which is better thick frag tape or thin?

[ignorus]

I recently got ahold of some thickest frag tape and some of the thinest ones. I was wondering which one works best?

btw, has anyone ever taped heatsinks to their hard drive?

 

[Tonec]

From my limited experience, the thin frag tape was less than ideal. The thick frag tape should work better for chips that are not perfectly flat.

 

[cautery]

This is long.... but complete, and should help...

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Generally, there are three commercially available (all be it in quantity) versions of 3M thermally conductive adhesive transfer tape: 10mil, 5mil, and 2mil.

No one tape is suitable for all applications. It depends on the qualities of the mating surfaces, and frankly, how much effort/attention to detail you are willing to invest to get a better thermal interface.

Thicker tape is generally easier to work with than thinner tape. 2mil thermal tape takes a LOT of patience to work with.

NOW.... The thinner the tape is, the better thermal performance you will tend to get (assuming the tape is not TOO thin... explained below). This is simply the case in all thermal interfaces.... Thermal impedance is directly propotional to the thickness of the thermal interface material.

To me, the ONLY governing factor in which 3M tape I would use is the mating surfaces of the thermal interface, as I am willing to expend the necessary time, effort, attention to detail (Read: Massively Anal Retentive) necessary to achieve the best possible results.

There are two major qualities of a thermal interface (and a whole bunch of minor ones):

Surface flatness, which I quantify with a "runout" measurement.

Surface smoothness, which I quantify by hand lapping the surface to the desired smoothness.

Of the two, flat surfaces are the most important.

Runout is the difference in surface "height" above a reference plane expressed in thousandths of an inch (in height) per inch of travel perpendicular to the reference and measured plane. It is generally easiest to measure this with a dial indicator.

The generally accepted maximum runout value for a GOOD thermal interface is 0.001"/inch.... meaning that for every inch "traveled" across the face of the heatsink, there is only an approximate 0.001" change in vertical relief.

BTW, the runout on a NEW PIII Coppermine is often WELL IN EXCESS of this amount. On measuring my two new PIII-700s, I found that they BOTH had runouts greater than 0.009"/inch. That is HUGE....

Surface smoothness comes into play AFTER the surface is flat. I generally will not settle for less than a surface smoothness achieved by progressive wet lapping of the surface to a minimum of 1500 grit wet/dry paper. If it is a critical juncture, I will continue the lapping progress to better than 3000 grit, and then through 3 progressively finer polishing compounds. I STRESS progressive as skipping large ranges in paper/polish grades will likely sacrifice surface quality, as the higher grade may not be able to remove the "scratches" left by the much lower grade.

Now that I have explained runout and quality of thermal interface mating surfaces, here is the general rule I follow in determining the minimum thickness thermal tape to use. You can always use a thicker than necessary tape, if you can afford to give up a little performance in the thermal interface.

The thermal tape should be NO LESS than the TOTAL runout measurement. For instance.... ASSUME that BOTH mating surfaces have runouts of 0.001"/inch. The TOTAL runout measurment would be 2 x 0.001"/inch = 0.002"/inch. Thus the MINIMUM SAFE thickness of thermal tape is approximately 0.002"/inch.... or 2mil 3M tape...

The reason we use the TOTAL measurement is that the possibility exists (actually it's quite probable), that the lowest "valleys" in the two opposing surfaces MAY end up opposing each other causing a 0.002" air gap. To fill that up, you need at least a 0.002" layer of thermal interface material. Make sense?

So, IF you have the patience, AND you have mating surfaces flat and smooth to within 0.002" TOTAL runout, you can use the 2mil 3M tape, otherwise move up to the 5 or 10mil stuff...

There is a whole lot of stuff to know about thermal interfaces.... especially if you are totally obsessive and anal about getting the best performance possible from the materials and tools at hand.

Hope this helps,