shouldn't be a need to guess about level of cure. get samples to your
fab's laminate supplier for testing.

At 05:53 AM 7/6/2010, Guy Ramsey wrote:
>My experience reflects Werner's logic.
>Though once cooled in most applications the warp (bow toward the heat) is
>quite insignificant.
>
>My only experience that reflects Paul's logic was determined to be
>improperly cured board laminate.
>If heating one side of the board to Tg results in permanent warp, edges
>curled toward the heated side, center bowed away from the heat, I would
>expect improperly cured laminate.
>
>Guy
>
>
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier
>Sent: Friday, July 02, 2010 1:20 PM
>To: [log in to unmask]
>Subject: Re: [TN] Question about FR4 Curing
>
>  Eric,
>You need to follow that logic to its conclusion.
>Top surface expands more than bottom, thus upwards warpage.
>The pregpreg layers cure more towards completion, freezing in some of that
>warpage which remains after cooling.
>Dishing the other way would indicate that perhaps you ave differences in the
>lay-up
>Werner
>
>
>
>
>
>
>
>
>-----Original Message-----
>From: Eric Christison <[log in to unmask]>
>To: [log in to unmask]
>Sent: Fri, Jul 2, 2010 12:10 pm
>Subject: Re: [TN] Question about FR4 Curing
>
>
>Thanks Guy.
>
>Re part two. My question is more to do with what happens to the material
>once it returns to room temperature? If some curing has taken place will the
>matrix have changed in volume?
>
>I should explain that the substrate forms the bottom of a very small box so
>all four edges can be regarded as being rigidly fixed. Normally the
>substrate becomes dished with the centre pushed into the box wrt the edges.
>Sometimes though, the substrate dishes the other way which is what is
>puzzling me.
>
>I have a hypothesis that during flex attach the top surface of the laminate
>sees ~150 deg C but the bottom surface stays quite cool. In this scenario
>you can imagine that the matrix near the top surface may cure while the
>matrix near the bottom stays as it was. If the curing caused an increase in
>volume of the matrix then the top surface would be bigger than the bottom
>surface and so the surface would bow upwards i.e. in the opposite direction
>to the one you'd expect.
>
>Regards,
>
>
>On 02/07/2010 16:40, Guy Ramsey wrote:
> > Our friends at Rockwell have a chart in one of their presentations that
>show
> > CTE of various common laminates up to a typical reflow temp.
> > At room temp CTE of them is about 25ppm /degree C. But, by 150 C things
>get
> > less predictable. And can be 75 ppm / degree.
> >
> > The thing is you are trying to attach a flex. What is its CTE. The
> > difference between the two? It is important.
> >
> > Part two of your question The Z axis expansion of the woven substrate will
>
> > be higher than the "x-y" advertized CTE. So, no I would not expect the
> > material to shrink. What happens of the flex expands wildly above 100 C,
>you
> > might think the substrate shrank . . .
> >
> > Guy
> >
> > -----Original Message-----
> > From: TechNet [mailto:[log in to unmask]] On Behalf Of Eric Christison
> > Sent: Friday, July 02, 2010 11:12 AM
> > To: [log in to unmask]
> > Subject: [TN] Question about FR4 Curing
> >
> > I'm wondering about an issue we have with the dimensional stability of a
> > substrate made of FR4.
> >
> > We're finding it's deforming during a flex attach operation, not by much
> > but enough to cause us a problem. We use ACF and a hot bar. The
> > substrate sees a temperature of 155 deg C for about 10 seconds. Someone
> > suggested that the movement may be due to incomplete curing of the
> > substrate.
> >
> > I'm wondering if a temperature of 150 deg C for 10 seconds is likley to
> > have any effect at all on the matrix.
> >
> > If it did, would the volume of the matrix increase, decrease or stay
> > much the same?
> >
> > I realise that we'll see deformation due the application of pressure as
> > well but in some instances the substrate seems to be moving in the wrong
> > direction.
> >
> > Thanks,
> >
> >
>-- Eric Christison
>Consumer&  Micro group
>Imaging Division
>
>STMicroelectronics (R&D) Ltd
>33 Pinkhill
>Edinburgh EH12 7BF
>United Kingdom
>
>Tel:    +44 (0)131 336 6165
>Fax:    + 44 (0)131 336 6001
>
>
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