Sensitivity levels for moisture intake during storage and handling determine the maximum time a surface mount device can be exposed to ambient conditions before the soldering process. In the manufacturing of printed circuit boards, msl stands for moisture sensitivity level, a rating that dictates the floor life of non-hermetic semiconductor packages. If a component absorbs too much moisture from the air, the trapped water can turn into steam during the high temperatures of the reflow oven.
This rapid expansion causes internal delamination or the cracking of the plastic package, a phenomenon known as popcorn effect. The rating system ranges from level 1, where the part is not sensitive to moisture, to level 6, which requires mandatory baking before use.
When a component has exceeded its allowable floor life, it must undergo a controlled baking process to remove the absorbed moisture. This involve placing the parts in an industrial oven at a temperature typically between forty and one hundred and twenty five degrees celsius for a specified number of hours. The duration and temperature of the bake depend on the specific msl rating and the thickness of the component body.
This process must be done carefully to avoid damaging the components through heat stress or causing oxidation on the solder leads. Once the bake is complete, the parts must be used within a very short window or sealed in a moisture barrier bag. Manufacturers keep detailed logs of when the bags are opened and how long the parts have been exposed to the factory environment.
The physical construction of the component determines its vulnerability to moisture ingress. Plastic encapsulated microcircuits are the most common parts that have an assigned msl because the epoxy molding compound is naturally hygroscopic. Larger packages with large internal dies are often more susceptible to delamination because the internal stresses are higher.
During the development of a new product, engineers select components with lower sensitivity levels when possible to simplify the assembly process. If a high sensitivity part is required, the assembly house must have a dry storage cabinet to keep the components at a low humidity level between production runs. This protection is necessary to ensure that the internal bonds of the semiconductor remain intact after soldering.
Adhering to the guidelines associated with this rating is a mandatory part of any quality management system in electronics assembly. A failure caused by moisture ingress is often hidden, as the internal cracks may not result in an immediate electrical fault but will cause the device to fail after it has been shipped to a customer. Standard inspection methods like x-ray may not always detect the subtle delamination caused by the popcorn effect.
High reliability industries often use scanning acoustic microscopy to verify that the internal structure of the package is solid. By strictly following the msl procedures, the manufacturer eliminates a major source of latent defects. This discipline ensures that the finished product meets its expected reliability targets.
The management of moisture is a silent but vital aspect of hardware production.
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