JEDEC Moisture Sensitivity Levels, Explained
A surface-mount part can absorb enough moisture from ordinary room air to crack itself during reflow — the so-called popcorn effect. MSL ratings are how the industry manages that risk. Here's what the numbers mean and how to handle them.
Plastic-encapsulated surface-mount components are hygroscopic: they slowly absorb moisture from the air. Run one through a reflow oven with too much trapped moisture and that moisture flashes to steam, delaminating or cracking the package from the inside — "popcorning." The Moisture Sensitivity Level is the rating system that keeps that from happening, and it becomes especially important for parts that have been sitting in storage.
What MSL Measures
Defined by the JEDEC standards J-STD-020 (classification) and J-STD-033 (handling, packing, and shipping), the Moisture Sensitivity Level rates how long a device can be exposed to ambient conditions — its floor life — after the sealed dry-pack is opened, before it must be reflowed or re-baked.
The Levels
MSL runs from 1 (least sensitive) to 6 (most sensitive), defined by floor life at standard conditions:
- MSL 1 — unlimited floor life; no special handling
- MSL 2 — 1 year
- MSL 2a — 4 weeks
- MSL 3 — 168 hours
- MSL 4 — 72 hours
- MSL 5 — 48 hours
- MSL 5a — 24 hours
- MSL 6 — bake required before use; floor life as labeled
The higher the number, the less time the part can spend out of its moisture-barrier bag before it needs intervention.
Floor Life, Dry-Pack, and Baking
MSD parts ship in a moisture-barrier bag with desiccant and a humidity indicator card. Open the bag and the floor-life clock starts. Exceed it — or find the indicator card has tripped — and the parts must be baked to drive the moisture out before reflow, following the time-and-temperature schedules in J-STD-033. Baking resets the clock; resealing with fresh desiccant pauses it.
Why This Matters for Obsolete and Last-Time-Buy Stock
Here's where moisture sensitivity collides with obsolescence. Parts warehoused for years on a last-time buy, or sourced from long-held excess, can sit well past their floor life or arrive with a spent humidity indicator. None of that means the parts are bad — it means they need proper handling. A disciplined distributor stores MSD parts in controlled humidity, uses nitrogen storage for long-term lots, tracks bag integrity, and bakes when required. It's a quiet reminder that how a part was stored matters as much as where it came from.
RH Electronics handles and packages ESD- and moisture-sensitive devices to spec, including nitrogen storage for long-term lots. Submit a requirement or see our quality process.