Because Tg is not the "melting temperature".
Tg is the "glass transition temperature", which is a little harder to
understand.
For example, I think the Tg of PTFE is around 25C, but it melts at 300C
!!!)
This might help clarify: For any material, when you change the temperature
1degree, the material dimensions change a certain amount. When you plot
several changes you get a general ratio that is called the CTE, the
coefficient of thermal expansion. For any material the CTE is fairly
predictable within a certain range of temperature, and you can estimate
the dimensional change by knowing the temperature change. But at a certain
temperature the estimate becomes unreliable, because the material does
something radical. You might not know exactly which way it will go, some
materials get more plastic, others get more rigid. But at this point is is
called the glass transition temperature. It doesn't necessarily mean the
circuit will fail or that your design cannot function indefinitely.
I believe a study has shown that some (supposedly) "high temperature"
materials actually perform worse at extreme temperatures. The reason is
because although the Tg for regular FR4 is lower than the "high temp", it
actually holds up better at extreme temperatures than the "high temp"
stuff. It doesn't break own as fast for each extra degree of temp. Even
though the high temp has a higher Tg, it breaks down faster once it gets
above that point. not good.
For this reason, many are starting to use a different value to rate
materials, which you will hear more about as more designs are going
lead-free. The value is Td, which is the DECOMPOSTION temperature. The Td
is probably a better indicator of you material performance at high
temperatures than Tg.
hope that helps,
Jack
Chris Ball
<chris.ball@VALEO
.COM>
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Subject
Please respond to [DC] IPC-4101
"(Designers
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Caterpillar: Confidential Green Retain Until: 05/22/2005
Retention Category: G90 -
General
Matters/Administration
Hi-
In IPC-4101, why is a temperature range specified rather than a rated max,
like components? Is melting the glass ever a desirable thing? If not,
wouldn't it be less confusing to just specify IPC-4101/21 at 110C MIN? Who
needs a guarantee that the glass will melt at 150C? In bare board fab, do
you sometimes need to T the G?
If that's a stupid question, tell me why. Good designers aren't afraid to
be slapped with the facts, and neither am I ;-/
I see the range and think I can get that material rated at 110C or 150C and
maybe some points between.
-Chris
<NITPICK>
P.S. If Tg could be as low as 110C, a fair-to-middlin' designer would make
sure the board never SEES 110C...
</NITPICK>
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