Layers of Thermal Interface Compound

[Transient1]

I have a situation where the bottom of an electrical component must be covered in one compound (k~1.25 W/m-K) in order to encapsulate the tin on its base. This is for a space application. The component's thermal path through the leads is poor and it requires a direct bond to the PCB. The same material that is used to protect the bottom of the component cannot be used due to difficulties appling it to the board (its very low viscosity).
So we use our standard component to PCB thermal compound (~.63 W/m-K). There is not a chemical reaction between the two different compounds (they've come in contact before). I assume the interface coverage is very good (no voiding). So the thermal resistance is just the path through two materials of different thermal conductivity. Is there anything I might be missing with this application? (By the way the combined bond line thickness is between .030 and .040 mils).

 

[tkordyban]

The only other thing I would recommend is to make sure the two interface compounds have good wetting with each other. A brute force way to check this is to try bonding them together, and then peel them apart. How the bond breaks between them will give you a good idea of how well the surfaces of the two materials were in contact. If the wetting looks good, then there will be good thermal contact. If you see areas of the joint that did not bond well, then you probably will have extra thermal resistance in the joint, which will be hard to quantify. If you see that, then you probably want to look for materials that like to stick to each other.

Tony Kordyban

 

[IRstuff]

If this is a space application, you might need to be certain that your compound doesn't outgas and deteriorate in a vacuum. Outgassing can affect vehicle stability and possibly contaminate optical sensors.

TTFN

FAQ731-376

 

[Transient1]

Tkordyban,

Thanks for the test idea. I might even have that data lying around all ready.

IRstuff,

Thanks for the input. Both are space qualified and have acceptable outgassing levels.