Active heat sink, fan-on-sink and other such names all define a heat sink type that contains a fan which directs air onto the semiconductor it is cooling (click to see a side by side example of an active heat sink with a passive heat sink). Are these kinds of heat sinks all they are purported to be or are their downsides to these heat sinks too?
First, let’s look at some benefits to using active heat sinks
- You can direct air flow to the hottest semiconductor’s in your system
- You may be able to makeup for poor internal air flow system design where you simply can’t get enough air to your processor and so a local air mover assist is needed.
- They may make sense if you design add-in boards with a hot main processor and you are not sure where your add-in board is going to end up being used and you must insure adequete airflow.
- Active heat sinks are a terrific choice for directed air onto prototype boards or chips during bench debug. Rather than taking a small desktop fan and blowing it on your alpha board full of probes, clamping or taping on an active heat sink onto the target processor can be an excellent way to spot cool your hot chip while you debug.
As with any active device introduced into a system, there are downsides as well.
- Active heat sinks can have fan failures, resulting in the components they are cooling overheating or kicking into a thermal shutdown.
- Active heat sinks are many times (though not always) very tall, limiting the kinds of cases or chassis they may be used in.
- Active heat sinks may not necessarily be increased in size to take advantage of more surface area for convection cooling.
- Active heat sinks can be expensive and choosing one can be tricky. The least expensive version may house fans using substandard components. The better ones may use fans with ball bearings but the price can put this solution out of a designers target cost budget.
If you are starting your system design from the beginning, the best move is to design the system to optimize cooling.  Insure your PCB boards are designed in such a way as to optimize for thermal design. Check your system to design for the best system fan placement. Create your system design so that flow is designed without blockages or potential for air pooling.  Finally, you want to make sure that if you do need heat sinks, that the combination of size, thermal interface material and heat sink attach are used to get the most that you can out of a passive heat sink approach.