Explaining MSL (Moisture Sensitivity Levels)

You may be looking at one of our parts and wondering, what’s the deal with the MSL statement?

MSLexample

Like an opened bag of chips, electrical components too can absorb moisture from the environment. This can be an issue when these components are fed through a reflow machine because the intense heat from the soldering process causes rapid release and expansion of the trapped moisture. (See a Reflow machine in action here) This internal humid gas tries to escape the component immediately and in doing so wreaks havoc on the die, housing and/or internal circuitry. This can lead to failure of not only the component but compromising the integrity of the board.

The Moisture Sensitivity Level – known in the electronics industry as simply “MSL” – helps identify how long a component can be exposed to 60-85% relative humidity at temperatures less than 86 degrees F before it becomes compromised for the reflow soldering process. The range starts out at MSL 1, known as ‘unlimited’ or unaffected and each incremental level represents a duration threshold.


Moisture Classification Chart from JEDEC J-STD-033B.1

The majority of parts affected by moisture are semiconductor based such as ICs, sensors and LEDs. But moisture sensitivity can be present with unexpected parts like nylon connector # 478-6169-1-ND. When in doubt, consult the part parameters or manufacturer datasheet.

The electronic industry has addressed this issue in JEDEC standard J-STD-033B by setting standards for handling, packaging, shipping and use of parts with moisture sensitivities. MSL-affected parts are packaged in moisture barrier bags and shipped with humidity indicators and appropriate MSL labels. The humidity indicator is a representation of the parts exposure and acts as a visual guide. Desiccant packs can help remove any excess moisture while sealed in the bag.

Desiccant # SCP648-ND , Moisture bag # SCP385-ND , humidity indicator # SCP444-ND , MSL Label # SCP333-ND

So what happens if parts absorb too much moisture and exceed their MSL ratings?

Parts that have exceeded their exposure limits are put into an oven at a low temperature to help draw out the built-up moisture.

Wait, I thought heat causes issues in these parts?

While the reflow machine delivers intense heat to melt solder, the oven slowly and gently draws that moisture out of the component to “reset” it.

The length of time the component must be in the oven is dependent on the thickness of the component itself, MSL and the baking temperature. It is not uncommon for parts to take days for the baking process.

In-depth baking guidelines and procedures are available in the JEDEC J-STD-033B file here: https://www.digikey.com/en/pdf/s/scs/scs-jedec-jstd033b-standards

How do I identify if MSL parts are compromised on my PCB?

Parts can visually display something called “the popcorn effect” in which the housing may have cracks or some type of visible imperfection after the reflow process. Sometimes the damage needs to be seen by microscope or X-ray to identify damage due to moisture build up.

I do my soldering by hand. Should I be concerned about MSL parts?

JEDEC standard states that component body temperature should not exceed 200 degrees C while reworking a board to minimize moisture damage. As an example, 63/37 solder melts at about ~185 degrees C.

If a MSL part is listed as 1, does that mean its waterproof?

No. You could think of the MSL as help identifying how sensitive components are to the humidity within a lab or warehouse; whereas waterproof or IP-rated products are for more exposed situations like outdoors.

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Excellent summary!

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