What, that they spread heat? ...if they are polished aluminum with e=1.3.
Heat spreaders are a conduction mass sink device, and surface quality
has nothing to do with it.
You bond it to the surface of the box, just as was stated, and then you
attach the source, just like you would attach your transistor or whatever
source to any other sinking device.
The ideal spreader is thick enough to spread the heat over the target
width or a width that is wide enough to make the external interface
efficient enough to sink the source OVER TIME. It is possible to make it
a small, pyramidal enlargement of the original heat source package
conduction tab/surface. There is a point at which you have overcome the
thin box wall issue and need no larger spreader.
On the OUTSIDE, however, you are in the realm of conduction cooling as
you need to sink that internal spreader footprint plus a slight bit
larger into a MASS that either leads to MORE mass, or an efficient fin
array or chiller source.
In reality, all sinks, regardless of type, be they convection cooled
finned devices or copper tubed modern jobs, are "conduction cooling
elements" from the POV of the heat source. THAT is the device that needs
to have it being bled off as fast as it is being created via dissipation.
SO the FIRST thing it needs is a mass of a determined size that has
fast conduction properties.
Then, the bleeder mechanism of choice weighs in. This is why many newer
sinks for CPUs have been a copper slug, press fitted into an Aluminum
finned array. I have also seen, and collected discarded solid copper
sinks.
So, if the finned sink likes the fast conduction slug interface to the
heat source AND the finned array, then the INSIDE of the box having a
good, fast conduction mass in it to make up for the issue the thin wall
would cause is the right way to conduct the heat to the outer sinking
mass, and subsequent bleeding (sinking) mechanism (fins, more mass, etc).
I have one here.
http://www.flickr.com/photos/nonlin/4773788504/in/pool-98328567@N00/
It is an inch and a quarter by an inch and an eight solid Copper slug,
surrounded by the best Aluminum fin array I have seen in a while.
The first AMD cpus that came out running very hot, and if the sink was
not intimately mated the CPU would fry and take out the whole MOBO with
it.