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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