Thermally Conductive Silicones Explained

Thermally conductive silicones move heat away from sensitive components such as the central processing units (CPUs) on printed circuit boards (PCBs). These elastomers can be cut from sheets, molded into parts, or applied as pastes. Although some thermally conductive silicones are used at the PCB level, others are used within electronic enclosures. Applications include the battery compartments on electric vehicles (EVs), the end effectors on industrial robots, and lighting systems for illumination and agriculture.

Elasto Proxy uses water jet cutting to convert thermally conductive silicones from sheet materials into thermal pads, a type of gasket. There’s no tooling to pay for or wait for, and this cutting method is ideal for low to medium volumes. As part of the value that we offer, Elasto Proxy can supply you with thermal pads that have a taped backing for peel-and-stick installation. You could use thermal pastes instead of pads, but pastes are messy and difficult to apply with the right thickness for some applications.

Contact us to discuss your application or keep reading to learn how thermally conductive silicones work and how to use them. We’ll start with a basic definition of heat, consider how heat moves, and examine heat sources and heat sinks. You’ll also learn about thermal conductivity (TC) and how it’s measured. In addition, you’ll compare thermal pads that contain fabric to unsupported thermal pads that do not. Silicones are known for their high-temperature resistance, but there’s much more to them than that.

What is heat – and how does it behave?

Heat is the flow of thermal energy. It’s responsible for the temperature within a system, such as a PCB that’s populated with electronic components or a lighting assembly that contains light-emitting diodes (LEDs), compact fluorescent lights (CFLs), or high-intensity discharge (HID) bulbs. High heat can damage these systems and cause them to stop working or malfunction. In the case of electric vehicles, excessive heat can cause EV batteries to ignite.

Because it’s the flow of thermal energy, heat doesn’t stay still. Instead, it moves between objects – ones that have different temperatures. According to the second law of thermodynamics, heat always flows from a hotter object to a colder object. Seen another way, it’s impossible for heat to flow from a cooler object to a warmer object. By putting this scientific principle to work, you can design thermal management solutions that move heat away from sensitive objects.

How does heat move?

Heat moves in three different ways:

• Conduction relies upon direct contact between two materials.
• Convection uses a fluid such as water or air to move heat.
• Radiation transfers heat through electromagnetic waves.     

The appliances and cooking utensils in your kitchen provide some examples. Let’s say you put a saucepan that’s filled with water and noodles on a hot stove. Conduction between the burner and the saucepan causes the pan to heat up. Convection between water (a fluid) and the noodles causes your food to cook. Convection also occurs when you put a steak in the oven. Heat from coils moves through the air (a fluid) and causes your steak to cook.

Radiation, the third form of heat transfer, is also right there in your kitchen. When you put a cold piece of pizza in a microwave oven, electromagnetic waves (i.e., microwaves) cook your food. Yet, ovens that use radiation aren’t only used in kitchens. Across the electronics industry, infrared (IR) ovens are used to cure adhesives so that they acquire their end-use properties. These IR ovens use wavelengths from a different part of the electromagnetic spectrum, but the method of heat transfer is still radiation.

What are heat sources and heat sinks?

Let’s stay with the electronics industry since it provides some examples of heat sources and heat sinks, terms you need to know in order to understand thermally conductive silicones. At the PCB level, CPUs and other chips produce heat and, therefore, are heat sources. We know that if this heat isn’t dissipated, electronic circuits can malfunction or fail. That’s why your desktop computer has a fan, but convection alone doesn’t provide all of the cooling that’s needed.

A heat sink increases the flow of heat away from the heat source and absorbs some of this heat. If you open up the case on your desktop computer and look at the motherboard (the PCB that contains your CPU), you’ll probably see a metal object with fins. Because metal is a good conductor of heat, heat sinks like this are usually made of aluminum or copper. With some help from your computer’s fan, hotter air moves across the cooler metal fins, which absorb some of the heat. But that’s still just convection.

Where do thermally conductive silicones fit in?

Normally, silicone rubber is a thermal insulator. It has a low thermal conductivity and does not easily transfer heat between objects. If you’re familiar with some of Elasto Proxy’s other products, you might know about SH-500-58-ZMT engine bay insulation. It contains a layer of silicone foam that reduces the movement of heat between the engine compartment and the cabin. This insulation is mounted on a metal firewall that would otherwise conduct heat from the engine.

So, how can silicones be thermally conductive? It involves changing their recipe. Like other types of elastomers, silicone compounds contain ingredients such as fillers, pigments, and additives. Still, the easiest way to understand these compounds is to return to the kitchen. This time it’s for dessert. Like ice cream, silicones are made by mixing ingredients together and then putting this mixture through a final process. With ice cream, that process is freezing. With silicones, that process is curing.  

Now think about the difference between vanilla ice cream and strawberry ice cream. The process of making them starts out the same. However, it’s the addition of strawberries that makes strawberry ice cream taste the way that it does – and not like vanilla. With thermally conductive silicones, it’s the addition of fillers like aluminum oxide and boron nitride that give the compound the thermal conductivity it would otherwise lack. Unlike plain silicone rubber, these fillers have high thermal conductivity.    

How is thermal conductivity measured – and what about temperature?

It’s easy to talk about high vs. low thermal conductivity, but you can’t just rely on comparisons. You need to know specific values (or ranges of values) and units of measure. Thermal conductivity (TC) measures a material’s ability to transfer heat and is expressed in watts per meter Kelvin (w/mK).  As a rule, materials with higher thermal conductivity have a faster rate of heat flow than materials with lower thermal conductivity. Keep that in mind if your design needs to move heat quickly or can do so slowly.   

Also, note that thermal conductivity is measured at a particular temperature – and that TC varies by temperature. Therefore, you need to identify the temperature range for your application. For example, different materials have different thermal conductivity measurements at 25°C (77°F). Aluminum and copper both have higher levels of thermal conductivity than lead or nickel, other common metals. Gold is even more thermally conductive, but it’s too expensive to use for heat sinks.     

Price isn’t the only other consideration, however. Filling a silicone with aluminum oxide, boron nitride, or other materials increases its density and, therefore, hardness. If you’ve designed gaskets before, it helps to think about door seals. If the rubber material is too hard, the door won’t shut or stay shut. If the rubber material is too soft, the door gasket will over-compress and admit the elements. With thermally conductive silicones, sheets between 10 (softer) and 85 (harder) durometer Shore A are available.

But how do thermally conductive silicones work?

Think back to what you’ve learned about how heat moves and the relationship between a heat source and heat sink. Thermally conductive silicones aren’t heat sinks themselves, but they help with cooling by drawing heat away. But why do heat sinks need this “help” if they’re made from materials, such as aluminum and copper, that already have high thermal conductivity? After all, the filler materials for thermally conductive silicones can’t match the w/mK values for metals.

Like door and window gaskets, thermal pads that are made from thermally conductive silicones are designed to fill gaps. More specifically, they fill air gaps between the heat source and the heat sink. Air is a fluid that moves heat, but its thermal conductivity is just 0.024 W/mK at 0°C (32°F) – and air’s TC doesn’t get a whole lot better as temperature increases. Therefore, it’s important to fill the tiny air gaps caused by surfaces that aren’t perfectly flat and smooth.

Let’s take a trip back to the kitchen to consider this point. Have you ever tried boiling water in a saucepan with a dented bottom? The saucepan takes longer to heat up, and the water takes longer to boil. It’s because there’s irregular surface-to-surface contact between the top of the burner and the bottom of the saucepan. Even with the best of saucepans, there’s always imperfect surface-to-surface contact; however, a pan with a dented bottom won’t even touch the burner in places.

What else is important to know about thermally conductive silicones?

Thermally conductive silicones can be reinforced with fabric for added strength. This fabric is added when the materials are made into sheets, or when thermally conductive silicones are molded in parts like  O-rings or grommets. As a rule, supported materials have faster rates of heat transfer. That’s important to keep in mind if your application requires not just added strength, but the more rapid dissipation of heat away from sensitive components.

Finally, you’ll need answers to the following questions.

• What is the thermal conductivity (w/mK) that’s required?
• What is the temperature range?
• What is the material density (hardness)?
• Do you need supported or unsupported thermally conductive silicones?
• How many parts do you need?
• How fast do you need them?

There’s a lot to consider, but the thermal pads you need are just a few clicks away. Contact Elasto Proxy to request a quote or talk to our team about your application.