Stacking thermal pads may seem like a quick fix to filling larger gaps faster during thermal management. However, this is not always the case – it comes with many thermal and mechanical challenges. From trapped air and inconsistent compression to inefficient heat transfer.
In this article, we will explain why stacking thermal pads is rarely recommended in most applications.
Why Stacking Thermal Pads is Not Recommended
· Additional Interfaces Increase Thermal Resistance.
If stacking thermal pads is used, they create an additional thermal interface. These interfaces add to the overall thermal pad thermal resistance and decrease the heat transfer efficiency. This means increased junction temperatures and thermal variability in assemblies for OEMs and electronics manufacturers, particularly in high-power or compact designs.
· Air Gaps Can Form Between the Pads
Surface conformity is not always ideal for stacked layers. Miniscule gaps occur between layers even with “tack thermal pads,” and the thermal conductivity is substantially reduced because air is a poor heat conductor. These air gaps cause thermal bottlenecks and result in susceptibility to localized overheating and reduced reliability in critical electronic systems.
· Stacking Changes Compression Specification
Thermal pads are specially designed to have a certain thermal pad thickness and thermal pad compression characteristic (between 10% and 30% compression range). If several pads are stacked, this changes and uniform pressure distribution is more difficult to obtain. This can lead to undercompression (poor contact) and/or overcompression (stress in material) which will have negative consequences for long-term performance and repeatability in production.
· Pads May Shift or Delaminate During Assembly
In high volume production, stacked thermal pads can move, misalign, and even move apart in part. The more layers, the more unstable they are, even if tack thermal pads are used to help placement. Thermal pad thermal resistance may increase and reliability will be reduced due to movement during assembly or operation.
· Long-Term Reliability Becomes Unpredictable
Thermal cycling leads to expansion, contraction and material fatigue over time. These effects are magnified over multiple interfaces with stacking thermal pads which can lead to delamination and degradation in performance. When thermal resistance increases over time, rather than staying constant, instead of the normal use of the product, the product may begin to heat up, thereby decreasing its lifespan or resulting in field failures.
FAQ
Are there rare circumstances where you can stack thermal pads?
Yes—rarely. Thermal pads can be temporarily stacked when the proper thickness is not available for prototyping or for quick fixes in the field. This is not desirable for production, however, because of higher thermal pad thermal resistance, non-uniform thermal pad compression and reliability concerns over the long term.
How thick should a thermal pad be?
It depends on the clearance between the components and the compression ratio needed. Most designs are designed to achieve 10 to 30% thermal pad compression for optimum contact. With proper selection of thermal pad thickness, stacked thermal pads are not required and all thermal performance is uniform.
What thickness of thermal pad should you use?
Create the gap between the heat source and heatsink under real assembly conditions and choose a pad that has an assembly range that fits. Tolerances, surface flatness and the thermal resistance for thermal pads should also be considered by engineers for efficient heat transfer without overloading components.
Can you put two thermal pads together?
Technically yes, but it is not recommended. Stacked thermal pads create more interface resistance, air gaps (which is a risk) and change the compression characteristics of the thermal pad. There is always one single, well-defined pad that performs the best and most consistently.
Can you use thermal pads alongside thermal paste?
Yes, but only under certain circumstances. The microscopic surface imperfections can be filled with thermal paste, and the pad will cover up larger gaps. But it’s essential to be engineered properly, as too much material can lead to an increase in thermal pad thermal resistance and loss of contact efficiency rather than improvement.
Let Gallop Customize your Thermal Pads
Eliminate guesswork when choosing thermal pad design. At Gallop, we engineer custom thermal pad thickness that matches your exact gap dimensions. By replacing stacked thermal pads with a single engineered solution, you can optimize your thermal management.
Contact our engineering team today for a thermal interface material recommendation.
