Werner

Good to hear from you. I think there may be some confusion here. The original
post mentioned preheat which I take to mean the preheat before soldering. As it
is customary for this to be in the 90-110 deg C range on the top surface before
the [wave] soldering starts, the bottom surface is very likely to exceed the
Tg. After the soldering starts, this is inevitable for a few secs, even with
the highest Tg material. I liken an epoxy resin below Tg to a dry stone wall
and above Tg to the same but with a 5 cm gap between each stone. In this
condition, anything can enter between the "stones" (molecules) and is
imprisoned when the stones touch again, on cooling down. Absorption is obvious
but, with a long molecule with a lipophilic end group, then adsorption is also
a possibility. The combination can be fatal, but the higher the temperature,
the greater the risk.

Brian

[log in to unmask] wrote:

> Hi Brian,
> You not only have a variabilityof the Tg, but the Tg is a single number
> locating a temperature range of about 45C over which the transformation
> actually takes place.
> This becomes important fot accelerated reliability testing of SMT solder
> joints; you can not characterize the lading on the SJs if you run your test
> temperature into this Tg-range. Thus, I recommend that the top cycle
> temperature be Tg minus 25C.
> Werner Engelmaier

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