Overview of Moisture Sensitivity Level (MSL) in Electronics

PCB assembly is an industrial process, and like many other processes, certain components or materials can be sensitive to moisture. Some electronic components will uptake moisture from humid air, and when this occurs the components will need to be baked to remove excess moisture to ensure correct operation and high quality in assembly. To provide the information designers and assemblers need to properly handle components, JEDEC has defined a set of moisture sensitivity level (MSL) ratings.

An MSL rating, as well as the required reflow peak and floor like, should be available on product packaging for your components. Make sure you know which components will have higher-than-normal MSL ratings so that your parts can be properly handled and assembled.

What Are the MSL Classifications?

All MSL ratings for electronic components are specified in the J-STD-020 standard (revision F is current as of 2023). The goal of defining MSL ratings is to standardize the maximum allowable exposure for an electronic component to a humid environment. Contrary to what one would expect, the MSL rating for a component is not specified against a specific humidity value, but instead it is specified in terms of the amount of time the component is exposed to a humid environment.

There are 8 MSL classifications which are outlined in the table below. Most semiconductor products or packages will be rated at MSL 3 or higher. The values below are based on the assumption that humidity in the production environment ranges from 40% to 60%; a higher humidity may require lowering the safe exposure period.

When a component is pulled from a kit, the MSL data should be included on the packaging. Once removed from sealed vacuum packaging, the exposure period begins and the component can begin uptaking moisture from a humid environment. While one might assume all humidity should be eliminated from the assembly environment, humidity is actually needed to ensure solder paste can be processed correctly during assembly.

Baking After Exposure

If a component is exposed to a humid environment beyond its rated safe exposure time, then the component will need to be baked to remove moisture prior to assembly. The baking temperature is typically set to ensure effective moisture removal without damaging the component or compromising solderability. Common baking temperatures range from 100 to 125 °C, but the exact temperature can vary based on the component's specifications. Make sure to check the component datasheet to determine whether a part must be baked, and call out the requirement on an assembly drawing.

The baking duration depends on several factors, including the component's packaging material and the amount of absorbed moisture. It is also a good idea to use a specific type of oven which can maintain a low humidity during baking.

There are several reasons to perform this baking process prior to assembly whenever components are exposed for longer than their floor life:

Popcorning: One of the most common issues with moisture absorption in ICs is a phenomenon known as "popcorning." When the IC, which has absorbed moisture, is subjected to rapid heating during solder reflow, any moisture trapped in the packaging can expand rapidly. This can cause the IC package to delaminate or crack. This leaves visible external damage on the package.

Solderability: Excess moisture adsorption on conductors can decrease solderability of ICs onto a PCB. This can result in weak solder joints. Depending on the coating on the exterior of the metal leads, this can contribute to intermetallic growth over time.

Dielectric breakdown: Moisture can cause a reduction in the dielectric strength of the insulating materials inside the IC. This can lead to short circuits and other electrical failures.

Accelerated Wear-Out: The presence of moisture can accelerate wear-out mechanisms in the IC, reducing its operational lifespan.

If certain handling procedures are needed, these procedures should be identified in assembly notes in your fabrication & assembly drawings. Make sure to call out specific reference designators that require special handling and make it clear what degree of MSL compliance is required.

Whenever you are working with moisture sensitive designs and you need to call out specific handling practices for your assembly, make sure you create your design data package using the best set of PCB design features in OrCAD from Cadence. Only Cadence offers a comprehensive set of circuit, IC, and PCB design tools for any application and any level of complexity.

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