A thermal epoxy’s conductivity figure tells you almost nothing on its own. What matters is how many degrees the bond line adds between the hot component and its heat sink — and that depends as much on thickness and area as on the material.
Q: What is thermal epoxy, and how much does it help?
A: Thermal epoxy is a thermally conductive adhesive: an epoxy loaded with metal or ceramic filler so it both bonds a component to a heat sink and carries heat across the joint. Unfilled epoxy conducts poorly, around 0.14–0.2 W/m·K; filled thermal epoxies typically reach roughly 1–3 W/m·K. How much that helps depends on the bond line’s thermal resistance in °C/W, which you can calculate from conductivity, thickness, and area before building anything.
Filler choice — aluminum versus aluminum nitride, conductive versus insulating — is covered in our overview of Epo-Weld™ thermally conductive epoxy. This post covers the step that comes first: working out whether a bonded thermal joint fits your temperature budget at all.
The One Equation You Need
For a bond line of uniform thickness, conduction resistance is:
R = t / (k × A)
- R — thermal resistance in °C/W
- t — bond line thickness in meters
- k — thermal conductivity in W/m·K
- A — bonded area in square meters
Multiply R by the power flowing through the joint and you get the temperature rise across the bond line. Each variable has equal weight: halving the thickness does exactly as much as doubling the conductivity.
Worked Example 1: A Power Module on a Heat Sink
Consider a module with a 40 × 40 mm base (0.0016 m²) dissipating 50 W into a heat sink.
| Bond line | Conductivity | R (°C/W) | Rise at 50 W |
|---|---|---|---|
| 0.1 mm | 0.2 W/m·K (unfilled) | 0.31 | about 16°C |
| 0.1 mm | 1.0 W/m·K (filled) | 0.063 | about 3°C |
| 0.3 mm | 1.0 W/m·K (filled) | 0.19 | about 9°C |
Two lessons appear immediately. A filled thermal epoxy cuts the rise by a factor of five over an unfilled adhesive. And letting the bond line drift from 0.1 mm to 0.3 mm gives back more than half of that gain.
Worked Example 2: A Small Component, Concentrated Heat
Now take a 5 × 5 mm component (0.000025 m²) dissipating 5 W, bonded with a 0.05 mm line of 1.0 W/m·K epoxy:
R = 0.00005 / (1.0 × 0.000025) = 2.0 °C/W, or a 10°C rise.
One-tenth the power produces three times the temperature rise, because the area is 64 times smaller. On small, hot parts, thermal epoxy becomes the dominant resistance in the stack, and conductivity and bond line control stop being optional details.
Need help running these numbers for your assembly? Email Us with your power, footprint, and temperature limits.
Building the Full Budget
The bond line is one link in a series chain. For the power module above, assume:
- Maximum junction temperature: 150°C
- Ambient: 40°C
- Junction-to-case resistance: 0.5 °C/W (25°C rise at 50 W)
- Heat sink-to-ambient resistance: 1.4 °C/W (70°C rise at 50 W)
Those two alone consume 95°C of the 110°C available, leaving 15°C — a bond line budget of 0.3 °C/W. The unfilled adhesive at 0.31 °C/W fails with no margin; the filled epoxy at 0.1 mm passes with room to spare; at 0.3 mm it still passes but with far less headroom.
Real joints add interfacial contact resistance at each surface, which the simple equation ignores. Measured bond lines, tested by a method such as ASTM D5470, commonly run higher than the calculated bulk value — so aim for a calculated R well under the budget, not just under it.
Matching an Incure Grade to the Joint
The catalog’s structured fields for the Epo-Weld™ thermally conductive line show three different rheologies, which is what largely determines the bond line you can actually hold:
- Epo-Weld™ TC-9033 — aluminum-filled, thixotropic paste, D70–D80 hardness, 2,600 psi tensile shear, 12,000 psi flexural. The paste stays where it is placed, suiting vertical surfaces and parts that are positioned before clamping.
- Epo-Weld™ TC-9042 — aluminum-filled, 9,000–13,000 cP, D82–D92, 1,900 psi tensile shear, 16,000 psi flexural. The lower viscosity squeezes out to thinner, more uniform bond lines under light clamping — the direct route to a lower R.
- Epo-Weld™ TC-9051 — aluminum-nitride-filled and electrically insulating, 35,000–45,000 cP, 1,400 psi tensile shear. Use it where the component’s tab or case is electrically live and must be isolated from the sink.
The aluminum-filled grades make no electrical-insulation claim, so do not rely on them to isolate a live surface. Cure schedules run 24–48 hours at 77°F or 2 hours at 200°F for TC-9033 and TC-9051; TC-9042 uses 24 hours at 77°F plus 2 hours at 200°F. Request measured conductivity data for the grade you choose and plug it into the budget above.
When a Bonded Joint Is the Wrong Choice
Thermal epoxy creates a permanent structural joint. If the component must be removable for service, or if the heat sink is already held mechanically, a non-curing thermal grease gives a thinner interface without the bond — compare the options in our guide to Epo-Weld™ thermally conductive grease. Where the joint must also carry electrical current, a metal-filled thermally and electrically conductive adhesive is the better fit.
Verifying the Budget in Production
Hold the bond line the calculation assumed. Glass spacer beads set a minimum thickness, controlled clamping pressure limits the maximum, and a first-article cross-section confirms both. Then measure case temperature under full load on assembled units and compare it with the budgeted value; a result more than a few degrees high usually points to voids or a thicker-than-designed bond line.
To size a thermal epoxy joint for your heat load, Contact Our Team.
Visit www.incurelab.com for more information.