Are you in the automotive systems sector, a power electronics engineer, or just an industrial machinery operator looking to invest in a thermal interface material for your project?
Choosing the appropriate hardness for a thermal pad will definitely benefit your company.
In the following article, we’ll describe the compression properties, Shore hardness values, and factors to consider when selecting the hardness of the thermal pads.
Thermal Pad Hardness Explained
By definition, hardness is a measure of how resistant a material is to permanent deformation when a standard force is applied on its surface.
This process determines whether a thermal interface material conforms to irregularities, the compressive force your assembly fasteners require, and how well it maintains contact at the interface during countless thermal cycles.
High hardness implies stiff material that is not easily deformable; this offers better structural integrity in high-vibration conditions and requires more force.
Additionally, there is a relationship between hardness and thermal conductivity; as hardness increases, thermal conductivity increases but conformability decreases.
Thermal Pads Shore Hardness
A standardized durometer is an instrument that measures the resistance a pad offers when you apply a certain load, also known as Shore hardness.
· Shore 00 Hardness
We regard the Shore 00 as the correct scale for very soft, compressible thermal pads, such as a gel. It has a hardness range of 0 to 100, with low numbers for soft, putty-like materials and high numbers meaning hard rubbers.
Here, we consider anything with less than 45 Shore 00 to be a very soft and malleable material.
For devices such as graphics cards and mainboards that span components at varying levels, a Shore value range between 25 and 70 is most useful.
· Shore A Hardness
Apply Shore A rating to firm silicone rubber compounds and thermal pads above the upper limit of Shore 00 hardness. They provide improved rigidity, ease of handling during automated assembly procedures, and increased resistance to extrusion under mounting pressure.
Using it where the mounting pressure is even for assembling components with flat, parallel mating surfaces is ideal.
Thermal Pad Hardness and Compression
· Hardness and Compression Force
For a project to attain a certain deflection percentage, the compression force is directly proportional to the Shore hardness value. For instance, a soft Shore 00 25 pad will deflect 50% at low mounting pressure, whereas a 40 pad will need greater clamping pressure to attain the same degree of deflection.
· Compression Percentage
It is always recommended to use compression ratios between 10% and 30% to ensure good contact without putting undue mechanical stress on electronic components.
The soft Shore 00 40 material compresses to about 13% at 69 kPa (10 psi), to offer good thermal interface without damaging devices that are affected by pressure.
Along those lines, you need to make sure that you apply enough force during mounting.
· Compression and Surface Conformability
Did you know that surface irregularities, the height difference between adjacent elements, as well as the warping of the PCB, match Pads with softer material properties better than pads with higher hardness?
This implies that pads with 15 W/m·K conductivity, which do not meet compression and conformability requirements, have worse practical performance than pads with 5 W/m·K conductivity that fit the entire surface perfectly.
Soft Thermal Pads vs Hard Thermal Pads
| Characteristics | Soft Thermal Pads | Hard Thermal Pads |
| Surface Conformability |
| · Limited |
| Compression Force Required |
| · High |
| Gap-Filling Capability |
| · Low |
| Component Stress Risk |
| · Moderate |
| Handling During Assembly |
| · Easier to handle |
| Thermal Conductivity |
| · higher filler loading |
| Compression Set Risk |
| · Lower |
| Applications |
| · Flat heatsink interfaces, structural assemblies, high-vibration environments |
FAQs
Does thermal pad hardness affect the thermal conductivity?
Yes, when you increase the amount of fillers, the thermal conductivity improves, while hardness and compressibility decrease.
A hard, high-conducting pad with incomplete surface contact provides worse thermal performance than a softer and low-conducting pad.
How does thermal pad harness affect the performance?
The hardness of the material influences the conformability, the compression force, and the quality of the interface.
The softer materials have good flexibility and therefore provide low contact resistance at the lowest possible mounting pressure. The harder material, on the other hand, works well on flat surfaces with uniform pressure.
What should you consider when selecting thermal pad hardness?
You should check the surface flatness, mounting pressure, component sensitivity, gap size, and assembly method.
If the surface is uneven, the component is sensitive, or if there is little clamping pressure, then you will need a softer Shore 00 thermal pad.
Are soft thermal pads better for heat transfer?
Yes, soft pads have a higher conformity to rough surfaces with low contact resistance through air gaps.
While they may have less filler and lower thermal conductivity values, your surface conditions will determine the effectiveness of thermal transfer.
How does thermal pad hardness affect compression?
The harder pad will require more pressure to get the same deflection as a soft pad.
If your setup has hardware that cannot provide enough squeezes to compress a thermal pad beyond its 10-30% deflection range, then you may leave air between interfaces, which increase thermal resistance.
Can thermal pad be too soft or too hard
Yes, they can be both. Though too soft pads, when subjected to mounting force and thermal cycles, can be pressed out at the interface, reducing contact surface area.
Hard pads, on the other hand, would not be able to compensate for surface imperfections and will leave air spaces with poor thermal conductivity.
Final Thoughts
Ultimately, it is clear that hardness of the thermal pad is a parameter that directly defines the thermal performance of your product in practice, rather than simply an indication on the datasheet.
If you select your materials based on parameters such as hardness, conductivity, compression ratio, and flatness, you will get thermal pads that will provide the required thermal performance.
