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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