Since it is not feasible to form
a copper base plate with integral heatpipes, we can only hope that companies
are properly soldering joints and using high thermal conductivity solder for
that purpose. Without general access to the formula of solder used on a heatsink we can't
say either way... but just like thermal interface materials, different alloys
solder offer improved thermal conductivities.
There's a good article on electronics-cooling.com that goes into detail on the subject
of thermal conductivities of different solder alloys that we recommend for
further reading. The chart above breaks it down clearly. In pre-RoHS days the most common solder
used in heatsink manufacture was Tin-Lead
(SnPb), it has a thermal conductivity of 50 W/mk. For comparison's sake
Aluminum and Copper have thermal conductivities of 247 W/mK and 398 W/mK
respectively. Good quality thermal compound generally has a thermal conductivity
of between 1-4 W/mK, which is why it's so important to apply it sparingly!
In post-RoHS manufacturing Lead is most definitely out,
so it's quite possible heatsinks are being made with low melting
temperature Bismuth-Tin (BiSn) solder that has a relatively poor thermal
conductivity of 19 W/mK... so ask yourself - what's the thermal conductivity of the solder used in your heatsink?