It's All About the Base, 'Bout the Base, No Trouble.
Heat Sink Bases: a Good Foundation for a Great Solution
Heat sinks come in all shapes, sizes, styles, and materials. But a common feature with many heat sinks is the base: the part of the heat sink that contacts the device(s) and supports extra surface area, typically in the form of fins, for increased heat transfer. The base is where different parts of the solution are connected together and has a number of responsibilities to fulfill in order to be an effective solution.
Purpose of Heat Sink Bases
Interface to Device(s)
The base of the heat sink is where engineers typically contact their heat sources that need cooling. Most electrical devices are designed to be contacted on the top, where heat has the path of least resistance away from the silicon chip. The finish and quality of the device surface and heat sink base are critical, especially in cases where no thermal interface material (TIM) is permissible in the application. Smoother surfaces will form fewer and smaller air gaps, so this is why base flatness and surface finish help determine the heat sink's actual thermal performance.
Since there is no such thing as a perfect surface in the application of thermal management solutions, we use thermal interface material to decrease this contact resistance (I go into more depth about interface resistance in another blog post). In fact, thermal interface material is so important for the complete thermal management solution that there are entire companies solely focused on just TIMs.
Besides surface finish and the home of the interface material, the actual geometry of the base is crucial to the success of the heat sink as well. Applications that are reducing costs by heat sinking multiple heat sources to a single base may have a challenge ahead of them. Not all devices are created equal: devices come in different sizes and shapes. Bases can be machined, cast, or otherwise modified to accommodate variations in device geometry with features like pedestals, through holes, or countersunk pockets.
Heat sink bases have another critical function and that is conducting heat away from your devices. Spreading heat away from the heat sources as quickly as possible helps keep the temperature of the device down, thus increasing its overall lifetime and reliability. Spreading the heat also enables the heat to be passed more evenly to the fins that are most likely attached to the other side of the base. This improves both the effectiveness and efficiency of your heat sink by sharing the load across as many surfaces as possible and not just the fins directly above the heat sources.
A really effective way of improving the spreading capability of a heat sink base is to embed heat pipes. Aavid Genie allows simple straight heat pipes to be modeled in the base. To be honest, that is only a small chunk of what is possible with embedded heat pipes. To the point where I'll have a future blog post on the subject manner.
Interface to Extended Surfaces (A.K.A. the Fins)
The next key function of the heat sink base is to transfer the heat conducted into it from the devices to the fins. For extruded or skived fin heat sinks, this occurs naturally through conduction as the base and fins are constructed of the same piece. It's when we get a little crazy and start adding on fins.
Heat Sink Bases that have fins added onto them may need some extra features to accommodate the fins. For zipper fins and folded fins, the heat sink base just needs to be flat. Bonded and brazed fin heat sinks on the other hand, they require grooves to hold the fins and joining material in place.
Finally, heat sink bases also provide mechanical support for the device(s) attached, especially those on a printed circuit board (PCB). Bases add rigidity to a somewhat fragile PCB. This can prove crucial for mechanically demanding applications, like those in transportation or consumer electronics. The nature of these applications are harsh and devices can be subjected to mechanical shock and vibration and daily abuse from its users.
Not Just All About the Base
Despite the title of this blog post, your heat sink isn't all about the base. Fins, flow, thermal interface material and other features play an important role in determining the amount of heat transfer you have away from your device. But the base is the center of all the action and a good one will help keep you out of trouble.