What Is Moisture Sensitivity Level (MSL)?
Also known as: MSL, MSD, moisture sensitive device, floor life
Definition
Moisture Sensitivity Level (MSL) is a rating, defined in the J-STD-020 and J-STD-033 standards, that states how long a surface-mount component can be exposed to factory air before it must be baked, to avoid cracking during reflow.
Moisture Sensitivity Level (MSL) Explained
Plastic packaged parts absorb moisture from the air. In reflow, that moisture can turn to steam and cause popcorning, delamination or internal cracks. The MSL rating, from level 1 (unlimited floor life) through levels 2, 2a, 3, 4, 5, 5a and 6, sets the allowed floor life after the dry bag is opened. For example, level 3 allows 168 hours at factory conditions, and level 6 parts must be reflowed within the time stated on the label after bake.
Parts arrive in moisture barrier bags with a desiccant and humidity indicator card. Once opened, the clock starts. If the floor life is exceeded, the parts are baked at a specified temperature and time before use, or returned to dry storage in a dry cabinet that can pause or reset the clock under J-STD-033 rules.
Tracking this is a material control problem. Many factories record the bag open time on the reel label or in MES, with dry cabinet location. ERP inventory can carry MSL level as an item attribute, and lot or location records can flag material that needs baking before it is issued to a work order. Without this, floor life is tracked on paper.
For SMT planning, MSL also shapes kitting: a level 3 part kitted a week ahead of a build may expire on the shelf.
Why It Matters
- Moisture damage can be latent, passing test and failing later in the field.
- Floor life expiry leads to bake cycles that delay builds and can degrade solderability if repeated.
- Customers in aerospace, medical and automotive may require records of moisture control.
- MSL as an item attribute in the ERP lets kitting and issue logic warn about restricted parts.
In Practice
A level 3 BGA reel is opened Monday at 8:00. The floor life is 168 hours, but the line only uses part of the reel before the weekend. The operator returns it to a dry cabinet at 4 pm on Monday, and the MES logs 8 hours used. If it had been left out past 168 hours cumulative, the reel would need a bake before the next work order could use it.
Frequently Asked Questions
What does MSL 3 mean?
A part with MSL 3 has a floor life of 168 hours at factory conditions after the dry bag is opened, before it must be baked or re-stored.
What happens if floor life is exceeded?
The part must be baked per the standard before reflow, or scrapped if it cannot be recovered, because moisture can damage it during soldering.
Can an ERP track MSL?
It can hold the MSL level as an item attribute and flag restricted lots, while open-time clocks are usually tracked by MES or dry cabinet systems.
Related Terms
Reflow Soldering
Reflow soldering is the process that melts pre-applied solder paste in a controlled-temperature oven so surface-mount components bond to the board pads. The time and temperature curve is called the reflow profile.
Surface Mount Technology (SMT)
Surface Mount Technology (SMT) is the method of placing and soldering electronic components directly onto the surface of a printed circuit board, instead of inserting leads through holes. It is the dominant way modern electronics are assembled.
Traceability
Traceability is the ability to follow the history, location, and application of a part or material through the supply chain and production process, both backward to its sources and forward to every product and customer it reached.
WIP (Work in Process)
WIP (Work in Process) is inventory that has been released to production and has begun conversion but is not yet a finished good. It includes partially machined parts, subassemblies, and material sitting in queue between operations.
Safety Stock
Safety stock is buffer inventory carried above expected demand to protect against variability in demand and supply during replenishment lead time. In ERP it is netted out of available inventory so MRP plans replenishment before the buffer is consumed.
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