The "glass transition temperature" is the point of inflection where the
TCE is greater ... for instance, TCE at less than the Tg is (made up
numbers because it depends on the material) ~ 30ppm/degC and the TCE is
~160-200 ppm/degC at temperatures above the Tg.
These Tg numbers are useful to a point, as indicated; they are useful only
in conjunction with TD value, CTE value, etc.... The values are based on
standardized testing which is done on BARE laminates. These are tests for
the raw materials done by the laminate manufacturers.
For materials converted into multilayers by a board shop, which have been
through various chemical processes and high temperature lamination, the
final RATING of the board's Maximum Operating Temperature is a figure of
merit which comes from Underwriter's Laboratories. UL has been testing to
and giving MOT ratings at roughly 130 - 135Deg C - that is the maximum
in-use temperature at the board surface (including components,
environment, thermal transfers etc...) for "FR4".
It is this later number that you need to concern yourself with for this
particular question. If your question is which laminate can better
withstand the heat of the
assembly process, then you need to look at the Tg, TD, CTE ... For in-use
maximum temperature - that is controlled by UL and your supplier can tell
you what their boards' MOT rating is ..
Tg's of 135 - 140 are fairly typical for today's ordinary FR4s; "high Tg"
means different things to different people: some spec 160C min; others 170
or 175... some 200C. TD is a new marker to try to predict what will happen
at lead-free soldering temperatures. It turns up that within the 135-140C
Tg family of FR4s, those with higher time to decomposition "last" better
than those that have lower TDs. TD is very much affected by the curing
agents in the resin system ... trying to pin down the min TD for lead-free
assembly is currently being studied. High Tg FR4s can have high or low TDs
... it's a different failure mechanism.
At most commercial operating temperatures the Tg does not come into play.
The original question was for a material capable at 120C continuous, which
is below the temperature induced expansion inflection point (Tg) of FR4.
The MOT, which combines the board, the environment, and the thermal
transfer of the components, is now the figure of merit.
(anyone who knows more and can add on, feel free).
Regards,
Valerie
"Jack C. Olson" <[log in to unmask]>
Sent by: DesignerCouncil <[log in to unmask]>
04/22/2005 03:40 PM
Please respond to "(Designers Council Forum)"; Please respond to "Jack C.
Olson"
To: [log in to unmask]
cc:
Subject: Re: [DC] IPC-4101
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>
Sent by: To
DesignerCouncil To
<DesignerCouncil@ [log in to unmask]
ipc.org> cc
04/22/2005 12:54
PM
Subject
Please respond to [DC] IPC-4101
"(Designers
Council Forum)"
<DesignerCouncil@
IPC.ORG>; Please
respond to
[log in to unmask]
COM
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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