Thermal materials for handheld ultrasound
Handheld ultrasound devices are becoming ever more compact, yet the density of electronic components inside keeps increasing. Thin thermal pads transfer heat from the transmit circuitry and processor to the housing, keeping the ultrasound probe cool and safe in direct contact with the patient's skin.
Medical device safety standards strictly limit the surface temperature of parts that contact the body, making thermal management one of the biggest challenges in designing portable ultrasound systems.
Where heat is generated in an ultrasound device
Heat comes mainly from the high-voltage pulse transmit circuitry that drives the piezoelectric elements, the receive amplifiers and the image processing chip. Within the probe, the piezoelectric elements themselves also generate heat during continuous operation. IEC 60601-2-37 limits the surface temperature of probes in contact with skin to about 43°C, requiring an effective heat path from the components to the frame and housing.
Requirements for thermal materials
- Thin and soft to fit the tight spaces inside the probe and main unit.
- Low compression force that does not stress components and fine-pitch solder joints.
- Good electrical insulation, as the transmit circuitry operates at up to hundreds of volts.
- Low outgassing that does not affect the acoustic matching layer and lens.
| Location | Recommended material | Thickness | Thermal conductivity |
|---|---|---|---|
| Image processing chip | Ultra-soft gap pad | 0.5–1.0 mm | 3–5 W/m·K |
| High-voltage transmit circuit | Insulating silicone pad | 0.2–0.3 mm | 1.5–3 W/m·K |
| Back of piezoelectric elements | Curing thermal adhesive | 0.1–0.3 mm | 1–2 W/m·K |
| Heat spreading to the housing | Graphite sheet | 25–70 µm | Above 1,000 W/m·K in-plane |
Adding graphite for heat spreading
Synthetic graphite sheets have very high in-plane thermal conductivity, spreading heat from hot spots across the entire housing area and avoiding localized hot zones at the handle. Graphite is usually laminated with an insulating PET film and a thin adhesive for easy assembly.
Design validation testing
Probe surface temperature should be measured in the highest-power scanning mode over an extended period, both in air and in contact with a tissue phantom, to ensure an adequate safety margin.
Early thermal simulation helps determine the optimal placement of thermal pads, limiting the number of prototype iterations and shortening time to market.
AMS supplies Bergquist gap pads, graphite sheets and thermal adhesives for medical devices, along with cutting to drawing. Contact AMS Vietnam for advice on thermal solutions for ultrasound devices.
