At 04:43 PM 3/7/97 -0700, you wrote:
>Please reply offline.
>We currently use an ceramic epoxy located 10 mm back from the
>thermocouple contact point to attach our thermocouples. I was
>wondering if anyone has come up with a more reliable method besides
>high temp solder.
>
>Thank you in advance
>
>Dave C
>
>
>Attachment Converted: d:\eudora\attach\Attachin
>
Hi Dave,
I know you won't want to hear this, but I strongly recommend hi-temp solder
as the attachnent method. All other scenarios I've seen lead to potential
for excessive error. Following are recommendations for thermocouple
attachment which will lead to excellent accuracy and reliability:
- Use .010" dia (30 ga.) thermocouples for all but fine pitch (0.5mm and
below). For these use .005" dia ( 36 ga.) wire.
- Use glass cloth insulation rather than teflon where longevity is
required, or in IR-predominant reflow environments (teflon can melt).
Adhesives also will bond to the glass, allowing leads to be tacked to boards.
- Use a high-Pb solder with a small "plastic range"; We use one that is Pb
97.5, Ag 1.5, balance Sn, liquidus/solidus at approx. 309 C (no plastic range).
- Prepare the joint by using solder-wick; remove as much of the low-temp
solder from the joint as possible, then add high-temp solder. Wick out the
high-temp solder and repeat the procedure. This will eliminate nearly all
remaining low-melting material, and ensure thermocouple does not become
"unstuck". This requires a high-output iron (NOT high temperature, high
HEAT TRANSFER).
- Solder thermocouple to joint. Use minimal solder. For fine pitch I
recommend making a sharp 90 degree bend about 1 mm back from the junction,
and inserting the junction between the leads from behind. Bridge over the
junction with solder. verify retention of junction in solder and use a sharp
point to separate thermocouple leads. Bare leads must not come in contact
with each other at any point other than the junction.
- Tack the leads to the board to provide strain relief. This is easiest to
accomplish by using old thermocouple wire and unused through-holes. Loop
wire around thermocouple leads, pass through holes, twist on opposite side.
A wrap of Kapton tape will keep wire from abrading/cutting the insulation.
NOTE: this technique should not be used with teflon-insulated thermocouples.
Use adhesive to "capture" leads instead.
- Remember that many ovens have some non-uniformity, especially across the
conveyor width, some especially near the edge rails. I strongly recommend
testing for oven loading" effects using organic (not metal) loading plates
of essentially the same mass as assemblies. You'll need approx 10 plates to
do a proper test).
The above information assumes reflow processing; it will also work just fine
for wave solder (top side) but will fail after a couple of passes on bottom
side due to dissolution of the high-Pb solder in the pot.
Note that I have sent this to you directly, as well as to technet, as I
believe this is a topic of general interest, and one where there is much
misunderstanding and misinformation. I hope you find the information useful.
Regards,
Fritz Byle
Rockwell Automation/Allen-Bradley
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