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Originally Posted by BillA
along the bottom ~30, and about the same vertically on the left
counting from lower left to upper right is what will define the 'depth' and the fringes become too dense to count,
their curvature can be seen going from upper left to lower right, the upper right corner is quite 'high'
- will be very apparent from the grease (assuming the die is flat)
added a detail image with more contrast
compare to this http://www.thermal-management-testing.com/PF3fringe.jpg
(a 10mm sq is not much larger than that central area)
of course this is only half of the equation, the die/CPU relative flatness is an issue of equal significance
- consider the case of one convex and the other concave ?
- what is the more typical case is both convex (certain if hand lapped - other than by N8 or Steve at PolarFlo)
the effect on the wb's apparent C/W is what is huge, more so than the actual dimensions might suggest (w/o testing)
any appraisal of wb C/Ws is incomplete w/o understanding the contribution of flatness,
could be due to this or that or whatever, assumptions are a tricky business |
I count ~23-25 but will call it 30 for easy and to give benefit of the doubt.
Can see how convex to convex or concave to concave would magnify thermal resistance and how it would need to be accounted for.
Suppose <10 microns variation for contact surfaces.(more later) Worst we could see would be about ~20 microns metal to metal gape.
How does claimed thermal resistance of AS5 factor in then?
http://www.arcticsilver.com/as5.htm
http://www.arcticsilver.com/ceramique.htm
Claimed thermal resistance of AS5
<0.0045°C-in^2/Watt (0.001 inch layer)
and ASC
<0.007°C-in^2/Watt (0.001 inch layer)
0.001 in = 25.4 microns
Quote:
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Originally Posted by BillA
and restudying the original makes me think my initial assumption was wrong, i.e. backwards
the band across the die area is probably high, not low
I suspect that the 'low' areas are out of focal length
- no way to verify now, have a viewer but not that wb
what was I saying about assumptions ?, lol
same effect though re heat transfer
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If we could guarentee (in a reasonable manner) <10 microns variation on a contact surface. Couldn't we apply the claimed thermal resistance across an area as worst case to better derive total thremal resistance?
If claimed thermal resistance is bogus. (Opinion: at least worth testing throughly.) Couldn't we start by defining what the actual thermal resistance (as per worst case thickness) of something like AS5 is?
As a first step anyway.
Or am I makeing too big an assumption?