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Component Moisture Question?



Component Moisture Question?
We mistakenly assembled circuit boards using BGA components that were not properly stored. Can we now rebake the assemblies?
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
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
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
And welcome to Board Talk with Jim Hall and Phil Zarrow, of ITM Consulting, although we're affectionately known as The Assembly Brothers in this venue.

Today we're coming to you from the ITM Elegante Ball Room high atop Mt. Rialto and we're here to talk about electronic assembly.

Materials, equipment, components, practices, procedures, and whatever else comes to mind.

I believe today Jim, we have a component procedure related question.

Jim
Yes this is from M.M. We mistakenly assembled circuit boards using BGA components that were not properly stored.

The BGA components had moisture tags that show indication of moisture exposure. All the assemblies passed testing.

Can we now rebake the assemblies to ensure that they will not have moisture related failures on these devices, or should we remove and replace the suspect BGA components?

Phil
I think this is a case of closing the barn door after the horse is out, or the BGA is out. Am I correct?

Jim
Absolutely Phil. This is a classic example of MSD potential damage.

A part was not stored properly. It was exposed to moisture. We didn't realize it. We reflowed it. The answer to your first question is no.

Rebaking at this point will do nothing to help that part. The damage occurs during reflow if there's too much moisture in there it can crack, delaminate and all the other bad things we know that can happen to the internal structure of moisture sensitive components. So no.

The question is, you say all the assemblies passed testing so there's no dead in the water failures. Was there any damage inside the parts? The best way to look for the most common types of moisture sensitive damage, which is cracking and delamination, is using scanning acoustic microscopy.

Better known as C-SAM. It's a non-destructive test. If you have it available, it's not real expensive or time consuming and again you can find it if you do the parts that are suspect you can see a lot of the typical kinds of damage.

You may want to x-ray them to look for potential cracks in wire bonds that may not have been caught during your quality testing for be it in-circuit or functional.

But, otherwise yeah, pull off all the parts and replace them. Good luck and you've learned, as my brother said, you've got to follow these procedures.

Phil
I just want to add that a lot of people feel that if they didn't get corner lifting on the corner bonds of the component that they're free and clear. We want to point out that the popcorn effect doesn't always occur.

That sometimes you don't get enough belly bulge during the reflow expansion to actually lift the corner leads. So just because you haven't lifted the corner leads doesn't mean those symptoms my brother Jim has mentioned aren't there.

Jim
Yes, the scariest thing about moisture sensitivity damage is that we have no idea how big the problem is.

Because if as Phil just illuminated, if you don't get this catastrophic pop-corning during reflow that lifts the corner, that cracks the packages, or we've seen actually parts being blown right off the surface of the circuit board, you can get situations where they look okay, they test okay but you've got some latent damage inside that has reduced the reliability of the product and you get failure in start-up, burn-in, early infant mortality, or worst of all intermittent.

Because you've got some delamination that's not a failure but the part overheats or you get some connections that aren't reliable and you get intermittence.

Typically if they've gotten out of the factory very few people die and fault analyze to the internal component level. So think about all the intermittent problems that are out in the field. Some percentage of those are probably due to moisture sensitivity damage during reflow in assembly.

But we have no idea how much or how big the problem is. So keep those moisture sensitive parts in their moisture barrier bags or in their dry cabinets.  Keep track of them and don't put them on a board if you're not sure.

Bake the parts before you reflow them. Baking them after reflow does no good whatsoever.

Phil
You have been listening to Board Talk and whatever you do ...

Jim
Don't solder like my brother.

Phil
Don't solder like my brother either.


Comments

Moisture sensitive devices are a class of their own. The manufacturer stipulated instructions on storage and handling moisture devices have to be meticulously followed. If the moisture ingress is very less and is at a benign site in the package these can escape on baking without affecting the device function and properties. However, if a large quantity of moisture has crept in, it can lead to de-laminations at the interface of die, the lead frame, bond wires, heat sink paddle, the in-built PCB of the BGA with the moulding compound.

Although, the said BGA devices have already undergone Reflow and is reported functioning properly does not imply that the device will work forever. The damages induced can pop up at a later stage.

Hence, I strongly recommend to remove the BGAs after baking the PCB and go for replacing the BGAs which are stored and handled as per manufacturer instructions. Prior to replacing the BGAs, one has to bake the PCB. If the devices have crossed the floor life prescribed by the manufacturer, the devices shall be subjected to baking. Bake out regimes stipulated by IPC-TM-650 shall be followed.
Dr. KUTTIYIL THOMAS OOMMEN THARAKAN, VSSC
I would agree last option R&R, avoid a BGA pad repair. A nice long reflow soak should drive off moisture. I would still consider baking the CCA if it was to be wave soldered (preheat much shorter, shocker).
Rob, Amherst
BGA's can be usually checked using even 2D x-ray (3D oblique and CT x-ray are superior methods, however. You are looking for "voids" if the number of voids is deemed acceptable, the BGA's are likely fine.
Bill Peterson, Hirose Electric
I can only add that I would discuss it with the customer (whether it is your external or internal one). Tell them what happened and try to come up with a reasonable solution (decision) together.
Mark Kostinovsky, Schlumberger

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