How to Select the Right Thermal Gap Filler Pad

When designing the next breakthrough device, managing heat is one of those challenges that can make or break your project. That’s where thermal gap filler pads come in. These precision-cut pads form a bridge between your heat-generating components and their heat sinks, using high thermal conductivity to channel heat away more efficiently than empty air. 

However, not all thermal gap filler pads are created equal. To select the right pads for your application, you’ll need to consider the following specs and characteristics: 

• Material type 

• Hardness and Conformability 

• Thickness and compression 

• Thermal conductivity 

• Dielectric properties 

• Environmental and regulatory requirements 

• Manufacturing and assembly  

In this post, we’ll break each of these down so you can make the best choice for your project. If you have questions, our team at ISP is here to help. Give us a call at (800) 984-1811 or reach out online. 

What are Thermal Gap Filler Pads made of? 

Thermal gap filler pads are typically made from silicone-based or non-silicone materials. 

• Silicone-based pads are the go-to for many applications because they can handle a wide range of temperatures. The downside is they can outgas volatile compounds that might not be ideal for sensitive electronics.  

• Non-silicone alternatives, like acrylic-based pads, are a great choice when you need to avoid any potential silicone contamination.  

Hardness and Conformability (Shore Value) 

A pad’s softness, or hardness, is critical for real-world performance. A lower Shore hardness value means a softer, more conformable pad. 

• Softer pads excel at filling small air voids on component and heat sink surfaces. This superior “surface wetting” minimizes thermal resistance but can make the pads more delicate to handle during assembly. 

• Firmer pads are more robust and easier to handle, especially for automated pick-and-place systems. The trade-off is that they may not conform as well to highly irregular surfaces. 

Getting the Right Fit: Thickness and Compression 

A thermal gap filler pad needs to be just the right thickness to fully displace the air between the component and the heat sink. While it might seem like thicker is better, a thinner pad allows heat to travel a shorter distance, making the transfer more efficient. More material equals more thermal resistance.  

You also need to consider how much the pad will compress. A good rule of thumb is a compression rate between 20% and 50%. You can find the compression rate using this formula: 

(Original Thickness – Compressed Thickness) / Original Thickness * 100% 

Be careful not to over-compress, as this can degrade performance of the pad or damage the components.  

How Well Does It Transfer Heat? 

Thermal conductivity (TC) measures a material’s inherent ability to transfer heat, measured in Watts per meter-Kelvin (W/m·K). The higher the number, the better the heat transfer. For demanding jobs, a TC of 5 W/m·K or higher is often preferred. Keep in mind that pads with higher TC values usually cost more. 

However, what matters most in your device is thermal impedance. This is the total resistance to heat flow, which includes the pad’s conductivity and the contact resistance at the surfaces. A softer, lower-TC pad can sometimes have a lower total impedance than a harder, high-TC pad because it conforms better.  

Electrical Properties Matter Too 

Even though you’re managing heat, a pad’s electrical properties can be crucial depending on your application. Some pads have a high dielectric breakdown strength, meaning they can withstand a strong electric field before they stop being an insulator. In radio frequency applications, a pad with a low dielectric constant is beneficial to prevent unwanted electrical interference. 

Environmental and Regulatory Requirements 

Depending on your product’s requirements, you may need a thermal gap filler pad that meets certain safety and environmental standards. Look for pads with a UL 94 V-0 flame rating, and those that comply with REACH and RoHS regulations for managing hazardous substances. It’s also wise to consider how the material will hold up over time, especially when exposed to high temperatures. Silicone-based pads, for instance, are known for their excellent stability even after long-term heat exposure.  

Manufacturing and Assembly 

Thermal gap filler pads can be cut into any custom shape you need, from simple squares to complex designs for specialized electronics. For small runs or prototypes, manual placement works fine. But if you’re scaling up, look for pads that work with pick-and-place systems. To make assembly even easier, consider pads that are laminated with a pressure-sensitive adhesive (PSA) backing.  

Working with a capable converter is key. They can deliver pads cut to your exact specifications in packaging that optimizes your manufacturing process. Speaking of which… 

Need a Hand? 

That’s where we come in. At ISP, we work with top-tier suppliers like Polymer Science and Henkel to source quality materials, then convert them into ready-to-install thermal gap filler pads tailored to your needs. We can work with your materials too, and we also offer protective packaging and PSA lamination so you get exactly what you need without juggling multiple vendors. 

To find the perfect thermal gap filler pad for your project, contact ISP online or give us a call at (800) 984-1811.