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October 2004

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Subject:
From:
"Kerr, Bryan" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Kerr, Bryan
Date:
Mon, 25 Oct 2004 16:20:06 +0100
Content-Type:
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Hi All

The key to good, accurate thermal profiling is the thermal connection
between the thermocouple and what you are trying to measure. The t/c
junction must be at the same temperature as the solder joint you are
measuring and there can be wide variation in the temperature of a specific
joint depending on PEC design, e.g. a small chip cap joint to a large earth
plane joint. After trying many techniques over the years, the one which most
truly reflects the actual joint temperature is to use a high melting point
solder and "envelop" the end of the 0.005" thermocouple. Try not to use too
much solder otherwise you will add to the thermal demand of the solder
joint. We often do not have scrap boards to play with either and find that
we can, in these cases, use a bare board and fit a dummy component or
components using HMP. You can then desolder and clean up afterwards. This
means putting a board through reflow several times however they should
withstand this OK.
Best plan is to plan to measure components with both the highest and the
lowest thermal demand to get a good picture of the delta T across the
assembly.
I would avoid the use of any tapes since this method of fixing is only good
for measuring flat surfaces and not complex solder geometries.

Bozo is a bit strong though !

Cheers

Bryan Kerr
Principal Quality Engineer
Process and Materials Laboratory
AMS Hillend Scotland

-----Original Message-----
From: Dehoyos, Ramon [mailto:[log in to unmask]]
Sent: 25 October 2004 15:00
To: [log in to unmask]
Subject: Re: [TN] A poll of sorts...concerning thermocouple attachment


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        The trick is to make sure that the TC wires are molded by hand to
lay by themselves on the board and secured with the Kapton or any hitemp
tape. They should not by force down with the tape. When the TC stay put time
after time through the oven the results will be about 2 degrees lower than
if they are soldered with high temp solder. If they are forced down with the
tape, the wires will spring up when the glue becomes weak by the heat.  So
Dr. ZapFart does not know what he is talking about.
        It is just my opinion,
        Ramon

-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Steve Gregory
Sent: Saturday, October 23, 2004 12:10 AM
To: [log in to unmask]
Subject: Re: [TN] A poll of sorts...concerning thermocouple attachment


Hi Phil,

I guess I'm what you call a "Bozo". I use Kapton tape quite often when I do
my profiling. I however, don't cover the thermocouple tip with it, but just
use  it to secure the wires.

My thermocouple tip is shaped and placed with tweezers to be at  the solder
joint I want to monitor,and then the wire is secured along the  assembly at
places so as not to disturb the rest of the components. The  thermocouple
tip
isn't covered by anything...high temp solder, epoxy, aluminum  tape, or
anything.

Just because I use Kapton tape, doesn't mean that I'm a "Bozo"...

-Steve Gregory-

According to Dr. Zapfardt in the October issue of Circuits Assembly  ("On
The
Forefront")attaching the TC with Kapton tape is for bozos  who really are
not
interested in actually getting accurate profiles, but  are just killing
time.
I couldn't have said it better  myself.

Phil Zarrow
ITM Consulting
Durham, NH   USA
www.ITMconsulting.org





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