Takes a bit of time to get used to this, but you can actually use your thermocouple tip as a pad. Which means you drill the pcb, and run the wires of the t/c to the nearest pad. Gently Remove the pad, squeeze the thermocouple tip and place it in position, secure it on the pcb with epoxy etc.
Depending on the size of your cga, you may want to take temperature reading in more than 3 points... I guess this will be limited by the number of t/c ports on your DRS machine.
Also, check the accuracy of t/c ports on your rework station, ideally you'd want a difference of ---oC (your requirement or airvac spec) between readings, consistent from one trial to the next.
Keep one t/c on top of the package, and you may want to have a reading of temperature at the most sensible adjacent site near the reworked component.
Regards,
Liviu
--------------------------
Sent from my BlackBerry Wireless Handheld (www.BlackBerry.net)
-----Original Message-----
From: Robert J. Vega <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Sent: Thu Nov 07 15:00:35 2002
Subject: Re: [TN] profiling BGAs
How would you recommend placing the thermocouples and creating a profile
for a ceramic CGA? I'm farily new at this process, and nobody here has
ever reworked a CGA. We've been running into difficulty where the top of
our package is getting very hot before our solder joints are reaching
reflow. Right now we have the tip of our t/c slid between columns and
placed as close to the solder joints as we could, and then taped in place.
We're using two t/c, and averaging the values. Also, we're using a DRS-24
rework station. According to what you say below, could we be actually be
reaching reflow, but because of the way our t/c's are placed, we're not
registering it? If so, how could we better place them? If not, has anyone
else had this problem?
Robert
Guy Ramsey <[log in to unmask]>@ipc.org> on 11/07/2002 01:30:08 PM
Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
to Guy Ramsey <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
To: [log in to unmask]
cc:
Subject: Re: [TN] profiling BGAs
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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