|Ask the Experts|
August 26, 2020
Moisture Sensitivity Level for Bare Boards
I am trying to ease our way through a NADCAP audit and I am having some trouble with MSL, specifically with regard to bare PCBs. All of the documentation I have been able to find, as well as IPC-1601, state that bare boards must be considered a MSD. But what MSL level is assigned to bare boards?
The larger question is determining what level of moisture protection we should flow-down in our requirements for our suppliers and implement in our plant. I imagine that the number of layers in the board may play a part in the MSL determination, as well as the board materials but does anyone have any idea what the general bare board MSL would be?
|Expert Panel Responses|
The moisture sensitivity of printed board design will depend on the resin system. Whether anyone has assigned an MSL number to "resin systems" I cannot say.
I would suggest that the most common MSL value assigned to components that have not be assessed for MSL is level 3.
None of this will matter much if you keep your boards in the sealed packaging (MMB) from the manufacture (for no more than 1 year), or in a dry atmosphere cabinet at 5%RH or less, after you break the seal, per J-STD-033. Then, once out of the sealed factory sealed bag or out of the dry atmosphere cabinet, get them through solder ASAP, that same day, or back into the dry atmosphere cabinet.
Senior Project Engineer
Electronic Controls Design Inc
You are correct that the MSL of the bare board can vary; the factors affecting moisture sensitivity, roughly in order of importance, are:
To make matters worse, damage can occur that affects the long-term reliability of the assembly that is not easily detected with post-assembly testing. Because moisture sensitivity depends on several factors and can vary so widely, it's not possible to give generic guidance on assigning MSL levels that has any real value.
The best guidance one can give is to be conservative and specify that all raw PWBs are handled and stored in such a way as to ensure that they are dry coming to the assembly line, and that there are not unusual delays between processes.
Your PCB supplier will be able to provide you with this information. Unfortunately, these requirements will vary depending on the materials used in construction. The RoHS compliant materials now in common use are way more moisture sensitive than some of the older materials that were used when tin\lead production dominated the industry.
One easy way to avoid tracking PCB is to simply build a PCB baking step into your production process. Again, your PCB supplier will be able to give you recommended temps and times.
Supply Chain Engineer
We do not include PCB's in the MSL program. JSTD-033 is specific to surface mount devices. We do have a separate board handling procedure that includes similar baking requirements to the JSTD-033, but is instead based on the IPC-1601. No MSL is required. This was accepted during the NADCAP certification process
Director of Corporate Quality Assurance
Delta Group Electronics Inc.
I have not seen a MSL value assigned to bare boards. However, there is some relationship between the surface finish of the PCB and the amount of moisture allowed.
My recommendation is to maintain all PCB's in their package until the time for build. If there is any leftover PCB's, place them in a bag with moisture indicator & desiccant and seal the bag. Attach a label to track the times the bag has been opened, just in case you need to bake the PCB's.
Senior Manufacturing Engineer
The JEDEC industry standard (J-STD-033) for electronic components requires they must also be stored and handled correctly. Specifically, attention should to moisture sensitivity level (MSL) ratings provided by the component manufacturer. There are eight levels of MSL (1, 2, 2a, 3, 4, 5, 5a and 6), and each denotes how long a component can be left out of its original packaging prior to reflow.
Your question on how to determine what level of moisture protection you should flow-down in your requirements for your PCB suppliers and implement in your plant is slightly more complicated. The IPC-1601 provides suggestions for proper handling, packaging materials and methods, environmental conditions, and storage for printed boards.
These guidelines are intended to protect printed boards from contamination, physical damage, solderability degradation, electrostatic discharge (ESD) (when necessary), and moisture uptake. IPC-1601 Section 5 "Printed Board Receiving, Storage and Assembly" reflects language addressing Humidity Indicator Card(HIC) and Moisture Barrier Bag (MBB) for the time of exposure to ambient conditions that should be controlled to minimize moisture uptake.
If bags are opened under factory ambient conditions (not exceeding 30 degrees C/60% RH) and a procedure to follow if they are. Also, the storage location stock room dry packaging required and a twelve month shelf life expectation if stored at less than 40 degrees and 90%RH ( IPC/JEDEC J-STD-033). The Maximum Acceptable Moisture Content(MAMC) states the maximum acceptable moisture content will be between 0.1% and 0.5% of moisture weight to resin weight.
To allow for the inevitable ambient exposure during normal assembly and processing at the user, the printed board fabricator should ensure that moisture content is below the MAMC before placing the printed board into protective packaging. Baking considerations if required should be below the MAMC for safe reflow considerations. As for a specific MSL rating for PCB board type I have no answer unfortunately.
VP of Advanced Technical Operations
FR4 with a Tg iY 170 degrees C PCB's are considered to be sensitive to humidity, such PCB's should be treated as Moisture Sensitive Devices. Should be stored in dry place with desiccant and humidity indicator card (similar as MSL components packing, max 30%RH). With this, a transparent dry pack bags should be used to control and limit humidity within the transport and storage period.
Usually the bare boards can be packed in pink wrap with desiccant to protect them against moisture. Based on the number of layers and materials used, they can be a MSL of 3 or higher.
You can use the MSL3 as your default if not indicated differently by the supplier.
It is recommended though to have your supply chain request this from the suppliers and they will come to your plant in the appropriate packaging.
Engineering and Operations Management
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