Moisture Sensitivity Levels (MSL) in PCB Assembly What You Need to Know
In the world of PCB Assembly, ensuring component reliability and long-term performance is critical. As devices become more powerful, complex, and sensitive, improper storage and handling can result in unforeseen great losses. One often-overlooked but absolutely essential factor in maintaining assembly quality is the Moisture Sensitivity Level (MSL) of electronic components.
So what is MSL? Moisture Sensitivity Levels is a classification system developed by JEDEC that indicates how susceptible a surface mount device is to moisture-induced damage during reflow soldering. Components will naturally absorb moisture from the ambient environment. When these moisture saturated components are exposed to temperatures seen in a reflow oven, the moisture inside rapidly expands into steam. This rapid expansion creates immense pressure inside the component that could cause significant damage to the device. Common symptoms of induced damage seen in moisture sensitive parts are internal damage, delamination, and “popcorning” — a phenomenon where the built up pressure cracks the package.
Components Exposed
Every minute your component is exposed to the environment,
your risk increases.
How do I interpret this chart? The moisture barrier bag should have a label on it designating what level it is. Given that information, you can then see how long can the part be exposed to the environmental conditions listed above and still be considered acceptable without baking. If these devices are kept in a 5% humidity dry box, then the timer is stopped. It is important to consider that during shipping, components can be subjected to significantly higher temperatures and humidities that can deteriorate items at a faster rate than what is listed.
One way to remedy moisture compromised devices is through baking. Baking parameters can vary with different components, but is generally safe to follow the chart above. It should be emphasized that excessive baking and can cause solderability issues on certain components and printed circuit boards. You should always consult your local pcb assembly house if you have any doubts or questions about this process.
1. Dry Packing – MSL rated devices are typically sealed in MBB (Moisture Barrier Bags). MBBs are there to prevent moisture from penetrating the bag and saturating the internal environment. A (HIC) humidity indicator card will also be placed with the component to monitor exposure and alert team members of a levels prior to opening the bag. Along with the HIC, a desiccant pack is also inserted into the bag to soak up some extra moisture that might be present while initially packaging the device.
2. Receiving & Storing – MSL rated devices need to be stored in a controlled environment. A dry cabinet with 5% humidity is the perfect storage for such components. 5% humidity can also be achieved in certain ovens with nitrogen purging and can be used to store these devices as well. Opened packages should be resealed or placed in dry storage quickly to ensure compliance with table referenced above.
3. Tracking Floor Life – By tracking the floor life of opened packages, one can safeguard against components being left outside dry storage.
For more information on Moisture Sensitive Level Components and how to best handle them, please check out IPC/JEDEC-J-STD-STD-020 !
