Have you ever been so hot you thought you might melt like a popsicle? Summer down here in SoCal gets pretty toasty and makes me wish I had a few freeze pops. When I get home from work I love to blast the AC and pretend that I live in an igloo. However, the only thing worse than boiling in the heat is seeing all my money evaporate away in cooling bills. Other people take a different approach to cooling; they use geometric shapes or heat exchangers to passively cool their houses. I may laugh whenever I see an ugly geodesic dome home, but the owners are the ones who get to keep their cool and their money. Cooling your PCB can be just as annoying as cooling your house. Active heat mitigation systems suck up electricity and take up too much space. Lucky for us, PCBs can also benefit from passive cooling. Some of the primary powerless cooling techniques include natural convection, heat spreaders, and heat exchangers. There’s even a groundbreaking droplet cooling technique that could come out in the next few years.
Benefits of Passive Over Active
Usually, I prefer an active cooling technique, like air conditioning, over a passive technique, like sweating profusely. The problem with active cooling in PCBs is that they usually take up too much power and too much space.
Power usage always seems to creep up on you. I never realize that I’ve spent several hundred dollars on cooling my house until I get the bill. The same thing happens in circuits. The primary ICs are already pulling tons of Watts, so who cares if you put in a low wattage fan. Well, if your application is low power, the extra draw could mean a much shorter battery life.
Some designers have the privilege of using as much space as they need. Unfortunately, I am not one of them. Space is a precious commodity on most of my projects. This means that I need to minimize space consuming active cooling components, like fans, to make room for circuits.
Passive cooling systems use natural phenomena to cool down boards, meaning they don’t need any more power to work. They’re also generally smaller than active systems, though that can vary based on which design you choose.
Heat sinks look cool and well help keep your PCB cool too.
Passive Cooling Systems
There are certainly a few other cooling techniques than the ones I’ll mention here. However, these are some of the most common chilling schemes.
Natural Convection/Heat Sinks - Natural convection is what it sounds like. While you don’t necessarily have to use heat sinks to take advantage of this, they are well suited to it. When arranged correctly, heat sinks can use the natural airflow to dissipate heat without using power. If you want to design the optimal heat sink you’ll need to crunch a few numbers to get efficient airflow and minimize the boundary layer. This solution is extremely simple but takes up more space than other passive approaches.
Conduction and Radiation/Heat Spreaders - Where convection involves heat transfer in fluids, radiation and conduction deal with solids. Heat spreaders normally are not the backbone of your thermal resistance management, but instead augment it. Basically, any piece of metal acts as a heat spreader since it will conduct thermal energy. To best use conduction and radiation, you should try to direct as much heat as possible to the outside layers of your board, where it can be radiated more efficiently. You can use thermal vias to conduct heat, instead of electricity, to the outside of multilayer boards. Large power and ground planes will also help dissipate heat because bigger areas will radiate more effectively.
Heat Exchangers - Heat exchangers are often used in conventional AC units. They use a refrigerant and a vaporization/condensation cycle to transfer heat from one area to another. In PCBs, there are two main types of heat exchangers: heat pipes and cold plates. The main difference between these is the form factor; heat pipes are thin and cold plates are wide. This kind of system can be very small, and it transfers heat efficiently. It also has a low chance of failure since it is a self-contained system with no mechanical parts.
Droplet Cooling - This experimental kind of cooling involves the same kind of process found in heat exchangers. The main difference is that the fluid in heat exchangers travels a relatively long distance horizontally, whereas here the fluid travels a short distance vertically. The nice thing about this system is that it automatically reacts to hotspots that can form on integrated circuits (ICs). In addition, it can conduct and radiate thermal energy like a heat spreader for power dissapation, but with greater effect, because it can conduct in both the horizontal and vertical directions.
This is what happens when you don’t have any kind of cooling.
I may not be able to help you beat the summer heat, but these techniques will help you cool off your boards. All of these passive processes will save your circuit power, and will generally use less space than an active system. If power and space are big concerns for you, try one of these systems.
Now that you’ve got a great cooling system, you need a PCB material to go with it. If you’re going to be designing boards, I would recommend you use CircuitStudio. Its plethora of advanced tools will help you keep your cool while making layouts.
Have more questions about thermal pad management? Call an expert at Altium.