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February 21, 2023

Metal alloys for heat sinks

Metals for heat sinks
Metal alloys for heat sinks  TO220 transistor package clamp
TO-220 Package dual clamp bar  

Heat sink tutorial

Thermal conductivity

Usually a heat sink is used to spread the heat away from a thermal source in order to reduce the temperature of the source. Typical sources in my world are power semiconductors and power resistors.

This means that thermal conductivity is the most important property of the heat sink. Thermal conductivity of a heat sink is a function of cross section and the material property (thermal conductivity) of the metal.

In metals heat is transferred though conduction electrons and phonons. The phonon (lattice vibration) contribution is similar to that of an insulator with the same weight atoms, so the phonons are not insignificant. For example the thermal conductivity of aluminum oxide is 30, versus aluminum metal at 220, so about 10-15% of the thermal conductivity is due to the phonons. But for most of the thermal conductivity the electrons are doing most of the heavy lifting.

Thermal conductivity of a metal depends on how far conduction electrons, can travel without scattering. Conduction electrons can be scattered by phonons, which is why thermal conductivity gets worse as the temperature goes up. The more phonons traveling through the metal, the higher the probability of having them run into a conduction electron.

Conduction electrons and phonons are also scattered by impurities. An electron traveling through an aluminum crystal will be scattered if it runs into an impurity atom in the lattice, such as zinc or copper, or even an aluminum atom of a different isotope. They can also be scattered by defects in the aluminum crystal. So if an aluminum atom is missing, or is displaced from the crystal lattice that site will scatter electrons moving past it.

Conduction electrons and phonons are also scattered by interfaces. Interfaces include the edges of the metal, but also crystal domain edges.

Here I should point out that the same thing that makes a metal strong also lowers its thermal conductivity. A pure crystal is soft because it is easy to slide the crystal layers over one another. Impurities and crystal boundaries will "pin" the crystal layers to keep them from sliding. Crystal defects introduced by deforming the metal (work-hardening) strengthen the metal by using crystal defects to keep the atomic layers from sliding. And will also create scattering centers to lower the thermal (and electrical) conductivity.

So the highest thermal conductivity metals will be pure and annealed with large crystal domains.

Aluminum heat sinks

Metal alloys for heat sinksMetals for heat sinks
Table 1. Aluminum wrought alloys arranged by thermal conductivity. In annealed state except 4032-T6. The strength of tempered versions can be considerably higher than listed here. For flat stamped heat sinks I usually specify 1100, it is one of the best for thermal conductivity, it is readily available, inexpensive and best of all easily remembered. For heat sinks that will need extensive machining I use 2024 or 6101.
Aluminum Alloys Thermal Conductivity
Tensile Strength
Extrudability Forming Main alloying element
1350 234 83 100 + 99.5% Aluminum
1060 231 69 100 + 99.6% Aluminum
1050 222 105-145 100 + 99.5%
1100 222 90 100 + 99.0%
6101 220 97 50 + 1% Si+Mg+B
3003 193 110 60 ++ 1% Manganese
2024 193 186 10 + 4% Copper
2017 193 179 10 + 4% Copper
6061 180 124 50 + 2% Mg+Si+Cu
7075 173 228 5 - 9.7% Cu+Mg+Zn
2219 171 172 10 + 6% Copper
3105 171 117 60 + 1% Mn+Mg
4043 163 145     5% Silicon
4032-T6 154 379     12% Silicon
2011 151 379 10 + 5% Copper
5052 138 180 30 + 2% Magnesium
5456 117 310 30 + 5% Magnesium
8090 95 340     2% Li 2% Cu

Copper heat sinks

copper heat sink examplecopper heat sink
Table 2. Copper has great thermal conductivity, but add any alloying elements to it and the thermal conductivity drops fast, as you can expect. Two percent Beryllium cuts the thermal conductivity by 2/3.
Copper Alloys Thermal Conductivity
Tensile Strength
C10100 394 221     Oxygen free high conductivity
C15000 367 255     0.15% Zirconium
C18200 324 234     1% Chromium
C17000 107 483     1.7% Beryllium
C26000 121 338     Cartridge brass
C35000 116 324     Leaded brass
C46400 116 400     Naval brass
C51000 46 485     Tin bronze
C75200 33 414-1000 MPa     Nickel silver

Steel heat sinks

Table 3. Sometimes sheet steel is already in your product, and if you can get away with using it as a heat sink it saves money and complexity.
Steel Thermal Conductivity
Tensile Strength
1006 65 295     Hot rolled
1010 52 325     Hot rolled
1018 52 325     Hot rolled
1020 52 380     Hot rolled
1070 51 640     Cold drawn
2205 Stainless 19 750     Annealed
410 Stainless 25 520     Annealed
316 Stainless 16 586     Annealed
304 Stainless 16 586     Annealed

Zinc heat sinks

zinc heat sink examplezinc heat sink example

Table 4. Zinc can be die cast to make complicated shapes at low cost and so is attractive as a heat sink. Injection molding machines are made specifically to mold zinc just like plastic. The thermal conductivity of zinc is moderate because it has large percentages of five different isotopes that cause electron scattering. Because of this the thermal conductivity doesn't change much when small percentages of alloying metals are added.
Zinc Thermal Conductivity
Ultimate Tensile Strength
Pure Zinc 125 145      
Zamak 3 113 268     4% Aluminum
Zamak 5 108 331     4%Al 1%Ci
KS 105 200     4%Al 3%Cu

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