Heatsink Fin to Base Thermal Resistance- Epoxy vs Braze 1

[bparris]

I'm investigating the trade-offs between bonding fins to a heatsink using thermally conductive epoxy or brazing. Epoxy is desirable because of cost but I need to quantify the impact on the thermal resistance of the fin-base interface. I can think of some basic hand calculations but I was wondering if there are any articles with correlations for this type of thing. I also need to find a thermally conductive epoxy that can withstand exposure to jet fuel and chemical solvents. Thanks.

 

[IRstuff]

Please do not cross post

TTFN

 

[sreid]

An alternative is R-Theta's swaged fin heatsinks.

http://www.r-theta.com/

 

[bparris]

IRstuff - I did not cross post. The two threads I started are quite different.

 

[MikeHalloran]

I agree that your posts are different ... but not a lot.

I'd also worry about finding an epoxy that will stay bonded through the required military range of storage and use temperatures, and thermal cycling.

I remember the US Navy as being very, very conservative about materials; will they even allow epoxy in this service?



Mike Halloran
Pembroke Pines, FL, USA

 

[bparris]

They allow epoxy if it has been tested to show chemical resistance to jet fuel, chemical solvents, etc.

This thread is regarding a thermal / heat transfer question, the other is a materials compatibility issue. They are fundamentally different and each one lends itself to a different forum. Is there a better way to have handled this?

 

[volpe2]

In general thermal epoxies have a thermal conductivity of around 5 W/m*K. An aluminum braze allow will have a thermal conductivity of around 150 W/m*K.

Thermal resistance R
Material thickness, L
Thermal conductivity, k
Cross sectional area, A

R = L/(k*A)

In order to make up for the lower thermal conductivity the epoxy will have to be much thinner, or over a larger area. Well designed brazed joints are .001/.002 thick, which makes it difficult to get the same preformance from epoxy.

Press fit fins are are another option. Conductive grease can be used and the interfernce fit will force it to very thin.

sreid's option also looks viable.

Matt Evans
Moorestown, NJ

 

[thermalguy]

There is lots of good input here, but here's another take on the subject. The Navy uses epoxies all the time for bonding aircraft together, for coating flight decks, lining fuel tanks, coating sensors, bonding sonar arrays..etc., so theres nothing wrong in concept with the material. Epoxies are typically not bothered by salt spray and they can be very resistant to solvents like jet fuel. They are not going to conduct heat as well as metal regardless of filler. However, this might not have any significant impact on the heat sink performance. If the bond is thin and the area large the difference between brazing and thermal epoxy bonding could be so small that it's difficult to measure. You can easily calculate this using the resistances refered to elsewhere in this forum and add in the thermal resistance of the fin/air interface. There are two caveats however: 1)in very high rates of forced convection the fin to base resistance will increase as a percentage of the overall heat sink thermal resistance and 2.)Don't use the glue above the glass transition temperature of the adhesive since all the good properties like chemical resistance and mechanical strength change significantly.

In a heat sink