What moisture actually does to a package
Plastic packages absorb moisture from the air. That is not a defect — it is expected, and it is why parts are dried and sealed before shipping. The problem appears at reflow. Absorbed water turns to steam very quickly at soldering temperatures, and the pressure has nowhere to go. The result ranges from invisible internal delamination to a visible crack through the body, often called popcorning. The part may still function at the bench and fail later, or fail immediately.
Two consequences follow. The moisture state of a part is a process parameter, not a storage detail. And the risk is set by the package, the die, and how long the part has been exposed — which is exactly what the moisture sensitivity level summarises.
Reading the moisture sensitivity level
The MSL is a classification, defined in the J-STD-020 and J-STD-033 family of standards, that states how long a package can be exposed to ambient conditions after the sealed bag is opened before it needs to be dried. The floor life shortens sharply as the level rises: at the low-numbered levels it is effectively unlimited or measured in months, while the highest levels allow only a day or so. The common middle classification, MSL 3, is typically quoted as 168 hours — one week of accumulated exposure.
The essential point is that the level is a property of the part, printed by the manufacturer, and the authoritative number is the one on the label of the bag in front of you. Do not infer it from the package type, and do not assume two similar parts share it.
What the dry-pack label tells you
Moisture-sensitive parts ship in a sealed moisture barrier bag containing desiccant and a humidity indicator card. Three things on that bag matter:
- The MSL and the floor life, printed on the label
- The bag seal date, which starts the shelf-life clock before opening
- The humidity indicator card, whose spots change colour with moisture exposure
Read the indicator card against its own reference markings rather than by eye alone, and record the seal date when the bag arrives. Once the bag is opened, note the date. From that moment the floor life is counting, and it counts accumulated exposure — time on the line plus time back in a dry cabinet plus time in transit between the two still adds up.
When baking is required
- The floor life has been exceeded, whether in one stretch or accumulated
- The bag was opened and left open longer than the label allows
- The humidity indicator card shows exposure beyond its limit
- A sealed bag arrives damaged, punctured or already open
- Parts have been returned, reworked, or stored in uncontrolled conditions
- The storage history is simply unknown — an old reel of unclear provenance should be treated as exposed
The baking method that ruins reels
Baking removes absorbed moisture by driving it out with heat, and the standard drying temperature quoted for bare parts is high. That temperature is not suitable for parts on tape and reel, in tubes, or in trays. Carrier tape, cover tape, tubes and many trays are polymer components with far lower temperature limits, and a high-temperature bake will deform, shrink or embrittle them — turning a moisture problem into a feeding problem.
The practical rule: if the part is still in its carrier, use a low-temperature bake as specified for that carrier for a much longer duration, or remove the parts from the carrier and handle them as loose components. Either way, follow the drying schedule given for the package and carrier rather than choosing a temperature because it is faster. Reels should not be assumed to tolerate the same conditions as bare parts.
Preventing the problem instead
- Keep parts sealed until the line actually needs them
- Where a bag must be opened early, reseal it with fresh desiccant, or store the parts in a dry cabinet
- Log open dates and accumulated floor time per lot, so the decision is made from records rather than recollection
- Bake only the quantity needed for the next build instead of a whole lot
- Treat the humidity indicator card as a measurement, and keep it with the lot
How this connects to what you buy
Moisture sensitivity is one of the reasons the date code and the bag label are worth reading at goods-in — a lot with a long and unclear history carries a drying obligation you have not budgeted for. Our note on date codes and batch numbers covers that routine, and the guide to packaging formats explains why the carrier decides how a part can be handled and baked.
Checking a moisture-sensitive part
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