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#21
danpi - Posted 11:52 pm PDT 04/8/09 (1133 Posts)
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#22
dave_c - Posted 1:13 am PDT 04/9/09 (7529 Posts)
<table width="90%" cellspacing="1" cellpadding="3" border="0" align="center"><tr> <td><span class="genmed"><b>danpi wrote:</td> </tr> <tr> <td class="quote">A high quality silicone grease has a relatively narrow molecular weight distribution, thus there is no oil to "pump out". It should not degrade at temperatures on a CPU package. I agree that there are other polymers having somewhat higher thermal conductivity. Fine on the trade market jargon, I do not pretend to know or be able to sling that about.</td> </tr></table><span class="postbody">
Whether it is high or low quality silicone grease, ALL silicone greases pump out the silicone oil far sooner than AS5, Alumina, or Ceramique does.
It's not that the oil itself degrades, again you do not understand. The oil viscosity relative to temperature has a higher change and thus, after several thermal cyclings there are dryed out solids causing poor thermal conductivity.
I have personally pulled heatsinks off CPUs that had brown scorch marks on their pin-side because of this problem with silicone oil based greases.
Again, silicone oil based greases are not appropriate for long term use on high heat density parts. 100W per sq. CM is excessive for a silicone oil based grease. It WILL, ALWAYS, be a problem within the lifetime of a system with these parameters. There isn't any particularly high or low quality silicone oil that makes a difference, silicone oil was a near-enough perfect science 30 years ago if not longer.
It is not a matter of conductivity at all, it is a matter of what happens to conductivity when the liquid and solid portions separate, the solids then leave elevated islands of compound that are poor at heat conduction and even that only in areas that these solid deposits remain.
To put it simply, going beyond theory I have years of observation of this effect to go on. Once CP... [Truncated]
Whether it is high or low quality silicone grease, ALL silicone greases pump out the silicone oil far sooner than AS5, Alumina, or Ceramique does.
It's not that the oil itself degrades, again you do not understand. The oil viscosity relative to temperature has a higher change and thus, after several thermal cyclings there are dryed out solids causing poor thermal conductivity.
I have personally pulled heatsinks off CPUs that had brown scorch marks on their pin-side because of this problem with silicone oil based greases.
Again, silicone oil based greases are not appropriate for long term use on high heat density parts. 100W per sq. CM is excessive for a silicone oil based grease. It WILL, ALWAYS, be a problem within the lifetime of a system with these parameters. There isn't any particularly high or low quality silicone oil that makes a difference, silicone oil was a near-enough perfect science 30 years ago if not longer.
It is not a matter of conductivity at all, it is a matter of what happens to conductivity when the liquid and solid portions separate, the solids then leave elevated islands of compound that are poor at heat conduction and even that only in areas that these solid deposits remain.
To put it simply, going beyond theory I have years of observation of this effect to go on. Once CP... [Truncated]
#23
danpi - Posted 6:31 pm PDT 04/9/09 (1133 Posts)
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