silicone oils weeping from thermal interface pads

[nigelt]

Any thoughts on the possible risks about the observation that silicone oils weep out of what are often called thermal gap pads? Is this normal/typical?
Result is light oily film on top of the device and on mating surface of the pad.
The compression is 15-25% of the nominal thickness - which is reasonable.
The ambient temperature is 60C and a thermocouple between pad and device is approx. 80C. Pad is 15mm sq x 3mm thk and is soft with tacky surface when installed.

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[mcgyvr]

Typically one should be worried about silicone contamination during the assembly process as it can have negative effects on solder joints,etc...
"weeping" of the silicone can be seen with higher than specified pressures and temperatures.
I'd suggest contacting the manufacturer and seeing if they can maybe recommend the proper clamping pressure for your application/temperatures or possibly a different material.

Post assembly silicone contamination is typically a concern in "high reliability" applications (aerospace/telecom,etc...) where it might effect the performance of certain devices. Many standards require the use of non-silicone thermal products.

 

[drawoh]

nigelt,

Are you using the thermal gap pads as a seal?

Rubber pads are not really all that conductive. It is best to make them thin, to minimize temperature drop. If I want something sealed by a sheet gasket, I want a relatively thick pad so that I get an acceptable clamp all around the variable gap. The gap is variable because of your mechanical tolerances.

--
JHG

 

[nigelt]

Reply from the manufacturer (respected N.A. supplier) is that the moisture is silicone oil - as suspected. In their view it is not a concern - which is not a surprising POV.
The oil does not appear until the unit is in service. So assembly is complete. Cosmetically there is no risk, now it's just down to whether we are afraid of this or not. I'd hoped to find some who had faced the same issue and what their reasoning was for accept/reject!
The pad is present only for thermal purposes. The compression is within acceptable normal range - around 20%. The application is not "critical" in the classic sense however there is a commercial video lens within the enclosure.

From another supplier website....."Many optical, medical, and sensor applications cannot allow silicone particles to be located within their enclosures - a line of non-silicone based thermal pads/Thermal Interface Materials (TIM) to eliminate any worries regarding siloxane gasses."
Need to check these out!