Some things to consider:

1.  FR4 resin transition temperature is between 115C and 135C.  Reflow 
soldering temperatures are typically 215C to 255C.  Any internal stress in 
the board will be relieved by warp and twist when the board becomes plastic 
at the reflow temperature.  The board fab must use care when pressing to 
ensure that cooldown is controlled and uniform - otherwise internal stress 
is created which will try to relieve in the reflow process.

2.  Since reflow temperature is typically above resin transition 
temperature, the board must be properly supported during reflow.  If the 
jigs are not properly set, the board will deform during reflow.

3.  Component placement on thin boards is a factor because of weight during 
reflow, and because of component heat-sinking during reflow.  Again, the 
jig placement, and cooldown control are critical.

4.  Your details suggest you have tried to keep a balanced copper 
distribution.  If, however, there is substantial difference in the 
percentage of copper on the signal layers, you will see differential 
expansion during reflow.  That differential force will warp and twist the 
board - quick cooldown will set the twist.

 From your comment that the board tends to flatten after a period of time, 
it seems likely that one of items 2 thru 4 above is at play.  On the 
limited information available, I would look especially carefully at the 
copper balance in opposing layers.




At 04:52 PM 4/3/02 +0100, you 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)
>
>Layer stack up is two cores + two foils (sizes rounded to 1 decimal place)
>
>R/P     Layer   Type                    thou
>-       -       Resist          0.4
>R       1       Ni/Au 0.5oz     1.1
>R       1       Foil 0.5oz              0.7
>-       -       Prepreg 7630    7.0
>P       2       1oz Copper              1.4
>-       -       Core                    15.0
>R       3       1oz Copper              1.4
>-       -       Prepreg 1080    5.0
>-       -       Prepreg 1080    5.0
>R       4       1oz Copper              1.4
>-       -       Core                    15.0
>P       5       1oz Copper              1.4
>-       -       Prepreg 7630    7.0
>R       6       Foil 0.5oz              0.7
>R       6       Ni/Au 0.5oz             1.1
>-       -       Resist          0.4
>---------------------------------
>Total                           64 = 1.625mm
>
>Effects are:
>Boards are flat from production, but twist on heating in solder reflow or
>wave solder.
>
>An analysis of 5 board produced 1 that exceeded the IPC warpage
>specifications.
>
>Trouble is all but one was too twisted to fit into the rack without effort.
>
>We had the same problem with the alpha version, but here this was put down
>to an incomplete plane
>on the two plance layers, this has been changed to a full plane - no change
>to warp.
>
>Suggestions so far have been to:
>0:      Add copper to waste (breakout) parts layers
>1:      Change the breakout to a waste part scored along the two long edges.
>2:      Use a three core construction
>3:      Add copper hash to layers 3 and 4, (other either side of the two
>cores)
>4:      Change warp and weft of cores
>5:      Increase core thickness and decrease 1080 prepreg thickness.
>6:      Use 1.8 FR4 by increasing core thickness (undesireable)
>
>So far we've had no input from the manufacturer as to what (if any) of the
>above will be better, though
>they agree that all should have some affect (positive or negative) on bow
>and twist.
>
>We've also noticed that over a long period (weeks) the twist gets less.
>
>Regards
>
>Jason.
>
>
>-----Original Message-----
>From: Mike Reagan [mailto:[EMAIL PROTECTED]]
>Sent: 03 April 2002 16:43
>To: Protel EDA Forum
>Subject: Re: [PEDA] WANTED: PCB Expert (Off Topic)
>
>
>Jason
>Some of our advice is free
>What process is warping the boards?  Reflow or manufacturing? or upset
>employee?
>
>Mike Reagan
>EDSI
>
>
>
>----- Original Message -----
>From: Jason Morgan <[EMAIL PROTECTED]>
>To: 'Protel EDA Forum' <[EMAIL PROTECTED]>
>Sent: Wednesday, April 03, 2002 9:03 AM
>Subject: [PEDA] WANTED: PCB Expert (Off Topic)
>
>
> > Hi,
> >
> > We have a board warping problem and are looking for a PCB expert to help
> > resolve it.
> > (Preferably located in the UK, but not important)
> >
> > When I say expert, I mean *EXPERT*. The problem is quite complex and
>already
> > has baffled two manufacturers.
> >
> > We will pay the going rate for any consultation.
> >
> > Regards,
> >
> > Jason Morgan
> >

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