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Originally Posted by bigben2k
Ok, let's stir things up a bit... 
I've come across some unsubstantiated information that states that a TEC does NOT spread heat, it carries it quite linearly.
I have to assume, for the sake of common sense and for safety, that the heat spreads would flow ******d at ~ at 45 deg angle, from the heat source (CPU die). Given a 1/4" cold plate, the two ceramic (?) layers that constitute a TEC, and a water block base plate, what's the resulting area that actually requires cooling? Is it less than a typical 40 mm by 40 mm TEC?
Here's another mystery: does the cold plate bend or flex, when clamped, and by how much? Does it impact performance? |
A TEC as far as I know transfers heat in a linearly because it cannot be thought of as a single device.
A quote "A thermoelectric has analogous parts. At the cold junction, energy (heat) is absorbed by electrons as they pass from a low energy level in the p-type semiconductor element, to a higher energy level in the n-type semiconductor element. The power supply provides the energy to move the electrons through the system. At the hot junction, energy is expelled to a heat sink as electrons move from a high energy level element (n-type) to a lower energy level element (p-type)."
What that quote does not mention is that each TEC will have many of these couples (n>p) and that he heat will only move directly throught each pair and cannot spread to other sets. So a proper heat spreader is required on both the hot and cold sides for the device to function well.
As far as mounting goes there is flex involved and manufactures standards referring to same. Here is a quote on assembly
"Procedure for Assembling (Type L) Lapped Modules to Heat Exchangers
IMPORTANT: When two or more thermoelectric devices are mounted between a common plate, the thermoelectric devices thicknesses should vary no more than 0.0015-inches. Contact our Engineering Department for more information on close tolerance lapped thermoelectric devices.
Step 1. Prepare cold plate and heat sink surfaces as follows:
A) Grind or lap flat within +/- 0.001" in module area.
B) Locate bolt holes as close as possible to opposite edges of module (1/8" clearance recommended, 1/2" maximum), in the same plane line as the heat exchanger fins. This orientation utilizes the additional structural strength of the fins to prevent bowing. Drill clearance holes on one surface and drill and tap opposite surface accordingly (see sketch). If a spacer block is used to increase distance between surfaces, performance is greater if the spacer block is on cold side of system.
C) Remove all burrs, chips and foreign matter in thermoelectric module area.
Step 2. Thoroughly clean and degrease thermoelectric module, heat exchanger and cold surface.
Step 3. Apply a thin continuous film of thermal grease (Wakefield Engineering Type 120 or Dow Type 340) to module hot side surface and to module area on heat exchanger.
Step 4. Locate module on heat exchanger, hot side down.
Step 5. Gently oscillate module back and forth, exerting uniform downward pressure, noting efflux of thermal compound around edges of module. Continue motion until resistance is felt.
Step 6. Repeat Step #3 for cold side surface and cold plate.
Step 7. Position cold plate on module.
Step 8. Repeat Step #5, sliding cold plate instead of module. Be particularly careful to maintain uniform pressure. Keep the module centered between the screws, or uneven compression will result.
Step 9. Before bolting, best results are obtained by preloading in compression the cold plate/heat exchanger/module assembly, applying a light load in line with center of module, using clamp or weights. For two module assemblies, use 3 screws located on module center line, with middle screw located between modules. To preload, torque middle screw first. Bolt carefully, by applying torque in small increments, alternating between screws. Use a torque limiting screw driver. The recommended compression for a TEC assembly is 150 to 300 pounds per square inch of module surface area. Using the following equation you can solve for torque per screw:
T = (C x D x F x in²) / (# of screws)
T = torque per screw (inch-pounds)
C = torque coefficient (0.20 as received, 0.15 lubricated)
D = nominal screw size (4/40 = 0.112, 6/32 = 0.138, 8/32 = 0.164)
F = Force (lbs / in²)
in² = Module surface area (length x width)
Check torque after one hour and retighten if necessary. Use Stainless Steel Screws, fiber insulating shoulder washers, and steel spring (Belleville or split lock type) washers (see sketch in Assembly Tips).
CAUTION!
1. To assure good thermal grease interfaces there should be no bowing of either surface due to torquing. To prevent bowing, apply less torque if one or both surfaces are less than 1/8 inch thick copper or 1/4 inch thick aluminum.
2. Lead wires are soldered to module tabs with bismuth/tin solder (136°C). If lead wire replacement is necessary, use bismuth/tin solder.
DO NOT use lead / tin solder (180°C)."
If you are interested in Peltiers I reccomend going to Melcors site and downloading their Aztec software which includes all kinds of info and a program to select what you need. They are fun to think about and play with but are not very eficient. If you want to cool a 100w chip to 0 deg c it can be done but expect to heat a 400 sf room in the middle of winter with your rig and the resultant rize in your electric bill.