked about is the visible one — the "popcorn" crack — but the expensive one is the invisible internal delamination that passes inspection and fails in the field.
MSL ratings are a clock, not a category
A device's moisture-sensitivity level sets how long it may sit exposed to ambient air before it must be baked or reflowed. That is a floor-life budget measured in hours or days, and it is consumed by everything: the time in your stockroom, the time in ours, the time on the line between opening the bag and the board entering the oven. An MSL-3 part with a 168-hour budget that has been sitting on an open bench for a fortnight has spent it.
What controlled handling looks like
Temperature- and humidity-controlled storage, so the clock runs slowly or not at all. Dry storage for opened parts. Bake-out to reset the budget on devices that have exceeded it. And floor-life tracking, so the elapsed exposure is a recorded number rather than somebody's recollection. All of it under an ESD-safe regime, because the two controls live in the same stockroom and the same discipline.
Why this belongs in the supplier conversation
Nothing here is visible on a finished board. You cannot inspect for it after the fact, and X-ray will not show you a delaminated die. It is a process you either have or do not, upstream of every other quality control. Worth asking any prospective manufacturer how they track floor life, and what happens to a reel that has run out of it.
Every component reaches our line in spec: dry-stored, baked where required and floor-life tracked, in temperature- and humidity-controlled storage. It is the least visible thing we do and one of the most consequential.