Thermal Pads vs Thermal Paste: How Best to Beat the Heat

When it comes to hot days, you can either hide from the heat, or enjoy it. For those who hide from the heat, you can usually find them inside with the shades drawn and parked in front of an air conditioner. For those who embrace the heat however, they are the ones floating on the water nestled in an inflatable pink flamingo, or lounging by the pool working on their tan. As for me, I’ve done both and am happy fitting into both worlds.

Your printed circuit board doesn’t enjoy the heat, though; not one little bit. It is designed to operate up to a certain temperature that shouldn’t be exceeded, and bad things can happen to it if there is too much heat. There are some things that you can do to help dissipate the thermal problems on your circuit board though. Using thermal pads or paste to conduct the heat from hot parts into heat sinks are good options. Let’s take a closer look at thermal pads vs thermal paste to see how they can help.

What are Thermal Pads vs Thermal Paste?

The first thing we will do is to define what thermal pads are in this context. For the PCB layout designer, a thermal pad usually means a large area of metal that either a device solders to, or a heat sink bolts onto. Thermal pad is also known as a “thermal relief pad,” which are small voids in a metal plane around a thru-hole pin. In both of these cases, thermal pads are physical features that are designed into the PCB to help manage heat either for cooling components, or to assist in the soldering process of a thru-hole pin.

The thermal pads that we are talking about here though are small pieces of thermally conductive material that transfers heat between objects on a circuit board. These pads are made from electrically insulating materials that conduct heat, such as a combination of silicone and ceramics, and are tacky allowing them to be easily applied to a PCB assembly. Thermal pads are normally positioned between a hot component and a heat sink to help conduct the heat from the component into the sink. Thermal pads can be much easier to work with then thermal paste which is messy and has to be applied with a syringe.

Thermal paste being applied with a syringe

More About Thermal Paste

Thermal paste, also known as thermal grease or heat sink compound, conducts heat in the same way as a thermal pad does. Thermal paste is made out of similar materials as the thermal pads, but in a liquid form that can be manipulated to fit the area that it is needed in. By applying thermal paste between a hot component and its heat sink, the paste will fill in any air gaps between the two. These gaps if left unfilled would act as thermal insulators which in turn would hinder the heat from being conducted into the heat sink. By filling these gaps though, the thermal paste maximizes the heat transfer and its eventual dissipation through the heat sink.

The big advantage to thermal paste is its ability to be easily spread. It will conform to uneven surfaces very well, and fill large gaps more evenly than thermal pads. This makes thermal paste more universal when working with unusual shapes and configurations of components. Another advantage is that with the ability to apply it as a thin layer, thermal paste can provide better thermal conductivity then the thicker pads.

The right design tools, like OrCAD PCB Designer, will give you the control you need

What’s the Best Course of Action for Thermal Pads vs Thermal Paste?

So which is better, thermal pads or thermal paste? Each has its pros and cons, so let’s review those:

Thermal Pads:

• The application is clean and without mess.

• The material is easily worked with, and can be cut to specific sizes.

• Thermal pads come in a broad range of materials allowing for custom applications.

• Thermal pads are more expensive however, and cutting and fitting them will add to your circuit board’s manufacturing time.

Thermal Paste:

• Thermal paste is inexpensive compared to thermal pads.

• Paste has proven to be very reliable in its different applications.

• Paste is easily applied, fills uneven gaps, and provides a much thinner interface which provides for better thermal conductivity.

• On the other hand, thermal paste is messy and can dry out, plus it isn’t as mechanically robust as a pad.

The truth is that there isn’t one clear winner over the other as each has its strengths and weaknesses. It all depends on the specific application. The important thing then is to carefully evaluate the thermal needs of your board, and then choose which one will best serve those needs. You may even find that you need to use a combination of the two in order to give yourself the best thermal coverage.

In addition to using thermal pads or thermal paste, you also need to design your PCB with proper thru-hole and surface mount pads, trace routing, power planes, and thermal relief pads. This will go a long way in helping you to manage the thermal conditions of your circuit board. To do this you need to work with PCB layout tools that will give the maximum flexibility in setting up these different design parameters. You need precise control over your PCB pad shapes and land patterns, as well as design rules that will control the size of traces and thermal relief pads.

The good news is that the PCB design tools that you need for this level of precision are already available from Cadence. Among their different high performance EDA tools is OrCAD PCB Designer, which has the power, flexibility, and control that you will need for good thermal management of your PCB designs.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.