MSL stands for Moisture Sensitivity Level. This classification means that a package is classified according the IPC/JEDEC J-STD-020 standard “Moisture/Reflow Sensitivity Classification for Nonhermetic Solid State Surface Mount Devices” so that the parts can be properly packaged, stored, and handled to avoid damage during assembly solder reflow attachment and/or repair operations.
This classification procedure applies to all nonhermetic solid state Surface Mount Devices (SMDs) in packages, which, because of absorbed moisture, could be sensitive to damage during solder reflow.
The vapor pressure of moisture inside a nonhermetic package increases greatly when the package is
exposed to the high temperature of solder reflow. Under certain conditions, this pressure can cause internal delamination of the packaging materials from the die and/or leadframe/substrate, internal cracks that do not extend to the outside of the package, bond damage, wire necking, bond lifting, die lifting, thin film cracking, or cratering beneath the bonds. In the most severe case, the stress can result in external package cracks. This is commonly referred to as the ‘‘popcorn’’ phenomenon because the internal stress causes the package to bulge and then crack with an audible ‘‘pop.’’ SMDs are more susceptible
to this problem than through-hole parts because they are exposed to higher temperatures during reflow soldering. The reason for this is that the soldering operation must occur on the same side of the board as the SMD device. For through-hole devices, the soldering operation occurs under the board that shields the devices from the hot solder.
