The drilling method only became standard practice because the clearance
under a BGA became too small for a thermocouple to be fitted betweeen BGA
and board. To me, this method always left me feeling a bit uncertain - was
the tip of the thermocouple making proper and consistent contact? Was it
acting as a heatsink for that solder ball? What affect did the thermally
conductive epoxy have, especially when it is less thermally conductive than
the solder material of the ball.

There are now some incredibly thin K-type thermocouples available from a
company in England. One can be fitted to the board surface between the pads
and soldred in place before the BGA is placed and soldered. They're quite
expensive at around $140, and I cannot remember the name of the name of the
Company that makes them right now. I'll let you know if I find it, as I'm
interested in these things myself.

Peter



"Tobias.Chang" <[log in to unmask]>  08/11/2002 12:25 AM
Sent by: TechNet <[log in to unmask]>

Please respond to "TechNet E-Mail Forum."; Please respond to "Tobias.Chang"

              To:  [log in to unmask]
              cc:  (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
              Subject: [TN] profiling BGAs








A couple quick questions regarding temperature profiling of BGA's at a
re-work station.

I posted maybe a month back and from the (2) responses I gather that
drilling holes to feed the thermocouples from under the board is the
standard practice.

I tried this, however the best I could do was *attempt* to ensure that
there
was contact between the tip of the thermocouple and a BGA solder ball.
(the
thermocouples came up from under the board, and I tried to place the BGA
such the tips of the thermocouples would be in contact with the side of a
BGA ball.  I have 3 thermocouples (middle, side, corner) under the BGA, and
the BGA is held down by chip bonder at 2 opposite corners)  When I tried
getting temperature values at the re-work station, I was getting
inconsistent numbers.

The other method I've tried was to use thermally conductive adhesive to
hold
a thermocouple between the chip and the board (with no drilling -> ie. the
thermocouple wire lies flat against the board and held down by the adhesive
and the chip).  This method seems to result in more consistent temperature
readings.

Can anyone comment on whether the adhesive method is used and reliable, or
if theres something I'm doing wrong with the drilled board method?

Thanks,

Toby Chang
Process Engineerin Co-op
Leitch Technology International Inc.

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