Assembly vendors "Palletize" boards that have a tendency to warp, basically it's a frame that goes with the board as it's assembled to keep it straighter.
Multiple layers of lamination also help to reduce and strengthen the boards.
Karl
> Date: Thu, 6 Feb 2014 14:15:32 -0800
> From: [log in to unmask]
> Subject: Re: [DC] PCB Bow and Twist controls
> To: [log in to unmask]
>
> Hi
> Here is one more thing to consider at the bare board fab level.
> Years ago (hand taped era) a fab shop showed us their facility for
> baking new shipments of laminate material.
> It was explained that they found varying levels of moisture in the the
> materials as received which was a significant cause of warpage in
> finished PCBs. They baked all new materials to drive out moisture,
> rejected any that warped during this process, and them stored the
> materials under a controlled climate.
> I don't know how much this applies to the new ROHS compliant materials
> but I suspect it still does.
> Greg Smith
>
>
> On 2/6/2014 7:52 AM, Jack Olson wrote:
> > just a "light discussion", huh? (smile)
> >
> > I'm not up to speed on the latest bare board fabrication processes,
> > and I can't directly answer the questions at the end of your post,
> > but I can offer a few comments:
> >
> > a) that's a pretty good list of factors you have collected, I can't think
> > of any others
> > b) I think the problem is harder to control for lower layer count. Once you
> > have a lot of copper layers, you are probably not going to see warping
> > unless something was done drastically wrong at the fabricator
> > c) Its fairly easy to compensate for copper balance, either the designer
> > can add copper fills in empty areas (if worried about affecting
> > performance, make a larger clearance like 100 mils or something, so it
> > really just fills the EMPTY areas) or by allowing the fabricator to add
> > thieving to the artwork
> > d) It is fairly difficult to compensate for asymmetrical layer stack,
> > unless it is a higher layer count
> > e) I disagree with the item 4) you listed. I think some bare board fabs
> > have tried to do that to pass incoming inspection, and then they warp at
> > assembly thermal. I'm pretty sure that is a bad practice!
> > f) I haven't heard of rails or tabs needing any particular kind of
> > attention related to warping as you suggest in item 7)
> > g) I don't think long narrow boards are necessarily more prone to warping,
> > but remember the spec allows a certain amount of warping per inch, so it
> > may appear to be more warped than other boards and still be acceptable (I
> > think its .7%, or 7 mils per inch? can't remember) anyway, you may be able
> > to see an obvious warp on a long board, that is still okay, is all I was
> > trying to say
> > h) yes, I know for sure that fabricators alternate the direction of the
> > weave for adjacent layers, and if they make a mistake you can have a
> > problem, but I should let a fabricator answer that one...
> >
> > hope that helps a little bit, sorry I can't answer your questions, Mr. Bill
> > Jack
> >
> >
> > On Wed, Feb 5, 2014 at 2:57 PM, Brooks, William <
> > [log in to unmask]> wrote:
> >
> >> I would like to open a light discussion on the control of bow and twist in
> >> FR4 PCB's...
> >>
> >> I'm aware that there are many factors that can contribute to printed board
> >> warping...
> >> Some are design related and some are fabrication material and process
> >> related
> >>
> >> These are some design related warp inducing features...
> >>
> >> 1) Asymmetrical stack up
> >> 2) Uneven distribution of copper
> >> 3) Very long and narrow boards...
> >>
> >>
> >> Some other suggestions I have heard of:(though not verified)
> >> 1) Copper fill or thieving on layers to distribute copper plating more
> >> evenly
> >> 2) Alternating the grain of the board layer materials to compensate for
> >> grain induced warpage
> >> 3) Reduce the resin content increase the glass fiber diameter to reinforce
> >> the strength and rigidity of the board
> >> 4) Flatten boards under heat and pressure to compensate for warp
> >> 5) Reduce lamination cycles by changing the design construction
> >> 6) Controlled cooling of boards after lamination to reduce stress
> >> 7) Pay attention to the geometry of breakaway tabs and support rails...
> >> add copper thieving to them so that they don't warp the board
> >>
> >> My questions...
> >> 1) What if any hard research has been done in this area and have there
> >> been any white papers published that I can access?
> >> 2) How do bare board manufacturers compensate for the tendency of certain
> >> boards to warp?
> >> 3) How much does the type of material and raw material manufacturer play
> >> into the equation?
> >> 4) What recommendations do you have to avoid the issue up front in your
> >> pcb designs?
> >>
> >> Are there others suggestions?
> >> Are there any great published articles out there that examine the subject?
> >> I look forward to your comments...
> >>
> >>
> >> Thanks,
> >>
> >> William Brooks, CID+
> >> Senior MTS (Contract)
> >> 2747 Loker Ave West
> >> Carlsbad, CA 92010-6603
> >> 760-930-7212
> >> Fax: 760.918.8332
> >> Mobile: 760.216.0170
> >> E-mail: [log in to unmask]
> >>
> >>
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