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IC Thermal Pad: Best Design and Usage Practices

Key Takeaways

  • Thermal pads and thermal paste are two methods for attaching a heatsink to an integrated circuit.

  • If you must use a heatsink, you might consider using a thermal pad over mechanical attachment or thermal pastes.

  • Other measures can be used to help dissipate heat, such as die-attached paddles, fans, or more complex active cooling measures.

IC thermal pad

Thermal pads provide a high thermal conductivity interface between an IC and a heat sink.

The first time I pulled a heatsink off an old CPU, I saw a thick slab of goop on the top of the chip. “It must be the glue they used,” I thought to myself. Little did I know, there is another option to thermal paste: high conductivity thermal pads and other thermal interface materials that draw heat away from a hot IC. These materials are easy to work with and provide a simple way to attach a large heat sink to an IC.

An IC thermal pad can provide sufficient heat dissipation away from a high-temperature component when compared to thermal paste, but thermal paste may be a better option in certain situations. The answer relies on whether you plan to replace the IC, the level of thermal conductivity Here’s when you might consider an alternative to a thermal pad for cooling your components.

IC Thermal Pad Options

Thermal pads for ICs attach directly to an integrated circuit and a heatsink. These materials, as well as thermal paste, are generally called thermal interface materials. These materials are intended to provide two functions:

  1. Act as an adhesive for attaching to a heatsink to an IC;

  2. Provide a high thermal conductivity path for heat to travel from the IC to the heatsink.

The effectiveness of an IC thermal pad depends on its thermal conductivity, and the thermal conductivity will determine the equilibrium temperature of the component while it operates. There are many different thermal pad materials available on the market, ranging from silicone materials to graphite and elastomers with diamond. Graphite is known to provide a low cost, high conductivity solution, but it also has known reliability problems with low lifetime. Silicone-based IC thermal pad materials are the current entry-level standard. Finally, elastomers with diamond have the highest thermal conductivity, but they also have the highest cost.

Thermal Pad vs. Thermal Paste

Thermal pads come packaged as a small sheet of material that can be cut down to the desired size. When a thermal pad is placed between the IC and the heatsink, any adhesive on the pad will bond to both surfaces and will leave behind some gaps. These small air gaps will provide slightly higher thermal resistance as they trap air, which has lower heat transfer conductivity. These materials may be firm at first, but they can become spongy once they reach the intended operating temperature. A clamp is often used to apply enough pressure to obtain sufficiently low thermal resistance.

CPU and IC thermal pad

Applying a thermal pad to a CPU

Thermal pastes are available in tubes and can be easily applied across the surface of an IC. The heatsink is simply pressed across the top of the paste and the paste eventually dries and hardens. These pastes require some solvents to remove, but they provide a very uniform, cohesive interface between the IC and the heatsink. In addition, these compounds can have very high thermal conductivity values, most notably pastes that include diamond.

IC thermal pad vs. thermal paste

Silicone-based thermal paste

There are other dimensions to consider when selecting a thermal pad, thermal paste, or another thermal compound for attaching a heatsink. The table below shows a more complete comparison of thermal pads and thermal pastes for attaching heatsinks to ICs.

 
 

Thermal pad

Thermal paste

Coverage

Leaves microcracks and bumps between the IC/heatsink and the pad

Very uniform coverage as the paste fills in all microscopic ridges on the IC and heatsink

Cleaning and replacement

Easy to apply, remove, and replace

Easy to apply, can be messy, and takes solvents to remove completely

Reliability and lifetime

Usually rated 1-3 years, can become brittle after repeated thermal cycling

Usually rated 3-5 years, known to outlast thermal pad materials

Mechanical strength

Weak; can be torn off by hand

Much stronger than thermal pad materials

Outgassing

Silicone-based materials can outgas, can pass the NASA Outgassing Specification

All pastes will outgas, but they can pass the NASA Outgassing Specification

 

Whether you attach a heatsink to your IC with a thermal pad or thermal paste, you should follow these two guidelines:

  1. Always ground out the heatsink as it can be a source of EMI. The heatsink can receive a current from a switching IC, and grounding the heatsink dumps any induced or displacement currents to ground.

  2. Determine whether you need compression with a mechanical clamp. This will protect the heatsink from shifting or damage, and it will keep the heatsink attached if the thermal material should fail.

  3. If you must attach a heatsink to the energized pad on a power FET or other power electronics, you should use an insulating thermal pad, sometimes called a dielectric pad. These materials provide high thermal conductivity with high electrical resistance, and they have high dielectric strength to prevent ESD.

How a Thermal Pad Affects Thermal Resistance and Steady-State Temperature

Choosing a thermal pad requires understanding the steady-state temperature your system needs to run at, in order to prevent component failure. When the thermal resistance of your interface material is lower, the steady-state temperature of the component will also be lower. The problem of determining the steady-state temperature is rather simple to solve with the time-independent heat equation. Simply follow the standard procedure for solving the associated Sturm-Liouville problem for the system in terms of the eigenmodes of the system’s Laplace equation.

If you want to consider a heatsink with a complex shape, you’ll need to take a numerical approach. Standard 3D thermal field solver packages can take your heatsink geometry and can be used to calculate the steady-state temperature. Datasheets will show you the thermal conductivity value of your thermal pad material, which can then be used in a thermal simulation to calculate the steady-state temperature of the component.

If you need to turbocharge your cooling strategy, then you need to use a fan with your heatsink to provide additional convective cooling. You could also use a heat spreader to provide the same effect. If you’re operating at the data center or high-performance computing level, you may consider adding liquid cooling or phase change cooling to critical components. These other cooling strategies can also be used in a thermal simulation to determine the steady-state temperature of your component.