Removing Warpage from PCBAs

Phil
And welcome to Board Talk with Jim Hall and Phil Zarrow of ITM Consulting, also known as the Assembly Brothers. Place and pick. In our never-ending quest to help people solve their assembly processes, situations and hopefully not to confuse them.

Today we have a question from E.J. E.J. says from a large lot, we have 20 populated boards with components on both sides that are warped. Components are varied height. What methods can we use to flatten them?

Jim
Syntax issues here, Phil. Is he saying the components are warped or the boards are warped? If it was components, it would probably have to big things like BGAs.

Frankly they would each have to be dealt with individually and I don't know any way to flatten components that have been warped. I suspect that for some reason 20 of the PCBs during the double- sided assembly process, the boards have become warped.

The first question it begs, as we put on our FA hats, is 20 populated boards in the process, what changed? What happened here? Was it the way the boards were stored or handled? Was it your reflow oven? Did something change there? Somebody lacking on support? That is one thing.

I don't know if there is a way with some fixturing that we can iron out these boards.

Jim
Going back to the causes, typically you think about warpage coming from inadequate board support, improper reflow profiles causing warpage during the process. But 20 boards, the thought I had Phil was one package of boards that wasn't packaged properly and came in with warpage, the bare PCB coming in.

I'm thinking 20, maybe that is the size of a package of boards that are coming in from your vendor, from your PCB fab. Maybe one package was damaged, got moisture in it or something else so that you got 20 raw boards coming in that either had warpage or had some variation in them such as moisture absorption which caused them then to warp during the assembly process.

Phil
Yeah. There is a lot of things we don't know. We aren't even sure if he is talking about boards or components that are warped either. But running on that assumption.

Jim
You have to accept the fact that it may be impossible to flatten out. If 20 of these PCBs are warped, it may be impossible to flatten them out and still maintain the reliability of your product.

To straighten them out, as Phil said, you can envision some sort of fixture much like a board support fixture that you would use to prevent warpage with pins that would contact the board in between, the components that are mounted on both sides, with fixtures that could put pressure on them to flatten out the board and leave them for some time, hoping that would flatten out or reduce the warpage. That may be possible.

In any case, if you try to do that you have to be conscious that you have solidified solder joints in that warped configuration. It may be the reality that if you try to straighten you are going to crack some of those solder joints.

It may not be possible to salvage these boards if you require a certain flatness in the final assembly.

Phil
I wouldn't be surprised if some of our listeners chime in with some methodologies, homeopathic or otherwise that they have used to de-warp boards. It will be interesting to see on this one. It is a tough one. I just wish we had more information.

Jim
That is a great lead in, Phil. All of you faithful followers of Board Talk, when we do this initially and these are presented on Circuitnet it is just us, our ideas good or bad. But over time people send in comments. We have a lot of very conscientious listeners who share their experiences.

This is a good case where someone may have had this problem, might have come up with a really good solution that doesn't come to our minds right at the moment.

E.J., or other people with similar problems, wait a week or a couple weeks and go back online and pull up this session on the archives and see if anyone has posted a suggestion.

There have been in the past many good suggestions. Corrections, for mistakes we made and so forth by our listeners. We thank you all for your active participation.

Phil
And on that note, you have been listening to Board Talk with Phil and Jim, the Assembly Brothers. As you go forth, no matter how warped you might be, please don't solder like my warped brother.

Jim
And don't solder like my brother.

Put warpage board in dry cabinet or in baking oven or burning chamber for 3 to 4 hrs on 45°c.

Compufab Inc. has been providing PCB Stiffeners to electronic companies world wide for over 40 years. we are very aware of the PCB warping issues. You must install a mechanical stiffener on the PCB prior to soldering. The board will stay flat, no warp. As your other respondents stated, you can not unwarp a warped PCB.

You will break solder joints and injure the components. It does not matter if it is a large PCB or a panel of tiny PCBS, mechanical stiffening is required to prevent warping from the wave soldering or the IR reflow. The initial PCB design requires the stiffener to be a part of the product, not an after thought. Sorry guys, but that is the way it is.

I agree with the comments from Edward and Pongtip. Uneven balancing of top and bottom Cu layers are known to give warpage. You must specify the max allowed warpage in the PCB manufacturing specification, so if the PCB manufacturer deliver warped boards you must reject them. Assembly on a already warped board will cause severe reduction of board reliability. If boards are flat before but warped after assembly and soldering, then you are in big trouble.

Don't even think about forcing the boards flat no matter how. If you anyway do it there is a high risk for PTH, MicroVia and/or solder joint cracking. The cracks may initially be so small that the boards can pass a final electrical test, but after a while in field the cracks will grow and create malfunction - temporary or permanent. Face the facts that you can't do anything to fix warpage problems on already manufactured PCBs or PCBAs unless you don't care about device reliability.

My best advice is to dispose the PCBAs, check you PCB layout and PCB Manufacturing specification and consider if you are using a sufficiently capable PCB manufacturer.

Warp correction after the fact faces a fundimental limit. The components and PCB are under residual stress. There is really only one way to straighten the boards and have comparatively stress free components. you must reflow the boards to the point of solder melt, then as the boards exit reflow, have a method to hold the boards flat as the solder hardens and then maintain flatness until the boards are cool. any method that does not reach melt, will result in additional stress and totally unreliable boards which should be scrapped.

If you clarify your requirements on the drawing, and if you design the board to allow a tolerance for acceptability (example: 90% Acceptability). Warping is going to happen, considering you are working with various layers with different thermal properties. As we go smaller and smaller with our technology, so do our materials. These two technologies are not always parallel in terms of advancement so of course we may have to work with less-than-ideal materials. Just because we have micro components, doesn’t mean our board materials are ideal for the smaller working parameters. Re-working should be not an inline process, only for MRB/NCR purposes.

There are many factors can make PCBA warpage. The provided info is not enough e.g. laminate type, Tg, copper density across PCB is uniform? Thick or Thin PCB? e.g. Laminate with less Tg or with not uniform Cu across the Fab -> can have warpage easily, Plus if have component with some dense area than the rest can be one factor, or some dense solder joints/leads in one area more than the rest etc. Also at what process the bd warp -> SMT or Wave Soldering or Reworking? These 20 bds passed any additional process than the rest?

***If at SMT reflow -> do not use any fixture to constraint the PCBA during reflow -> it will create another defects if the bd cannot freely expand in the reflow oven. It will not have any total solutions if we don't know what mentioned above.

***If the PCBA already warp -> do not use any fixture to bend them -> it will crack solder joint.

As the Assembly Brothers mentioned, there is no info given as to the size of the board, the extent of the warpage, or its location on the board. However, that being said, you may consider adding a stiffener. Provided there is sufficient clearance to do so, you would need to drill holes through the board in order to anchor the stiffener, which may pose a problem, and you may also need to use insulating washers or stand-offs, but with proper care and planning, this may be a viable solution to your problem.