Thanks for all the advice and suggestions.

Some more info that I missed from the first post:

We've been trying to solve this for some time. We've tried boards supplied
from more than one house and populated at more than one house, all with the
same sorts of results.

We're not the fab, so we have no control on the process, its up to their QA.

The profile is optimum (according to fab) and any change would result in bad
joints.

We've tried reflow and wave with the same or similar results.

We've tried with and without components with the same or similar results.

So the component layout and thermal profile probably are not to blame.

It must be the board design or the results of that combined with the
conventional layup for such a design.

I can't say who designed the rack, but to our knowlege their own cards fit
into the rack OK - even though our samples are twisted to a small degree.

We're prepared to try adding hashing to the inner layers - it seems logical,
but as the PCB house seem not to be sure we don't want to spend the money on
a run without all the available information (it costs 2k for each run).

I'll keep you all informed of the solution and the outcome, thanks once
again for all the help.

Jason


-----Original Message-----
From: Jon Elson [mailto:[EMAIL PROTECTED]]
Sent: 03 April 2002 21:53
To: Protel EDA Forum
Subject: Re: [PEDA] WANTED: PCB Expert (Off Topic)


Jason Morgan wrote:

> Many thanks,
>
> Details are as follows:
>
> 6 Layer 1.6 FR4
> 8" x 10" Board
>
> PCB support is a wasted rectangle 10mm wide along all edges, supporting
PCB
> at 2 or 3 points along each edge.  (First observation is that this should
> have copper layers)

A number of different ways to even out copper coverage on layers on
opposing sides of board were mentioned, and these are probably aimed
at solving residual stresses left in the board.  That sounds good, but may
be difficult to accomplish, dpending on board density, etc.

One thing that comes to mind is that the boards come out of the laminating
press flat, go through all the additional steps in PCB fabrication OK, but
then warp when YOU process them to attach components.  Are you sure
you have to heat the boards as hot, for as long as you are doing, to get
good soldering?  That may have something to do with it.

Finally, could you give the boards some mechanical support during the reflow
soldering?  Making some simple metal frames that hold the board edges
during the entire time it is heated might keep them flat as they cool.

Jon

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