Do Vacuum Packed PCBs Need Baking?

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Do Vacuum Packed PCBs Need Baking?

Do we need to bake PCBs that are vacuum packed? Can additional baking cause other problems?
Board Talk

Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
Process Troubleshooting, Failure Analysis, Process Audits, Process Set-up
CEM Selection/Qualification, SMT Training/Seminars, Legal Disputes

Phil Zarrow
With over 35 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis. He has vast experience in SMT equipment, materials and processes.

Jim Hall
A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics. He is a pioneer in the science of reflow.

Transcript

Phil
Welcome to Board Talk. We're the Assembly Brothers, Jim Hall and Phil Zarrow of ITM Consulting, and we're here to answer your questions on SMT processes, equipment, materials, procedures, or anything else you have on your mind.

Our question today is from G.K. The question is, "I understand from some EMS industry sources that PCB baking is not necessary if the PCB is vacuum packed by the PCB supplier. Is this true or do we still need to consider baking for our process?"

Jim
This is a really interesting and pertinent question, isn't it? The concept of moisture sensitivity of printed circuit boards is really getting a lot of attention.

In fact, the SMTA, which has had a Moisture Sensitivity Device Council for several years is now focusing most of its attention not on components, as has been the tradition, but on printed circuit boards.

We also see that the IPC is proposing to establish some specs. They have a council, an IPC D-35 Printed Board Storage and Handling subcommittee and they are preparing a draft spec, IPC 1601 Guidance Document for Storage and Handling Issues for PWB's and Assemblies.

And from reading the notes here, you can go to the website and check it out, what they're moving to is exactly the same kind of handling procedures both for suppliers, in this case PCB fab suppliers, and for assemblers in how you have to handle this.

Phil
This is long overdue because what we see a lot in the field is a lot of unnecessary baking and other times when baking should take place and isn't.

Jim
Well, what happens if you don't bake the board? What would cause a person to bake a board?

Phil
It's going to be that moisture gets trapped in the board and that moisture may cause delamination.

Moisture can also dilute your solder and fluxes, and so you're gonna see incidents of bad soldering, whether it's could be as radical as dewetting solder balls.

Good old solder balls are everywhere. The other side of the coin is we always advise not to do any unnecessary baking if you don't have to.

Jim
Solderability issues.

Phil
Solderability issues, depending on what the finish is.

Jim
I mean you can bake in nitrogen but that's another expense and so forth.

Phil
Exactly, so this is good that finally we're being led away from the seat of pants approach on this.

Jim
Just to quote a couple of notes from this Moisture Sensitivity Council notes, they're talking that printed circuit boards should be treated as an MSL Level 4 classification and not left open to the floor for more than 72 hours. But most PWB suppliers guarantee a shelf life for six months for an unopened package.

So going back to the original question, if you keep them sealed in their vacuum bags, they're good as long as you don't expose them on the floor for more than 72 hours. Now that's a very general statement.

In the notes, they also talk that different laminates that are going to be more sensitive than newer high Tg laminates specifically developed for lead-free assemblies.

They're going to absorb moisture faster and they would get an MSL rating that would be higher. But again, this is just theory so it sounds like if you can guarantee that your boards aren't exposed more than 72 hours before their final reflow, you'll probably be in good shape.

Phil
So you want to keep tuned to the IPC D-35 Printed Circuit Board Storage and Handling subcommittee.

Jim
IPC 1601.

Phil
So that's it for this session. You've just wasted probably less than five minutes of your very precious time with us, the Assembly Brothers.

And thank you for listening to Board Talk, this has been Phil and Jim, the Assembly Brothers of ITM Consulting, and remember ...

Jim
Don't solder like my brother.

Phil
And don't solder like my brother, either.

Jim
Keep the kids away from the solder pot.

Comments

Moisture can indeed wreak havoc inside the PCB. Rapid expansion of escaping moisture can cause delamination, but it can also throw the Z axis expansion rate thru the roof; stressing microvia's and other delicate electrical connections. Some of these via's can crack, and "heal" themselves after reflow, only to come back as gremlins when the board is warmed up when the circuits become active. Intermittent failures are often blamed on components, and few people troubleshoot to this level. It takes a trained eye and microsection equipment to do it right.

Brad Fern, Entrust Datacard

Your presentation is very timely, as we are currently trying to figure out "what went wrong" with some recent board builds. My group builds only a few prototype boards per year, so we rely on the expertise of our bare board and assembly vendors to provide us with the latest processes and procedures when it comes to making boards.

I had never heard (from the bare board vendor nor the assembly vendor) that baking the bare boards was something to consider with "older" boards. We received our new bare boards in June of 2008 - they were not in sealed bags, and we stored them on a shelf in our lab. We had 2 boards assembled within about 5 weeks, and they both worked fine.

Then at about 8 months out, we had 4 more assembled, and three of them worked. At 11 months out, we had 4 more assembled and only 1 of them worked. These are RoSH boards, 16-layers, with almost 100% SMDs, including 3 1136-bump BGAs.

The same vendor did all the assembly, presumably using the same process each time. We DID have the components baked, but it never occurred to me that the PWBs should have been baked, nor did the vendor mention that fact (this was around May 2009). The assembly vendor has since x-ray'd the bad boards, and not found any soldering defects.

Does this sound like a PWB moisture problem to you? How long has this problem been known? Would the PWB moisture-induced problems be something that can be seen? Does delamination also cause bad electrical connections? Can moisture crack the barrels inside vias? Where can I get more info on the problems associated with moisture in PWBs?

Thanks Much!

Jim Shoots, Xerox Corporation

As mentioned earlier on this topic, a Humidity Indicator Card should be present in the packaging. I reiterate this point, because I have visited many PCB suppliers in the USA and find that storing desiccant bags in an open container is common. Sometimes it is "a day's worth" in a small cardboard box, other times it is the whole 5-gal pail with no lid. In certain climates, the desiccant is saturated before it goes in the bag with your PCB's.

JAH, Garmin International

There can be two issues in my opinion. First, moisture may be in the board before vacuum sealing and not enough desiccate. Second, we have seen issues with laminate that has not been completely cured. "Green laminate" so to speak.

When soldered or reflowed it degases out the drilled or punched holes, producing defects. This problem can be eliminated or reduced by baking. A potential risk of baking then would be degrading OSP coatings if that process is used.

Brad Frederick, Kem-tron Inc

I received boards in a vacuum seal with a desiccant pack but NOT with a humidity indicator card. Perhaps a corner was cut on the edge of the sealed package in transit.

Anyway, we assembled the boards within 24 hours and we had delamination issues. From now on, we bake everything. At 50% RH, I should have had 7 days. I assume my RH was much higher.

Jeff Lynn, Colt Tech

I have some questions: What is the correct vacuum pressure to seal a bag of a moisture sensible component? Is the level of vacuum related to the level of sensitivity of the components? Where I can find some specification on the matter?

Norberto Barrera, EPIC Technologies

If the boards received do not have a desiccate pack and a indicator card included to indicate its present level, I have always stayed on the safe side and baked out the boards prior to assembly. It is a small price to pay to insure that the boards do not delaminate or bubble. Same goes for Jedec components. If packaging is opened a new desiccate pack and indicator card should be put in the bag and it should then be resealed.

Jim Holton, Endicott Interconnect

You are correct and don't need to bake if the package is vacuum packed with the assumption that the board is dry coming from the PCB supplier. You may want to add this assumption to your statement.

Harold Kleinfeldt

When measuring TMA, producing the results can give "unstable" laminate. What are the main causes of this (epoxy cure?) and what affect does moisture have on these results?

Paul