For packages that are sensitive to moisture, it is important to control the moisture content of the components. Moisture can seep into the package moulding compound from the ambient air, leading to high moisture levels that can damage the package during the reflow process. Therefore, it is necessary to dry moisture-sensitive components, seal them in a moisture-resistant bag, and only remove them from the bag immediately before assembling them to the PCB. The permissible time from opening the moisture-barrier bag until the final soldering process, as well as the duration of exposure to ambient humidity, is specified by the moisture sensitivity level (MSL). The IPC/JEDEC J-STD-033 standard defines eight different MSLs. Information about the MSLs of our products can be found on the "Moisture Sensitivity Caution Label" on the packing material. Additionally, IPC/JEDEC-J-STD-20 specifies the maximum reflow temperature that must not be exceeded during board assembly.
If moisture-sensitive components have been exposed to ambient air for longer than the specified time for their MSL, or if the humidity indicator card indicates excessive moisture after opening the dry package, the packages must be baked before the assembly process. Baking a package too often can cause solderability problems due to oxidation and/or intermetallic growth. It's important to note that the packing material may not be able to withstand the baking temperature, so it's essential to check imprints/labels on the packing for the maximum temperature. Depending on the reflow peak temperature, two levels of moisture sensitivity may be given. Tin-lead devices with a lower reflow peak temperature may withstand a longer soaking time, while lead-free devices with a higher reflow peak temperature may have a derated MSL.
Below is an example of the MSL classification as seen on a Microchip datasheet.
