Tim
Thanks for this info. Interesting. First time I've heard of non-aqueous solvents
having the same effect, although it is not unreasonable. However, this leads me
to another question. To make the prepreg, the glass cloth is run through a
solution of resin, which is then dried in the treater ovens. There is always
some solvent (I think ca. 1%) left in the prepreg. During pressing, this will
certainly vapourise, but it has nowhere to go, especially in the middle of the
sheets. Does this mean that samples taken from the middle of a sheet of laminate
will have a lower Tg than from the extreme edge????
The 5 deg C Tg range measured seems very small. I would opt for 10 or more being
more typical, as this is a function of the MW of each molecule, which is very
variable. A small range implies that the average MW is quite small, which may
not offer the best electrical properties, as I understand it.
Brian
Timothy Reeves wrote:
> I asked about this phenomenon last October on the Technet and got an answer
> from Kelly Schriver. He stated in reply that he and Jack Brand of Honeywell
> Materials Lab ran a series of tests in the 70s that showed using DSC that
> the Tg of standard FR-4 would typically drop from 115-120 C down to 100-105
> C after 4 weeks at 70% RH.
> Perhaps he's still on the 'Net and would be willing to look for the data he
> has. He also said that non-water solvents have a similar effect, and the
> solvents presence in the microcapillaries alters the expansion
> characteristics of the material.
> Tim Reeves
> > ----------
> > From: Brian Ellis
> > Sent: Monday, June 5, 2000 23:34
> > Subject: Re: Baking of boards
> >
> > Hans
> >
> > Offhand, no. I would suggest you contact the laminate manufacturer.
> > However, I do know that
> > there have been papers published on the subject, mainly from academic
> > sources totally
> > unrelated to electronics laminates. I think that the now defunct polymers
> > laboratory in the
> > Materials faculty of the Swiss Federal Institute of Technology, Lausanne
> > (EPFL) had a
> > student who did his dissertation on this subject about 15 years ago, under
> > Prof. Kausch,
> > but I'm not even sure of this. Otherwise, one of the large epoxy makers
> > may have data. I
> > don't know the mechanism, but it may have something to do with the
> > residual ionics from the
> > prepolymerisation reaction.
> >
> > Brian
> >
> > Hans Rohr wrote:
> >
> > > Brian,
> > >
> > > Do you know of any published data that shows a relationship between the
> > decrease in Tg
> > > with increasing board moisture content? I am doing a failure analysis
> > on a one year
> > > old board that has a measured Tg (by DSC) that is 12 degrees C lower
> > than when it was
> > > fabricated. The moisture content of the board is close to 8,000 ppm.
> > The DSC scan did
> > > not exhibit a double peak. A very small quantity of boards exhibited
> > massive
> > > delamination after SMD reflow. The remainder of the boards from the
> > fabrication lot
> > > were run several times over a 2 month period and exhibited no sign of
> > delamination.
> > >
> > > Hans Rohr
> > >
> > > Brian Ellis wrote:
> > >
> > > > Tim
> > > >
> > > > The thermal energy required to remove ab/adsorbed water molecules from
> > a
> > > > heterogeneous material is about the same, irrespective of the
> > temperature
> > > > (within reasonable limits and provided you do not have a phase change,
> > such as
> > > > exceeding the Tg, which, incidentally, drops by about 20 °C when the
> > board is
> > > > humid). If you use a vacuum oven, you have extra energy required to
> > turn the
> > > > pump and operate the traps. The pump runs hot, after a time, and this
> > heat is
> > > > dissipated into the atmosphere and not into the oven. If you use a
> > cryogenic
> > > > trap to prevent oil vapour from entering the oven or moisture from
> > emulsifying
> > > > the lubricating oil, this also requires energy to produce the removal
> > of heat.
> > > > If you use absorber/adsorber traps, they require energy to regenerate.
> > Overall,
> > > > vacuum drying requires 50-100% more energy than atmospheric pressure
> > drying,
> > > > albeit the temperature may be considerably lower.
> > > >
> > > > Brian
> > > >
> > > > Timothy Reeves wrote:
> > > >
> > > > > When you say that the energy is actually greater, you mean the
> > energy
> > > > > imparted to the escaping water molecules, right? But what does that
> > matter?
> > > > > What really matters is energy imparted to the board. The board
> > itself may
> > > > > actually decrease in temperature if a low enough pressure is
> > obtained. In
> > > > > that sense, Doug was right (but he should have said "less heat
> > [transferred
> > > > > to the polymer, glass, and copper]")
> > > > > I know... you were just trying to point out the misconception most
> > people
> > > > > have regarding temperature vs. heat.
> > > > > I love these technical arguments... :-)
> > > > > Tim Reeves
> > > > >
> > > > > > ----------
> > > > > > From: Brian Ellis
> > > > > > Sent: Thursday, June 1, 2000 7:54
> > > > > > Subject: Re: Baking of boards
> > > > > >
> > > > > > Sorry, Doug, the heat is more or less the same. The energy
> > required is
> > > > > > actually
> > > > > > greater. It is the temperature that is lower.
> > > > > >
> > > > > > Brian
> > > > > >
> > > > > > Douglas Pauls wrote:
> > > > > >
> > > > > > > If you are really concerned about the effects of heat on the
> > polymer
> > > > > > > qualities or the oxidation of the surfaces, consider using a
> > vacuum or
> > > > > > > vacuum-make operation. Gets the water out with less heat.
> > > > > > >
> > > > > > > Doug Pauls
> > > > > > >
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