Example Data Table
| Heat Rate |
Area |
Hot Face |
Cold Face |
Resistance |
Area Resistance |
Conductance |
| 120 W |
25 cm² |
72 °C |
60 °C |
0.1 K/W |
0.00025 m²·K/W |
4000 W/m²·K |
| 75 W |
10 cm² |
55 °C |
47 °C |
0.106667 K/W |
0.000106667 m²·K/W |
9375 W/m²·K |
| 200 W |
40 cm² |
90 °C |
70 °C |
0.1 K/W |
0.0004 m²·K/W |
2500 W/m²·K |
Formula Used
Temperature difference: ΔT = Thot − Tcold
Total contact thermal resistance: Rc = ΔT / Q
Area normalized resistance: R″c = Rc × A
Contact conductance: hc = 1 / R″c
Heat flux: q″ = Q / A
Layer resistance: Rlayer = t / (k × A)
Design resistance: Rdesign = Rc × (1 + margin / 100)
How to Use This Calculator
Enter the hot and cold surface temperatures at the contact joint.
Add the heat transfer rate through the interface.
Enter the apparent contact area and choose its unit.
Add the number of similar interfaces in series.
Enter layer thickness and conductivity when a pad, grease, foil, or insulator exists.
Use pressure and roughness fields as design records.
Press the calculate button. The result appears above the form.
Download the CSV or PDF report for your records.
Thermal Contact Resistance in Electrical Design
Contact thermal resistance describes the heat barrier at two touching surfaces. It matters in power modules, busbars, heat sinks, battery tabs, and enclosure joints. Real surfaces are not perfectly flat. They touch at small peaks. Air, oxide films, grease, and pressure change the effective path. A low value means heat crosses the joint easily. A high value means temperature rises quickly near the source.
Why Interfaces Need Care
Electrical equipment often fails because heat cannot leave active parts. The semiconductor junction may be sound, yet the mounting face may be poor. Bolts may be loose. Pads may be too thick. A polished surface may still trap air. This calculator helps compare the measured temperature drop with heat flow and area. It also separates a simple layer resistance when thickness and conductivity are known.
Inputs That Matter
The most important values are heat rate, hot face temperature, cold face temperature, and apparent contact area. Temperature difference may be entered in common units. Area may be entered in square meters, centimeters, millimeters, inches, or feet. The tool converts every value to SI units before calculation. Optional pressure and roughness fields record design conditions. They do not replace laboratory correlation data.
Reading the Results
The total resistance in kelvin per watt shows the full joint penalty. Area normalized resistance is useful when comparing different part sizes. Contact conductance shows how strongly the interface transfers heat per square meter. Heat flux shows loading intensity. The optional layer calculation estimates the drop across a pad, grease film, foil, or insulator. If the layer resistance is larger than the measured total, the inputs need review.
Practical Design Tips
Use realistic contact area, not the full outline when only part of the joint touches. Keep surfaces clean. Apply even clamping pressure. Avoid excessive interface thickness. Compare results at several heat loads when possible. Save the CSV for quick records. Use the PDF report for reviews and design notes. Treat calculated values as engineering guidance. Validate critical systems with testing and manufacturer data before release.
When to Recheck
Recheck the joint after vibration, cycling, or service. Small clamp changes can raise resistance. Trend results over time to catch hidden thermal problems early.
FAQs
What is contact thermal resistance?
It is the temperature drop across a contact joint divided by heat flow. It shows how strongly a surface joint blocks heat transfer.
Why does area normalized resistance matter?
It lets you compare different joint sizes. A larger plate may show lower K/W only because it has more area.
Can this calculator replace lab testing?
No. It gives engineering estimates from entered values. Critical electrical designs should be verified with testing and supplier data.
What does contact conductance mean?
Contact conductance is the inverse of area normalized resistance. Higher conductance means better heat flow through the interface.
Should I include thermal grease thickness?
Yes, when you know the film thickness and conductivity. The calculator can estimate the added layer resistance and temperature drop.
Why can contact-only resistance become negative?
This happens when the entered layer resistance is larger than the measured total resistance. Review area, thickness, conductivity, or temperature readings.
Does contact pressure change the result?
Pressure can strongly affect real contact resistance. This calculator records pressure, but it does not use a pressure correlation.
What units are best for electrical thermal work?
Kelvin per watt, square meter kelvin per watt, and watts per square meter kelvin are common for interface calculations.