Moisture Sensitivity Level (MSL)

Many of the datasheets and vendor websites mention about a specification called Moisture Sensitivity Level (MSL). Designers are busy verifying the electrical specifications and they tend to ignore the MSL specifications. As a result they could experience an effect called "popcorn" on their components. popcorn effect causes the components to bulge if not used as per the mentioned MSL specification. To get further into the details, all the components that we purchase from vendors are manufactured, packed at some specific conditions. These components are intended to be worked upon only under those conditions. One of the operating condition talks about the moisture. When a component is assembled, the soldering activity may take some duration and during this time any moisture tapped under the component could be unnoticed and this leads to a package damaging over a period of time and sometimes the component bulges. This is actually the popcorn effect. 

MSL specifies the duration for which a moisture sensitive component can be exposed to ambient conditions. The ambient condition is like 30degC at a specific RH level. The Moisture Sensitivity Level rating is done according to the JEDEC industry standard classifications (IPC-M-109, J-STD-20 and J-STD-33). The MSL specifies the components under various levels which indicates the sensitivity to moisture and time for which the specific component can be exposed. Below are the different levels as per J-STD-20.

Level 1 - The component can be exposed for anytime.

Level 2 - Not more than 1 year

Similary, there are levels like 2A,3,4,5,5A,6

Level 6 is the more stringent where the component need to be baked before use. 

Except Level 1 other levels are indicated for a RH of 60%.

The below is a snapshot from the RS232 Transceiver IC, TRS3122 from Texas Instruments which indicates the MSL 

This MSL is more relevant to SMT packages and more in low profile chipsets. This is because the atmospheric moisture may get tapped in and may cause serious damage later. Packing at the vendor side is the most critical stage and it is also the first step to avoid damages due to MSL. Vendors also should indicate the same on the package they ship to customers. So, from next time when we receive new components, we have to verify this indication before opening the package.