Phil,

I don't understand why you are using conductive epoxy to adhere the
thermocouple to the die.  Correct me if I am wrong but according to the
Seebeck effect, any dissimilar metals that are in contact generate a
thermoelectric circuit and thus current.  If you are adding additional
metals to a thermocouple system which is designed to have only two basis
metals- you are generating additional current which entirely skews the
results.  Please coorect me if I am wrong, but this is according to my
Omega temperature handbook.

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Steve-
One of the tests that I perform on reflow systems when I evaluate them
is an IC internal temperature test.  I have a board with a QFP84 mounted
on it.  I have drilled down to the die and attached a thermocouple to
the die with conductive epoxy and filled the hole up with non-conductive
epoxy.  I also attach a thermocouple to one of the lead/pad
interconnects of that device.  I run the board through a typical profile
and observe the difference between the lead/pad interconnect temperature
and the internal temperature (of the die).

Some of the older Convection/IR systems, particularly those with a high
amount of IR (such as the lamp systems) would take the internal
temperature above the lead temp. though some did not.  Of all of the
"newer" Convection Dominant (Forced Convection) reflow systems, all of
them allowed the die to remain cooler than the interconnection - usually
by at least 10 deg. C or more.  Only one machine (no longer in
production) brought the die up to equilibrium with the lead.

Hope this helps.

Phil Zarrow
ITM, Inc.
Durham, NH

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