We find that material shrinks after thermal excursions above Tg. I vote that fully cured material will exhibit a thickness reduction with each thermal excursion. We see this using a TMA. At an isotherm at 260°C, there is a downward slop in thickness until delamination. I do not believe that materials CTE will change significantly until the temperature hits Tg. After Tg the CTE is about oh 5 to 7 times higher than before Tg. Sincerely, Paul Reid Program Coordinator PWB Interconnect Solutions Inc. 235 Stafford Rd., West, Unit 103 Nepean, Ontario Canada, K2H 9C1 613 596 4244 ext. 229 Skype paul_reid_pwb [log in to unmask] <mailto:[log in to unmask]> -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Eric Christison Sent: July 2, 2010 12:10 PM To: [log in to unmask] 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 ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. 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