What is thermal conductivity explained simply?

Thermal conductivity indicates how well a material conducts heat. Its symbol is λ (lambda), the unit is W/(m·K). Metals like copper have high values (~400 W/(m·K)), insulators and air (~0.026 W/(m·K)) very low ones.

What unit does thermal conductivity have?

The SI unit is watts per meter and kelvin: W/(m·K). It describes the heat flow through 1 m of material thickness at a 1 K temperature difference. Synonyms are thermal conductance value or thermal conduction coefficient.

Which metal has the highest thermal conductivity?

Silver conducts best at ~429 W/(m·K), but it is too expensive for series use. In practice, copper at ~400 W/(m·K) is the best usable heat conductor, followed by aluminium (~235 W/(m·K)).

How high is the thermal conductivity of copper?

Pure copper is at about 398 to 401 W/(m·K), usually stated as ~400 W/(m·K). This makes copper, after silver, the best practically usable heat conductor, ideal for hotspots, copper inlays, and heat spreaders.

How high is the thermal conductivity of aluminium?

Pure aluminium reaches ~235 W/(m·K). Heat sink alloys range from 125 to 210 W/(m·K) depending on the alloy, typically 150 to 200. Aluminium is the standard heat sink material because it is light, inexpensive, and extrudable.

Is copper or aluminium better for cooling?

Copper conducts about twice as well, but it is heavier and more expensive. Aluminium is lighter, cheaper, and easier to manufacture. With high heat density and tight space, copper wins, otherwise aluminium. COOLTEC combines both through copper inlays in an aluminium base body.

How do you calculate thermal conductivity?

Via the formula λ = (Q̇ · d) / (A · ΔT): heat flow Q̇ [W] times thickness d [m], divided by area A [m²] and temperature difference ΔT [K]. Rearranged, Q̇ = λ · A · ΔT / d gives the heat flow through a component.

Is a high or low thermal conductivity better?

It depends on the goal: for cooling and electronics a high λ value is desirable, for insulation a low one. There is no blanket good or bad, what matters is dissipating heat versus insulating heat.

Why does stainless steel have such a low thermal conductivity?

The alloying elements chromium and nickel disturb the lattice structure and hinder electron transport, hence only ~15 to 16 W/(m·K). In return, stainless steel is very corrosion-resistant and suited to media-carrying channels and pipes, often combined with an aluminium base body.

What is the difference between thermal conductivity and the heat transfer coefficient (U-value)?

λ is a pure material property in W/(m·K). The U-value in W/(m²·K) describes a whole component and also depends on thickness and geometry. λ is therefore the base quantity from which the U-value and thermal resistance are derived.

Does thermal conductivity increase or decrease with temperature?

For pure metals, λ decreases slightly as temperature rises. For gases and amorphous materials it increases, for crystalline ceramics it usually decreases. For design purposes, a reference value at 20 °C is therefore usually sufficient.

What does the lambda value (λ) mean?

λ is the symbol for thermal conductivity in W/(m·K). A high λ value means good heat transport (metals), a low one good insulation. In cooling engineering: the higher the λ of the material, the better the heat dissipation.

Thermal Conductivity of Metals and Cooling Materials: Values, Table, and Formula

Thermal conductivity λ determines how well a material dissipates heat. This overview provides the definition, formula, and a table of λ values for cooling engineering.

What is thermal conductivity? Definition

Thermal conductivity indicates how well a material conducts heat. Its symbol is λ (lambda), the unit is watts per meter and kelvin, W/(m·K). A high λ value means good heat transport, for example copper at about 400 W/(m·K). A low value means good insulation, for example air at about 0.026 W/(m·K).

Symbol: λ (lambda). Common synonyms are thermal conductance value and thermal conduction coefficient.
Unit: W/(m·K), the heat flow through 1 m of material thickness at a 1 K temperature difference.
Rule of thumb: a high λ value means a good conductor (metals), a low λ value means a good insulator (air, foam).
Distinction: λ is a material constant. The heat transfer coefficient (U-value) and thermal resistance also depend on geometry and thickness.

Calculating thermal conductivity: formula and worked example

Thermal conductivity follows from Fourier's law. From a measured heat flow, λ can be determined as follows:

λ = (Q̇ · d) / (A · ΔT)

Q̇: heat flow [W]
d: material thickness [m]
A: cross-sectional area [m²]
ΔT: temperature difference [K]

Example: A 10 mm thick aluminium plate (λ = 200 W/(m·K)) with 0.01 m² area at a 5 K difference transfers Q̇ = λ · A · ΔT / d = 200 · 0.01 · 5 / 0.01 = 1000 W.

The thermal resistance of a layer follows as R_th = d / (λ · A) in K/W. Unlike λ, R_th and the U-value also depend on geometry and thickness.

Thermal conductivity table: values of important materials

The table lists typical λ values at around 20 °C, sorted in descending order and focused on metals and cooling materials. Report values and general physics constants are marked.

Material	Thermal conductivity λ (W/(m·K))	Relevance for cooling
Silver	~429	Highest conductivity of all metals, too expensive for series use and therefore a reference value only.
Copper (pure)	~400 (398-401)	Best practically usable heat conductor, ideal for hotspots, copper inlays, and heat spreaders. Heavy and expensive.
High-conductivity copper alloys (e.g. CuCrZr, CuAg)	305-394	Only slightly below pure copper, used where high conductivity and higher strength are needed together.
Aluminium (pure)	~235	Very good conductor at low weight, the basis of most heat sinks.
Aluminium alloys (heat sink)	125-210 (typ. 150-200)	Standard material for heat sinks and cold plates: light, inexpensive, extrudable, and machinable.
Graphite	130-160 tech. (up to 2000 in-plane)	Anisotropic and light, high in-plane conduction for heat spreading.
Sintered ceramic (AlN, BN)	100-200	Conducts heat and insulates electrically at the same time, ideal near sensitive circuits. SiC, by contrast, is a semiconductor.
Brass	~110 (approx. 80-120)	Medium conductivity, used more for connectors and fittings. Strongly alloy-dependent.
Steel (unalloyed)	~50	Clearly a poor conductor, used structurally rather than thermally.
Copper-nickel (CuNi)	~20-50	Significantly lower conductivity than pure copper, chosen for its corrosion resistance in pipes and corrosive coolants.
Stainless steel (V2A/V4A, austenitic)	~15-16	Poor heat conductor but very corrosion-resistant, for channels and pipes combined with an aluminium base body.
Thermal paste/pad (TIM)	~1-12	Fills microgaps at contact surfaces and lowers the contact resistance. Product-dependent, check the manufacturer value.
Water (coolant)	~0.6	Transports heat by convection, not by conduction. Reference for liquid cooling.
Air	~0.026	Very poor conductor, which is why heat has to be actively dissipated.

Thermal conductivity of copper (approx. 400 W/(m·K))

Pure copper conducts at about 398 to 401 W/(m·K) and is, after silver, the best practically usable heat conductor.
COOLTEC uses copper deliberately as an inlay and heat spreader to defuse hotspots.
The drawbacks are its high weight and material cost, so copper is usually applied selectively rather than over the whole area.

Copper heat sinks in detail →

Thermal conductivity of aluminium (approx. 235 W/(m·K))

Pure aluminium reaches about 235 W/(m·K). Heat sink alloys range from 125 to 210 W/(m·K) depending on the alloy, typically 150 to 200.
Aluminium is the standard material for heat sinks and cold plates: light, inexpensive, extrudable, and easy to machine.
Anodizing protects the surface against corrosion without noticeably reducing heat conduction.

Aluminium heat sinks in detail →

Copper or aluminium? Material comparison for heat sinks

Both materials dominate heat sink construction. The table compares the decisive properties.

Criterion	Copper	Aluminium
Thermal conductivity	~400 W/(m·K)	~200 W/(m·K)
Weight	high (dense material)	low
Material cost	higher	lower
Corrosion	resistant, oxidizes slowly	very resistant with an anodized layer
Manufacturing	more demanding to machine	easy to machine and extrude

Copper wins with high heat density and tight installation space, aluminium with weight and cost. COOLTEC combines both through copper inlays in an aluminium base body: high conductivity exactly where the heat load is greatest.

Stainless steel, brass, and other materials

Even poor heat conductors have their place in a cooling system when other properties matter.

Stainless steel (V2A/V4A) conducts at only about 15 to 16 W/(m·K), but it is very corrosion-resistant and suited to media-carrying channels and pipes.
Combined with an aluminium base body, stainless steel channels unite resistance and heat dissipation.
Brass and steel serve more as connection and structural materials, not for heat dissipation.
Graphite conducts up to 2000 W/(m·K) in-plane and is suited to heat spreading. Sintered ceramics such as AlN and BN conduct heat and insulate electrically at the same time, whereas SiC is a semiconductor.

High or low thermal conductivity: which is better?

There is no blanket good or bad. The trade-off is dissipating heat versus insulating heat.
For cooling and electronics, a high λ value is desirable.
For insulation, a low λ value is needed.
For pure metals, λ decreases slightly as temperature rises. For gases and amorphous materials it increases, for crystalline ceramics it usually decreases.

Choosing the right material for your cooling

In practice, λ alone does not decide, but the interplay of several requirements.

Besides thermal conductivity, corrosion resistance, weight, cost, and manufacturability matter.
Thermal interface materials (TIM) such as thermal paste and pads fill microgaps at contact surfaces and lower the thermal contact resistance.
COOLTEC manufactures cold plates, extruded aluminium heat sinks, and copper-inlay solutions and sizes them for your application.