Moisture sensitivity level

Moisture sensitivity level relates to the packaging and handling precautions for some semiconductors. The MSL is an electronic standard for the time period in which a moisture sensitive device can be exposed to ambient room conditions (30 °C/85%RH at Level 1; 30 °C/60%RH at all other levels).
Increasingly, semiconductors have been manufactured in smaller sizes. Components such as thin fine-pitch devices and ball grid arrays could be damaged during SMT reflow when moisture trapped inside the component expands.
The expansion of trapped moisture can result in internal separation (delamination) of the plastic from the die or lead-frame, wire bond damage, die damage, and internal cracks. Most of this damage is not visible on the component surface. In extreme cases, cracks will extend to the component surface. In the most severe cases, the component will bulge and pop. This is known as the "popcorn" effect. This occurs when part temperature rises rapidly to a high maximum during the soldering (assembly) process. This does not occur when part temperature rises slowly and to a low maximum during a baking (preheating) process.
IPC (Association Connecting Electronic Industries) created and released IPC-M-109, Moisture-sensitive Component Standards and Guideline Manual ("not active").
Moisture sensitive devices are packaged in a moisture barrier antistatic bag with a desiccant and a moisture indicator card which is sealed.
IPC-M-109 includes seven documents. Moisture/reflow sensitivity classification for plastic
Integrated circuit (IC) SMDs, there are eight levels of moisture sensitivity. Components must be mounted and reflowed within the allowable period of time (floor life out of the bag).
Practical.
MSL-specified parts must be baked before assembly if their exposure has exceeded the rating. Once assembled, MSL is generally no longer a factor.
References.
IPC/JEDEC J-STD-020E, Moisture/Reflow Sensitivity Classification for Nonhermetic Surface Mount Devices, December 2014.