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Moisture Sensitivity Levels (MSL) in PCB packaging
The Moisture Sensitivity Level of a PCB is an important aspect of the packaging process that can affect the quality of your assemblies, the reliability of components, and the overall productivity of your operations.
The improper handling of increasingly miniaturized and moisture sensitive surface-mount devices has led to several defects associated with the solder reflow process, including cracks, delamination and what is commonly referred to as the ‘popcorn’ effect.
By understanding how moisture sensitivity levels are classified and by establishing methods for labelling and controlling moisture exposure, one can significantly lower failure rates and better manage costs while improving compliance with the quality expectations of the customers and industry.
This article will be a comprehensive resource for MSL PCB packaging and will include MSL classifications, Industry Standards, Labeling Methods, and Best Practices to assure that your electronics manufacturing is done safely and reliably.
What Is Moisture Sensitivity Level (MSL) in PCB Packaging?
The Moisture Sensitivity Level (MSL) indicates how susceptible an Electronic Component is to absorbing moisture into the PCB Packaging when it’s stored or handled in the field. Most Surface Mount Devices (SMDs) have a type of packaging called plastic encapsulated, which will absorb moisture out of the surrounding air for many months. The MSL rating is the maximum amount of time (floor life) that a component can be exposed to room temperature and humidity conditions prior to additional drying methods, i.e., dry storage or oven bake being required.
When moisture absorbed into an SMD is heated to high temperatures during solder reflow, the moisture inside the package rapidly expands upon evaporation, creating internal stresses within the package. These internal stresses can cause cracks, delamination, or damage to the bonds between layers of the package, commonly known as the “popcorn” effect. The proper MSL management within PCB packaging is critical to avoid failures and ensure reliable assemblies.
Why MSL Is Important in PCB Packaging
The Moisture Sensitivity Level (MSL) is an important parameter in controlling the risk of moisture related issues at the time of PCB assembly per IPC/JEDEC guidelines. Components can incur internal structural damage due to solder reflow temperature when exposed beyond the allowed moisture exposure limits; however, there will typically be no visible signs of failure. Controlling the MSL properly will ensure the components are assembled according to the qualified environmental and time requirements outlined by all relevant industry standards.
As defined in the IPC/JEDEC handling standards, proper moisture sensitivity level control is needed to prevent:
1) Damage to the internal structural integrity of component packages resulting from moisture-induced delamination, internal cracking, and bonding degradation caused by reflow.
2) Integrity and reliability of the components through manufacturing and operational phases.
3) Qualified floor life and compliance with ambient conditions.
4) Latent failures which cannot be visually inspected or electrically tested.
5) Reduced rework, scrap and baking cycle costs, improved production efficiency and lower total cost of ownership.
6) Consistent solderability through thermal cycling of the packages used in the solder process.
Moisture Sensitivity Level Classifications
The Moisture Sensitivity Levels (MSL) are standardized classification systems developed by IPC/JEDEC to define an electronic component‘s maximum safe exposure to ambient conditions prior to solder reflow. The classification levels are based on “floor life”, which is measured under controlled environmental conditions of 30°C and 60% relative humidity. Lower MSL numbers represent less susceptibility to absorbing moisture and therefore, no specific moisture protection requirements, while higher MSL numbers necessitate careful moisture management, tighter limits to exposure times and strict adherence to baking prior to PCB assembly and soldering.
Below are common MSL classifications and descriptions:
- MSL 1 – Unlimited Floor Life
Components designated as MSL 1 are non-moisture sensitive and may be stored and handled at ambient temperatures and humidity without specialized moisture protection. - MSL 2 – Extended Floor Life
Components that meet this MSL designation can tolerate some degree of moisture exposure and generally provide a floor life of one year at typical storage conditions prior to the implementation of moisture control practices. - MSL 2a – Moderate Moisture Sensitivity
This is similar to the MSL 2 designation except that it allows a shorter exposure period than MSL 2 and requires more frequent monitoring during storage and assembly. - MSL 3 – Limited Floor Life
MSL 3 components have a substantially reduced floor life of approximately 168 hours (7 days) after which baking or dry storage will be required prior to reflow. - MSL 4 – Very Limited Floor Life
Components meeting the MSL 4 designation must be assembled from their original package within 72 hours of exposure. Due to extreme sensitivity to moisture, strict humidity controls must be applied to avoid damaging the components due to moisture absorption. - MSL 5 – Highly Moisture Sensitive
Components with an MSL designation of 5 have a floor life of approximately 24-48 hours and must be immediately assembled or subjected to controlled storage conditions once opened. - MSL 5a – Extremely Moisture Sensitive
Components with an MSL 5a designation have a floor life of 24 hours or less and must maintain continuous moisture protection throughout all stages of handling. - MSL 6 – Must Be Baked Prior To Assembly
Components with an MSL designation of 6 must be baked prior to assembly regardless of exposure time and cannot be stored at ambient conditions.
The MSL designations outlined above are intended to serve as the primary guide in selecting packaging methodologies, storage conditions, exposure tracking, and recovery procedures related to MSL PCB packaging.
Industry Standards for MSL
Control over the MSL of packaged PCBs is primarily driven by international standards developed within the electronics manufacturing community; these standards ensure consistent classification, safe processing and reliable solder performance among different manufacturers.
The standards collectively define how moisture sensitive components should be classified, tested, labeled, and stored during the assembly process, which forms the basis for managing moisture sensitivity levels in electronic manufacturing.
These major industry standards controlling MSL PCB Packaging are:
- IPC/JEDEC J-STD-020 – Moisture/Reflow Sensitivity Classification
Establishes and classifies moisture sensitivity levels and defines the reflow temperature profiles used to assess a component’s ability to withstand damage caused by moisture. Establishes test procedures to determine how a component responds to solder reflow after controlled moisture exposure. - IPC/JEDEC J-STD-033 – Handling, Packing, Shipping and Use
Provides specific details on the requirements for maintaining moisture control after a component has been classified with respect to its MSL. Includes details on moisture barrier bags, desiccant usage, humidity indicator cards, floor life tracking, baking procedures, and storage conditions for moisture-sensitive components. - JEDEC JESD22-A113 – Preconditioning of Plastic Surface-Mount Devices
Defines preconditioning techniques to be used in the qualification testing to simulate real-world moisture exposure before evaluating reliability. - IPC-A-610 – Acceptability of Electronic Assemblies
It is not an MSL classification standard; however, it does reference moisture related defects such as delamination and package cracking as part of the assembly inspection criteria. - IPC-7351 – Generic Requirements for Surface Mount Design and Land Patterns
Supports MSL Control by addressing design considerations for packages that can affect their moisture behavior and the reliability of the solder joints.
Thus, these industry standards collectively form the basis for establishing a comprehensive framework for defining and managing moisture sensitivity levels and controlling moisture in the PCB packaging and assembly processes.
Common Component Packages and Their MSL Ratings
The degree of sensitivity to moisture varies based upon the type of packaging used for a specific semiconductor device, along with the type of material(s) used to make the packaging and how thick the packaging is, and its internal construction. Packages that utilize ceramic or hermetically sealed enclosures (such as those used for ceramic and hermetically sealed packages) typically have an MSL rating of 1. These types of packages are resistant to absorbing moisture, thus making them suitable for use at lower MSL ratings. On the other hand, plastic encapsulated surface mount packaged components (such as SOICs and QFPs) will generally be rated between MSL 2 and MSL 3, depending on the physical dimensions of the packaged component and the composition of the moulding compound used to encapsulate the component.
Packages including QFN, BGA, CSP, and fine pitch thin packages are often rated at higher MSL ratings (typically between MSL 3 and MSL 5A). This is primarily because there are fewer pathways available for moisture diffusion within these packages, and therefore, less stress occurs internally during the soldering process (reflow), which results in a higher resistance to moisture absorption. By understanding the typical MSL ratings associated with different types of packaging, engineers can develop adequate practices and controls to implement proper storage, handling and assembly techniques when using MSL PCB packaging.
MSL Labelling and How to Read It
| Details about Label | Method of Reading | Uses |
| MSL Rating | Checks the sensitivity of moisture rating written on the label | Helps to know the floor life and handling rules |
| Bag Seal Date | Verify the Sealing Date on the Moisture Barrier Bag | Confirm the beginning of the storage life |
| Floor Life Statement | Review the allowable time for exposure to air after the bag has been opened | Keep track of the time that bag is exposed to air |
| Humidity Indicator Card (HIC) | Keep Track of change in color of humidity dots | If the limits have been exceeded, baking will be necessary |
| Bake Instructions | If exposure limits have been violated, review the temperature and duration guidelines | Apply the baking process only when exposure limits have been violated |
| Storage Condition Notes | Review the Dry Storage and/or Controlled Environment Instructions | Prevent additional absorption of moisture in the product. |
Best Practices for Different MSL Levels
In order to control moisture effectively in the MSL PCB packaging process, a variety of best practices for handling parts must be applied based on the part’s MSL level. The best way to protect against moisture-related failure and ensure assembly reliability is by following established handling procedures based on a part’s MSL level.
MSL Level Handling Procedures:
1) Handle all parts according to their respective MSL levels.
2) Allow Low-MSL Components Longer Exposure Times:
Low-MSL components can be exposed for a longer period to ambient conditions; however, they should always be stored in a dry environment with minimal contamination.
3) Apply Strict Controls for High-MSL Components:
All High-MSL components must be placed in a sealed Moisture Barrier Bag (MBB), contain desiccant material, and include a Humidity Indicator Card (HIC) after an MBB has been opened.
4) Track Floor Life Accurately:
Once packaging is open, track the cumulative exposure time to avoid exceeding the maximum allowed exposure time.
5) Utilize Controlled Storage Environments:
Dry Cabinets or Humidity-Controlled Storage allow for either pausing or extending the floor life of components.
6) Perform Baking When Necessary:
Components may need to be baked at an approved temperature prior to soldering if the floor life has expired or if excessive moisture exposure has occurred.
7) Minimize Repeated Exposure:
Avoid repeated exposure of components to air, as this will unnecessarily reduce the shelf life of the components, or expose them to moisture.
Conclusion
A manufacturer’s ability to manage moisture sensitivity levels (MSL) of components is an essential aspect of assuring that PCBs are packaged and assembled reliably.
Preventing popcorn defects from occurring as well as protecting against potential long-term electrical failure, is possible through the effective management of MSL.
Manufacturers can greatly reduce failures related to moisture by following established standards for MSL classification, accurately identifying and understanding MSL labeling, and employing optimal component handling procedures.
In addition to the increasing miniaturization and increased complexity of electronics, manufacturers will be required to effectively maintain MSL requirements as a key aspect of their modern PCB production.
Build Reliable PCBs with Proper MSL Control ; Partner with FS PCBA.
FAQs
Yes. Moisture related problems could exist internally and will not be detectable by a visual inspection of the product. They can also be the source of reliability problems that are caused by the product’s failure after it has been deployed.
Yes, excessive or improper baking can deteriorate the packaging material and solderability, so baking should only be done as required and to the standard limits.
Yes. All components, whether they have an immediate assembly process or a storage process, can absorb moisture while in storage; therefore, all components require MSL controls at every point of the supply chain.
Reference
https://www.electronics.org/system/files/technical_resource/E8%26S12_01.pdf
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