Fixing / flattening warped PCB's? - Page 1
Forum section: electronics / beginners

author: [Chrille]
author_rank: Contributor
timestamp: April 13, 2021, 06:39:38 pm
content: I recently bought a brand new PCB (quite large) and it turns out that it is slightly warped and I would like to fix it before I solder any components to it.Is there a good and reliable way to straighten PCB's and make them completely flat without any risk of damage to them? I Googled a bit and it seems that you might be able to straighten them by clamping them to something flat and put them in an oven with a temperature of ~125 degrees Celcius and leave it in there for an hour and then take it out of the oven and let it cool in room temperature. Is this a good method? Anyone here tried this? Perhaps there are other ways to do it?All suggestions and experiences welcome!

author: Benta
author_rank: Super Contributor
timestamp: April 13, 2021, 09:23:55 pm
content: I can't give you much hope.FR-4 is very form stable, being epoxy/glass based. Heating it will not bring anything until you reach a temperature (~300 C) that will start breaking the polymer chains, and that will leave you with a weakened PCB.I suggest that you either send it back, or live with it and do the soldering in a fixture, and afterwards firmly fix it mechanically into your chassis. There'll always be a bit of stress, but it should be negligible.First time I've ever heard of this issue. I hope the PCB was cheap.

author: wraper
author_rank: Unknown
timestamp: April 13, 2021, 09:28:46 pm
content: Quote from: Benta on April 13, 2021, 09:23:55 pmFR-4 has much lower Tg (glass transition temperature), generally in 130-170oC range. At temperature above that FR-4 is quite soft and will straighten by itself when lying on a flat surface. If you are going to reflow it in an oven, it will take a shape of underlaying surface without prior straightening.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 13, 2021, 09:33:38 pm
content: First of all, pay attention to copper balance. Uneven amounts on layer pairs (top/bottom, mid top/mid bottom, etc.) relieve internal stress unevenly, and that's where the warpage comes from.Or it's a manufacturing error, in which case just as well to send it back and demand fab to spec -- assuming you have controlled for the above case, your board spec requires it, and you paid for the service to follow that spec, of course. (Proto services give no regard for your specs: the specs are listed in their spec sheet. If their specs don't meet yours, get a custom run, or go somewhere with tighter proto specs.)As for where you are now, if you're not willing to scrap the parts, yeah, something that flexes the board is about the most you can hope for. Differential cooling may help (i.e. heating things up and quenching stretched areas), though I don't know how well this works with a laminate (it's a traditional sheetmetal method). I do find, when tinning copper clad for protos (i.e., not etched and soldermasked PCBs, just plain 2-sided copper clad), it ends up warped, presumably due to the shrinkage of freezing solder as I tin the surface a bit at a time, and probably some help from the copper and laminate shrinking differentially as I go. This usually cancels out once I've tinned both sides, or just flex it a bit to work out the stress. (Note: this must be done before placing components; flexing a board with chip components on it is not a great idea!)Oven time won't help: it was manufactured in a heated press, between flat platens. You can try preloading a bend to cancel it out, but if it's a warp rather than bow or twist, you'll have a hard time with that (you have to stretch everything that'snotsticking out).Time and temp isn't a bad idea; laminate creeps a bit over time (accelerated by heat), and takes a modest set on deformation. So if just flexing it doesn't help, heat might. It's just, getting the strain everywhere but the affected area, isn't going to be easy, I suspect.And, to clarify a bit on topology: zero Gaussian curvature is like any way you can bend a sheet of paper (without like, getting it wet or something). These are easy: simplly bend and twist, you can just flex these out, no problem. It's when it's nonzero that it's a problem. For domed curvature, it's that the middle is larger than the surroundings, so domes out from it; either the middle has to be shrunk, or the surround has to be stretched to match. You can squish down the dome to undo most of that, but you can't do it any further than pressing it down flat -- there's no "flatter than flat" you can force the material into, and in that flat shape, the material will still hold some stress, and spring back from that position. For saddle-shaped curvature, it's that the middle is smaller, and needs to be stretched out. (This can be done by doming it, at least in part.) In either case, it has to be stretched evenly, lest you just make things worse!You can also consider adding brackets or frame rails or a chassis or whatever, to enforce a more rigid shape on the board. This is probably a non-starter for assembly (good luck doing pick-and-place and reflow soldering like that..?), but good practice for final assembly, for obvious reasons.Tim

author: Benta
author_rank: Super Contributor
timestamp: April 13, 2021, 10:44:16 pm
content: Quote from: wraper on April 13, 2021, 09:28:46 pmI'm sorry, but this is contrary to the several decades where I've worked with FR-4.Your "glass transition temperature" has very little to do with the form stability (or the glass content) of FR-4. I routinely heat FR-4 to 180...200 C when joining FR-4 parts using epoxy adhesive, and the FR-4 stays in shape and just as hard under those conditions as in room temperature. I've never seen it warp or go soft.Perhaps bringing own experience is better than Wikipedia?

author: wraper
author_rank: Unknown
timestamp: April 13, 2021, 10:53:04 pm
content: Quote from: Benta on April 13, 2021, 10:44:16 pmMaybe because I see how FR-4 changes its shape in reflow oven? Of course if it is covered with copper which is mostly uninterrupted, likely it will keep its shape if force is not applied.QuoteNothing to do with glass content. As you mentioned Wikipedia, here is a link or youhttps://en.wikipedia.org/wiki/Glass_transitionFor FR-4 in particular:https://www.pcbway.com/blog/technology/What_does_Tg_mean_to_FR4_board_.htmlQuote

author: Benta
author_rank: Super Contributor
timestamp: April 13, 2021, 11:03:18 pm
content: Please don't cherry-pick or quote out of context. That's really bad style!I know exactly what I wrote and what I know about FR-4. Your web-search "proofs"? Well...

author: wraper
author_rank: Unknown
timestamp: April 13, 2021, 11:38:32 pm
content: Quote from: Benta on April 13, 2021, 11:03:18 pm

author: wraper
author_rank: Unknown
timestamp: April 13, 2021, 11:52:09 pm
content: Quote from: Benta on April 13, 2021, 11:03:18 pmOf course outright baselessly saying that my claims are based on Wikipedia and web search is a very good and polite stile.

author: jmelson
author_rank: Super Contributor
timestamp: April 14, 2021, 01:34:12 am
content: Quote from:  on April 13, 2021, 06:39:38 pmIf the problem is just twist, just use your hands and apply an opposite twist. Blank PC boards are extremely rugged, even multi-layer.I have done this many times, never had a problem. Newer Chinese PCB fabs seem to have figured out how to eliminate twist and bowing, which is great.But, I used to get a lot of boards that were not completely flat.Jon

author: jmelson
author_rank: Super Contributor
timestamp: April 14, 2021, 01:34:57 am
content: Quote from: wraper on April 13, 2021, 11:38:32 pmOh, well, I think that one is beyond hope!Jon

author: wraper
author_rank: Unknown
timestamp: April 14, 2021, 02:22:39 am
content: Quote from: jmelson on April 14, 2021, 01:34:57 amJust heat to 200-250oC and bend back without much effort and let it cool. It's quite extreme, but it's just to show that FR-4 can be deformed with little force when hot. Although I do not suggest anything other than twisting, since it's very easy to break it in half with barely any force.

author: AmnevaR
author_rank: Contributor
timestamp: April 14, 2021, 08:51:42 am
content: Quote from: jmelson on April 14, 2021, 01:34:12 amYes, the FR4 material is rugged, but PCB vias are not! If you bend a PCB too much to straighten it - some vias might crack!It will be very hard to find the cracked PCB via later, good luck with it