Moisture Sensitivity Level (MSL)

Moisture Sensitivity Level (MSL) is a standard defined by IPC/JEDEC J-STD-020 that indicates how susceptible a moisture/reflow-sensitive electronic component is to moisture-induced stress during storage, handling, and the PCB assembly process. The MSL rating system helps electronics manufacturers determine proper handling, packing, and use of moisture-sensitive devices (MSDs) to avoid damage caused by absorbed moisture during reflow soldering.

MSL ratings range from 1 to 6, with MSL 1 being the least moisture-sensitive and MSL 6 being the most sensitive. Components with higher MSL ratings require more careful handling and storage to prevent moisture absorption, which can lead to package cracking, delamination, and other reliability issues during reflow soldering.

The MSL of a component is determined by the component manufacturer through standardized testing. They expose the component to varying levels of temperature and humidity, then measure the amount of moisture absorbed. The component is then classified according to the IPC/JEDEC J-STD-020 standard.

Overview of MSL Levels

Here's a brief overview of each MSL rating:

• MSL 1: Unlimited floor life at ≤30°C/85% RH. No dry packing required.
• MSL 2: One year floor life at ≤30°C/60% RH. Dry packing required if exceeding this.
• MSL 2a: 4 weeks floor life at ≤30°C/60% RH. Dry packing required if exceeding this.
• MSL 3: 168 hours floor life at ≤30°C/60% RH. Dry packing required.
• MSL 4: 72 hours floor life at ≤30°C/60% RH. Dry packing required.
• MSL 5: 48 hours floor life at ≤30°C/60% RH. Dry packing required.
• MSL 5a: 24 hours floor life at ≤30°C/60% RH. Dry packing required.
• MSL 6: Mandatory bake before use. Dry packing required.

Floor life refers to the allowable time a component can be exposed to ambient conditions (≤30°C/60% RH) after being removed from its moisture barrier bag (MBB).

To manage moisture-sensitive components, electronics manufacturers must:

1. Ensure MSL-sensitive components are dry packed by the supplier in a sealed moisture barrier bag (MBB) with desiccant and a humidity indicator card (HIC).
2. Check the HIC upon receiving the components. If it indicates high humidity, bake components at the recommended temperature and time to remove excess moisture.
3. Store components in a low-humidity environment (≤5% RH). If floor life is exceeded, bake components before assembly.
4. Assemble PCBs containing MSDs promptly. Unused components should be resealed in the MBB.
5. If baking is required, follow the component manufacturer's recommendations for baking temperature and duration to avoid component damage.

By understanding and properly managing moisture sensitivity levels, electronics manufacturers can prevent moisture-related defects, improve product reliability, and reduce rework and scrap costs.

Soak/Bake Requirements for Each MSL Level

The IPC/JEDEC J-STD-020 standard specifies the following soak and bake requirements for each Moisture Sensitivity Level (MSL):

MSL 1

• Soak: 168 hours at 85°C/85% RH
• Bake: Not required

MSL 2

• Soak: 168 hours at 85°C/60% RH
• Bake: Not required if floor life is not exceeded

MSL 2a

• Soak: 696 hours at 30°C/60% RH or 120 hours at 60°C/60% RH
• Bake: Not required if floor life is not exceeded

MSL 3

• Soak: 192 hours at 30°C/60% RH or 40 hours at 60°C/60% RH
• Bake: Required if floor life is exceeded

MSL 4

• Soak: 96 hours at 30°C/60% RH or 20 hours at 60°C/60% RH
• Bake: Required if floor life is exceeded

MSL 5

• Soak: 72 hours at 30°C/60% RH or 15 hours at 60°C/60% RH
• Bake: Required if floor life is exceeded

MSL 5a

• Soak: 48 hours at 30°C/60% RH or 10 hours at 60°C/60% RH
• Bake: Required if floor life is exceeded

MSL 6

• Soak: Time on Label (TOL) at 30°C/60% RH
• Bake: Mandatory before use

The soak requirements are intended to simulate the worst-case moisture exposure a component may experience during storage and handling. The bake requirements are used to remove excess moisture from components that have exceeded their floor life or have been exposed to high humidity conditions.

Baking is typically performed at 125°C for 24 hours, but the exact temperature and duration may vary depending on the component manufacturer's recommendations. It is essential to follow these recommendations to avoid damaging the components during the baking process. Baking at 125°C removes excess moisture from the component packaging and molding compound, restoring it to a safe condition for reflow soldering. The bake time may be adjusted based on component characteristics like package density, provided correlation to the standard bake time is established.

By adhering to the appropriate soak and bake requirements for each MSL, electronics manufacturers can ensure that moisture-sensitive components are properly handled and processed, minimizing the risk of moisture-induced damage during PCB assembly.