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November 2002

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Subject:
From:
Guy Ramsey <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 7 Nov 2002 13:30:08 -0500
Content-Type:
text/plain
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text/plain (81 lines)
In some packages, and with some types of reflow nozzles, your method
will not accurately reflect what is going on. In most cases it will work
fine but with a heavy board, or with a ceramic BGA, heatsinked BGA and
nozzles that heat only top of the part you may be as much as 30 degrees
off the actual temperature of the solder and pad. Your method measures
the air or board temperature between the solder balls. Which will be
stable but may not indicate when you have reached reflow.

BGA balls are brought into reflow as heat is conducted though the
package. The balls temperature (and the package temperature) can be
significantly higher than the board or air temperature under the
package. Be sure that you have not exceeded the maximum package
temperature.

Normally, we use the method you described but in difficult situations we
have drilled.
I may try Ramon's method. It sounds like it would work, unless you are
looking at .8 mm or less. When the balls are too small and the package
standoff is minimized the 3 mil wire may be a problem.

Guy Ramsey
American Competitiveness Institute
Senior Lab Technician/Instructor
Ph: (610) 362-1200 x107
Fax: (610) 362-1290



> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Tobias.Chang
> Sent: Thursday, November 07, 2002 11:25 AM
> To: [log in to unmask]
> 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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