1) As a metal interface, they are thermally conductive and will spread heat quickly away from a heat source.
2) As a soldered attachment, they will form intermetallics with base metals. This bond is strong and will remain mechanically reliable for the lifetime of the device.
3) This solder bond will be electrically conductive.
If the primary reason for implementing a solder bond is purpose 1 (thermal attachment), a void-free solder bond is highly desirable. This will maximize the thermal dissipation through the high K solder.
Void-free soldering is typically achieved on gold-plated substrates. There are particular tactics for soldering to gold. Some of these involve the solder choice. Popular solder alloys include Au/Sn, tin alloys, or indium solders. Details regarding these material choices are outlined in our application note on soldering to gold.
For more information on assembly techniques when soldering to gold, check out my previous posting on soldering to gold.