What is the Proper Storage Condition For PCBs

Phil
Today's question is from a T.S.Y, we don't know where, but T.S.Y. asks, Is there a specification for proper conditions for the storage of PCBs?

Well the answer to your question is YES. Thank you for listening to Board Talk. We'll be back - actually, let's elaborate on this a bit, right Jim?

Jim
This is the second question we've had on this subject recently. The last one was on baking, but the answer is basically the same. The proper way to store your raw boards is in a moisture barrier bag that's vacuum packed and sealed. Otherwise, moisture can seep in, be absorbed by the circuit boards; and then when they get into the soldering operations, bad things can happen like delamination, cracking, and other things. So you don't want to expose your boards to moisture.

There aren't any real "hard" specs yet; although, the industry is realizing this. The IPC has formed a committee- D35 - to study it and is producing a draft document, spec IPC 1601, which is going to outline handling parameters. And what is being proposed or debated right now is that they're going to treat them just like moisture sensitive devices and components.

That means the PCBs will come sealed in a bag; and when you take them out of the bag, you will only have so much time to put them through all of the soldering operations. If you exceed that time, you have to bake them.

So I guess not having that spec cast for us yet, we could say, "Keep it in the bag. Keep them sealed. Don't take them out of the bag any longer then you have to." And if you know that you've got an open bag of boards, a partial bag left over after a build, and you need to - - you know you're not going to be able to assemble them for some time, either quickly reseal them into a moisture barrier bag just as you would MSD components or put them in a dry box.

Again, we don't have any hard numbers to compare that to. Some people have initially said, well I'm just going to treat a board like a Level 4 MSL, which would give you 72 hours out of the bag exposure time at nominal temperature and humidity conditions.

So it's a good point. The concern has been brought up, and I think the concern is because of the laminate materials for lead-free in the higher reflow and soldering temperatures. Lead-free is just making all the problems with delamination worse, so the moisture issue becomes more critical and now we're going to have to define it.

Jim
So keep it in the bag. And remember, don't solder like my brother.

Phil
And don't solder like my brother.

Jim
Keep those kids away from the flux pot.

Absolutely agree with Jim's recommendation. All of the boards we ship to customers are vacuum bagged with nitrogen and humidity indicators. Typically no more than 20 arrays per bag to help reduce the open bag issue after a production run.

I can't count how many times I've been contacted by a supplier who had delamination issues after reflow and wanted to RTV the assemblies thinking they got bad boards. All PCB's are hydroscopic, and people that process them need to get the skinny on how to handle boards if they intend to continue to build them. Investing in a vacuum sealer is mandatory, and they are not very expensive. The partially used bags are always that ones that delaminate...oh the irony.

Another thing to note is that immersion silver treated boards can tarnish if left exposed and subsequently they won't solder; so its double jeopardy with those boards.

The zipper will not be a 100% vacuum sealed. Also if you have foil lamination bag with zipper it Likely to fracture with constant zipper openings, but is a bag that is IPC complain. the aluminum metallizer polyester lamination bag is much flexible and resist much better but is not a IPC complain material.

This is a good topic as the proper moisture protection is not well understood even by some component manufacturers. One point to consider is that J-STD-033 recommends that Full Evacuation (vacuum) not be used as it prohibits the air inside the bag from circulating to the desiccant and thus limiting its effectiveness. "Light Air Evacuation" is recommended. If you can plainly see the bag sucked up against the parts (boards) inside, it is too much.

It is nice to know that IPC has constituted a committee and planning to bring out a standard. This is very much essential (both for Bare and Assembled PCBs. Yes - I agree that lot of inputs from experts are available and a vacuum sealed bag is also an answer. There are many other things which already have been indicated by professionals in the field which are valuable.

We require all our board vendors to bake out then seal all boards in Real moisture barrier bags, with desiccant and indicator cards. This works most of the time. Vendors in the states are really good at it, but China can be a problem. Adds a little to the cost, but this saves us a lot of hassle. Of course, you want to use them within a couple of months. Don't store them for two years and think they will be OK.

IPC-1601 ("...not needed/recommended...") and J-STD-033 ("...SHALL NOT...") clauses re: vacuuming/full evacuation before sealing. Clauses commonly violated by both board and part suppliers in my experience.

Ziplock-style MBBs are not mentioned in either document as being acceptable. For that matter, nor are they explicitly prohibited. However, a heat seal- the only method mentioned- is easily inspectable while a zip-seal is not. Also, the WVTR is determined by the MBB material and construction. I highly doubt a ziplock seal attains the same rating as the bag material itself.

So this begs the question of overspecification. Are there going to be differing MSLs for boards reflowed that are SnPb based vs SAC305 vs whatever?

This comes down to process engineering doing their job. Differing needs dictate differing processes which shouldn't be locked up in a single specification. One size does not fit all.

Specifications, unless really needed do little but drive added cost. I would encourage IPC to make this a HANDBOOK, GUIDELINE or a white paper rather than a specification.

You could do some people a big favor by alerting them that using black, static bags to store main boards containing button cells will discharge them since the the black plastic, being carbon loaded, is a resistor. We got a bunch of main boards with no memory retention due to this.

Are "Zip-lock" Moisture Barrier Bags a good alternative to store MSD components?

Regarding bare PCB care. What about PCBs that are completely done just waiting to be shiped but delayed. What sort of care do I need to provide to protect the units?