H is the rate at which heat is being transferred. This is also equal to the amount of heat, Q(heat), over the period of time(t). The higher the better, it's just like going fast in a car, except here we are just pushing heat out faster.
The second part of the equation is where all the fun begins. The symbol k is a property that comes from the metal involved. From the equation, you can see that if the value k is higher, we will be conducting more heat. The k values for aluminum, copper and silver are respectively: 235, 401 and 428. As you can see, copper and silver thoroughly pound aluminum into the ground, by almost doubling their conductivity. Most heatsinks on the market are made out of aluminum because it is a decent conductor of heat, but also because it is very easy to work with. There are a great many cheap ways to work with this material. However, copper is being used more often, some manufacturers are even making solid copper sinks, while others opt to include a copper bottom. What does all this mean to you? Decisions - and lots of them.
We've only seen one heatsink that uses silver, although we still haven't seen a solid silver heatsink, as cost would be quite a factor. Silver currently goes for about four to five dollars an ounce; copper and aluminum are by far less expensive. Aluminum costs about four cents per ounce, with copper costing only slightly more.
The letter A
A is the value for area, in our case surface area. From the equation, you can gather that the greater the area, the better the rate of heat transfer. DT is the difference in temperature observed on either side of the heatsink. The greater the difference, the better the rate. Dx is the thickness of the slab of metal. For simplicities sake, lets just assume we have a cube of metal. Dx would be the distance from the side that is being heated to the side that is releasing heat. In the case of real heatsinks, Dx would most likely be the distance from the heated side to the point where the fins begin. From the equation, we can see that having this value small would be beneficial. So if you ever see heatsinks that are really thick, stay away.
The properties of thermal conductivity are not the same as those of electrical conductivity. This is readily apparent if we look at speaker cables. Most good speaker cables are made from copper, normally oxygen free. As an electrical conductor copper is almost twice as good as aluminum, in turn silver is slightly better than copper. If we want to get really expensive, gold is an exceedingly good conductor of electricity, which explains why everyone wants to gold plate every connector.