Two failure modes, one variable
Humidity in an assembly plant has two effects that point in opposite directions.
Too dry: static charge builds, and ESD damages MOS gates, thin dielectrics and bare boards long before the product is powered. Most ESD control plans therefore set a floor around 30 % relative humidity and add ionisers where the local environment cannot hold it.
Too damp: plastic packages absorb moisture from the air. At reflow, that moisture turns to steam inside the package and the package cracks, delaminates or lifts a die — the "popcorn" failure. It often passes electrical test and fails in the field.
The resolution is not a compromise humidity level. It is a humidity floor for the room plus dry storage for the parts.
MSL and floor life
J-STD-033 classifies moisture sensitivity from level 1 (no restriction) to level 6 (bake before use). The practical ones in production:
| Level | Floor life at ≤30 °C / 60 % RH |
|---|---|
| 2 | 1 year |
| 2a | 4 weeks |
| 3 | 168 hours |
| 4 | 72 hours |
| 5 | 48 hours |
| 5a | 24 hours |
| 6 | bake required before use |
Floor life starts when the moisture-barrier bag is opened. A reel that is opened on Monday and used on Friday is a different risk from one opened on the same shift, even though the parts are identical.
Dry storage
Three options, often combined:
- Moisture-barrier bags with desiccant and a humidity indicator card. Cheap, effective, and awkward once opened: the clock starts.
- Dry cabinets at low relative humidity, which suit high-mix production where reels are opened repeatedly. This is the usual answer for a plant running many small builds.
- Baking ovens, for parts that have exceeded floor life or arrived with a damaged bag.
The choice is about access pattern as much as about protection: a bag protects a part nobody touches, a cabinet protects a part used every day.
Baking, correctly
Baking removes moisture, but it is not free. The schedule depends on package thickness, body material and whether the part is in a tray, tube or reel; the tables in J-STD-033 exist because the wrong bake can damage tape, labels and the parts themselves.
Two rules follow: bake to the schedule for the package, and use the part promptly afterwards or put it back into dry storage. A baked reel left on the line overnight is back where it started.
What the customer should see
For any build with area-array or fine-pitch packages, the process record should show MSL handling: bag-opening dates, cabinet logs, bake records for parts that exceeded floor life, and the reflow profile used. Ask for it in writing when comparing pcb assembly services; a supplier who cannot produce it is not controlling moisture.
That record is what distinguishes a supplier who controls moisture from one who has a dehumidifier. Workmanship is judged to IPC-A-610, and the gates that catch the resulting defects are described in the quality control note.
