Heatsink on aluminum lid?

[riverbeer]

Bought this project box from radio shack. It has a plastic and an aluminum lid. I have an ext. mount heatsink. Can I mount it directly to the alum. lid or do I need a cutout? If so, can I use the plastic lid? Thanks

 

[BadNewsBrewery]

So the objective is to minimize the thermal resistance. Aluminum is a fairly good conductor, but is not as good of a conductor as not having the aluminum there at all. My point - if you use high quality thermal paste and get the heat sink and SSR squarely mounted to the aluminum lid, it shouldn't be a big deal. Done right, there's a chance that the lid acts as part of the heat sink as well, though it will radiate heat back into the box, too.

Personally - I'd use the plastic lid and mount the SSR directly to the heatsink.

 

[Bensiff]

Yup. I'd go with the direct contact. Less room for trouble spots should the lid not be exactly flat or you don't get the torque on the screws even causing a slight uneven mount.

 

[alien]

I've tried this both ways. Having the SSR on the outside and screwing the SSR through the metal lid into the heatsink makes construction easier, but making a hole and mounting the SSR directly to the heatsink is better because there is only one surface contact instead of two, and because less heat is radiated back into the box.