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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