A little know-how about thermal management

Most of the time one would like to derive the resulting heat loss as well and quickly as possible to the heat sink (cooling element). For this purpose, the component is coupled as tightly as possible and over a short distance.

In the case of space problems (very tight, no room for large cooling elements at the location of heat generation, even distribution of heat over a large area, e. g., backlight illumination of screens), it may be necessary to use so-called "heat spreaders". These allow little heat through the material, but spread heat very quickly and well to a larger area. So one can direct the heat flow in the direction of a heat sink. Graphite foils achieve up to 1,500 W / mK thermal conductivity in the surface, the electrically insulating Temprion™ OHS™ up to 50W / mK.


 

Heat conduction is not just a matter of materials used

The heat transfer e. g. from the case of a power transistor to an aluminum heat sink can be quite bad. Surface roughness reduces the direct contact area necessary for heat conduction by about 60-80%.

Even a good heat-conducting, but "hard" insulation material can not completely fill the resulting air pockets. Especially high performance films like Kapton® MT have disadvantages.

By using thermally conductive waxes (PCM, Phase Change Material) one can avoid this disadvantage. The wax, which is solid at room temperature, melts during initial startup and fills the cavities. This results in a continuous heat path without interruptions by air inclusions. The thermally conductive phase change material is so thin that it hardly affects the heat conduction.

Total heat resistance = (resistance housing) + (resistance transition) + (resistance TIM) + (resistance transition) + (resistance heat sink)

The solution: Reduction of the heat transfer resistance through thermally conductive coatings!

 

 

 

 

 

 

Heat transfer as a function of thermal conductivity and material thickness

There is a direct, linear connection between the material thickness and the heat flow that can be transported through the material. If you want to transport the same amount of heat through a material of double thickness, the thermal conductivity must also be doubled.

Thermal resistance = (material thickness * 1000) / (thermal conductivity * surface)

[mm * 1000 / (W / m * K * mm²) = K / W

Advantage for Kapton MT +: very good thermal conductivity due to low material thickness and very good dielectric strength

Important: this statement refers only to the transport within the material - the heat transfer resistance is not taken into account (see above "How important are interface coatings")

Equiation:
(assumtion: area is constant)

Kapton MT = 0,45 W/m*K and 25µm

a silicone foil of 200µm needs 3,6 W/m*K
a gap-pad of 1,00 mm needs 18 W/m*K
a gap-pad of 2,00 mm needs 36 W/m*K

to transport the same quantity of heat per time!

 

How to measure thermal conductivity?

According to ASTM D5470, the thermal conductivity of a test material is defined as follows:

A heated metal block supplies the heat source. Closely above and below the heat-conducting material temperature sensors are mounted. Below is a metal block that represents the heat sink (cooled if required).

After a stable heat flow has set, the temperature difference generated by the test material is determined. From this, the thermal conductivity is calculated.

Advantage of this method over the laser flash method: the roughness on the surface of the material is also measured. Because in the real situation of installation, the contact resistance at the interfaces of the individual materials will always play a role.


How to choose the right material for heat management in electronics?

 

1.) Why does one need thermally conductive materials?

Modern electronic devices push more and more power in less and less space. The importance of efficient heat management is therefore more important.

In order to dissipate the resulting heat loss, better materials adapted to the requirements must be used. Because an old rule of thumb says that 10 ° C temperature reduction doubles the life of an electronic component.

The choice of the right thermally conductive material depends on the operation site and the desired effect. The following information shows the advantages and disadvantages of individual product groups and helps to make the right choice.

2.) Types of thermally conductive products

  • Heat conducting adhesive tapes
  • Phase change products
  • Thermally conductive casting compounds and thermal paste
  • Thermally conductive adhesives
  • Heat spreader
  • Thermally conductive pads, gap fillers, silicone rubber pads


3.) Overview of advantages and disadvantages

Thermally conductive adhesive tapes

Description

Advantage

Disadvantage

Thermally conductive adhesive tapes are intended to connect the heat-dissipating components to the heat sink. Acrylate or silicone adhesives can be used. They are applied to a substrate, e. g. Kapton® MT, aluminum foil.

- good adhesion

- Can be delivered as a stamped part or roll material

- Ideal for pre-assembly

- Low weight and very thin

- Good electrical insulation (when using Kapton®, for example)

- Average thermal conductivity (with polymer film, but very thin = length of the heat path)

- Adhesive force with shear forces may not be sufficient

Phase change products

Description

Advantage

Disadvantage

Phase Change products combine the qualities of heat-conducting paste with the easy handling of thermally conductive pads. A coating of wax, which is solid at room temperature, melts during operation above 50-60 ° C. As a result, the air pockets between the surfaces are filled and this significantly improves the heat transfer.

- Can be delivered as a stamped part or roll material

- Works like heat-conducting paste, but is much better to dose

- Clean and easy to use

- High thermal conductivity

Very good electrical insulation by carrier foil (e.g., Kapton® MT

- Only low adhesion, requires additional mechanical attachment

- Can not be used several times like heat-conducting paste (repair)

Thermally conductive casting compounds and heat-conducting paste (thermal grease)

Description

Advantage

Disadvantage

Heat-conducting pastes are usually silicone-based viscous masses with a ceramic filler. They are already in use for a very long time and offer a good reduction of the heat transfer resistance.

-High thermal conductivity

- Adapts very well to surface roughness

- Can be used again after repair

- Cheap, tested

- Correct dosage difficult

- Risk of contamination

- Mechanical attachment of the components necessary

- No electrical insulation (thermal paste)

Thermally conductive adhesives

Description

Advantage

Disadvantage

Silicone or epoxy resin based adhesive with very high final adhesion. The application and the dosage require a sophisticated production.

- High thermal conductivity

- Replacement of mechanical attachment

- Correct dosage difficult

- Limited temperature resistance (epoxy)

- No post-processing or repair possible

Heat spreader

Description

Advantage

Disadvantage

Special group of heat conducting materials whose aim is to distribute the heat loss of hot spots on the surface. It can be used to conduct heat to the location of the heat sink. Either metal foils or special products such as e. g. graphite foils.

- Can be delivered as customized punched parts

- High thermal conductivity on the surface

- Depending on the material there is hardly passing heat through the material.

- If not metal foil possibly expensive solution

- If pure metal foil no electrical insulation (by combination with insulation film no disadvantage)

Thermally conductive pads, gap fillers, silicone rubber pads

Description

Advantage

Disadvantage

Combines the benefits of heat-conducting paste and insulation material (e. g., classic mica). Silicone rubber with ceramic fillers is frequently used. The soft surfaces adapt well to bumps and bridge gaps. Easy to handle

- Can be delivered as customized punched parts

- Clean and easy to use

- Easy installation

- Good thermal conductivity

- Good dielectric strength

- Bridges gaps

- In order to achieve good heat conduction, the pads must be pressed considerably

- With good thermal conductivity and greater thickness an expensive solution

 

Thermal resistance, thermal impedance and specific thermal conductivity

In data sheets you always get different information about how thermally conductive materials are.

Common is the declaration of the specific thermal conductivity, which is a material-typical value independent of thickness and area: W / m * K

But also values like W / K or K * in² / W can be found. How do you convert these into each other (neglecting the heat transfer resistance from one material surface to another)?

The initial equation is

Thermal resistance = thickness / (area * specific thermal conductivity)

K / W = d / (A * (W / m * K)) => K / W = d * (m * K / W) / A

Specific thermal conductivity = W / m * K

W = watts
K = Kelvin
m = meters
A = area in m²
d = thickness in m

By changing the formula according to the one you are looking for you can easily compare the data from different data sheets: for example, the specification K / W * d = K * in² / W (if standardized to inches).

Rule of thumb: double material thickness for the same spec. thermal conductivity means half the amount of heat transfer. Or: with the same amount of heat that is to be transported, a twice as strong material and a twice as good spec. have thermal conductivity! Advantage for thin foils like Kapton MT +, even if the spec. thermal conductivity is just under 1 W / m * K.