Re: Thermal Insulator question

Chuck Harris
 

Thermal Conductivity
--------------------

Mica = 0.75W/mK
Silicone/fiberglass = 1.3-3.0W/mK

The unit of thermal conductivity is Watts per milliKelvin temperature rise.

So, higher W/mK is better conductivity.

Dielectric Strength
-------------------

Silicone/fiberglass dielectric strength is typically 300-400V/mil.
Mica dielectric strength is typically 5000KV/inch, or 5000V/mil.

Where 1 mil = 0.001 inch.

Mica was used at first because it was all we had. Later advances in
material science brought us the silicone-fiberglass composites, and
their improvement in thermal transfer. In addition, the incompressibility
of silicone rubber, and its tendency to flow, improves thermal contact
with irregular surfaces. The silicone rubber flows into the voids.

Mica's biggest advantages are very high dielectric strength and very
high temperature capability.

Silicone/fiberglass pads biggest advantages are ease of use, and
high thermal conductivity.

Thermal greases, such as silicone grease, or the white zinc-oxide/
silicone greases, are used to fill any air gaps between the device being
cooled, and the heatsink. The zinc-oxide was added to silicone grease
because it has a slightly better thermal conductivity. In some applications,
beryllium oxide ceramic powder, silver powder, or ??? powder is added to
silicone grease improve thermal conductivity even more.

Rule of thumb: The higher the thermal conductivity, the lower the electrical
resistance. IF you want super high thermal conductivity, bolt the device
directly to the heatsink.

-Chuck Harris

Dale H. Cook wrote:

As a rule I use silicone thermal pads only when heat dissipation needs are relatively
low. For more demanding applications for devices with isolated mounting I use thermal
compound, and for those without isolated mounting I use isolation bushings, mica, and
thermal compound. For most applications I use GC Type Z-9 zinc oxide-silicone
compound. For those rare occasions when bleed must be minimized I use GC Type 44
non-silicone compound. For the most demanding applications where heat transfer must
be maximized, such as with power MOSFETs in broadcast transmitters, I use Aavid
Thermalcote 249G compound, which has exceptional thermal conductivity (about four
times that of common zinc oxide-silicone compound) and is used by the top
manufacturer of AM and FM broadcast transmitters. I'm glad I don't buy it in five
gallon buckets as they do - it is expensive enough in small quantities ($10 for a one
ounce tube at Mouser).

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