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Originally Posted by RoboTech
Les and Incoherent,
Thanks for taking the time to look over my system and give it some thought... 
Any ideas why my C/W numbers seem a little high? Any suggestions for how I might improve?
I don't fully understand what the graphs you generated are showing or what the offset is. Any clarification would be appreciated. Thanks... |
Robo. It's not really that I think that your numbers are too high. The C/W number that you are generating is the CW for your setup, i.e. you are measuring the C/W of the waterblock + TIM + copper between die surface and the temperature sensor.
What we are trying to do is compare your number to other tests.
We hope that the waterblock part of the total C/W is relatively constant. The TIM is a bit of a variable, however based on the measurements that I have made, it is fairly consistant depending on the compound type. Lumiere is a fairly thin grease, the number that I
get for it stays pretty much the same over several mounts and pressures. It does change after a few days (settling from 0.0515 to about 0.0495). I think that applying my number to your data is not necessarily accurate but in the absence of anything else...
If we can accept this TIM value and apply it to your numbers, adjusting for the different die size we are left with the copper C/W. I did a quick calculation based on your drawing above using
waterloo to try and establish a number for this. Ending up with an offset for your setup consisting of TIM C/W + Cu C/W = 0.0765 C/W.
In my setup I am able to generate, from a couple of assumptions and several measurements, a "true" WB C/W. In the chart I attached above I have added the offset for your setup to my data for the MCW6000 and overlayed it onto your data for the MCW6002 (we are considering them to be essentially identical). It is basically what my numbers say your setup should give. It is pretty close. Too close, we would expect that the waterblock should give a different (lower) number due to the larger die size although this behaviour must be waterblock design dependent. What Les is showing is that your numbers are not matching the predicted value. The culprit is not necessarily your data, it might be my treatment of it - or of course my numbers, mounting pressure might be more significant than I think. However, I would reiterate my
concern about the probe active area and location.
A possible suggestion would be to make the die heat channel unstepped, i.e. make it 14x14 for a longer distance from the surface so that the sensor is in the 14x14 channel, that way we could be sure that the temperature gradient is fairly linear and hence easier to calculate what the offset
should be . Pain in the arse I know.