Reducing size and power without sacrificing efficiency and reliability
By Daniel West, Lead Technical Applications Engineer at KYOCERA AVX.
As active and passive electronic component manufacturers strive to reduce sizes and losses, power densities go up. This is a good thing, save for the increased thermal issues that can drive efficiency down and reduce reliability.
There have been quite a few advancements in thermal management solutions since gallium nitride (GaN) and silicon carbide (SiC) semiconductors. One of the latest differs a bit from grease or a spreader. It is a surface- mount device (SMD) ceramic chip that looks and feels like a multilayer ceramic capacitor (MLCC). The material set, however, is mostly composed of aluminum nitride (AlN) orberyllium oxide (BeO). It’s no surprise that these are both great thermal conductors, but it is their electrically insulating properties that provide a unique solution for ungrounded applications that need heat to be removed.
When terminated, the capacitance is sub-picofarad, down to a few femtofarads, which would minimally interfere with signals (if present) and isolate the surface from the circuit while providing a low thermal resistance path. Terminations on the ceramic thermal bridge arrive in the standard nickel/ tin finish. Silver over nickel is also offered for epoxy mounting, palladium-silver for applications that have non-magnetic requirements, and tin-lead-based plating for applications requiring tin whisker mitigation.
These devices from Kyocera- AVX also offer a full end termination style (think MLCC terminations) or a partially terminated end. These options will optimize heatflow through the body of the ceramic for a wide range of consumer, medical, and military applications. They arrive standard in EIA sizes from 0302 to 3737, along with custom arrangements to provide a specific cross- section for dialed-in heat flow.
A simple test was performed on a 1kΩ, 1W resistor to compare how much heat could be taken out of it using a thermal bridge and a metal fin. A value of 841mWwas applied to raise the temperature of the resistor close to its rated operating temperature of 125°C in the ambient test environment (left in the figure).
A metal fin was added to one termination of the resistor as an alternate thermal management solution, causing the temperature to fall from 123°C to 106°C (middle in the figure), which is not too bad. On the same termination of a different resistor, a thermal bridge was attached that had similar dimensions to the metal fin. The temperaturedecreased an additional 30°C more than the metal fin and settled around 78°C (right in the figure). Thus, the thermal bridge dropped the internal temperature of the resistor by close to 50°C, enabling either increased power handling or increased reliability, depending on how you look at it.
The test was expanded to determine what power was required to raise the resistor temperature to its rated 125°C (1.5W); also, to see how vias on the thermal bridge mounting pads affected heat flow. As you may have guessed, more vias are optimal and details can be obtained from Kyocera-AVX if needed.
While the ceramic thermal bridge may look unassuming, with added flexibility of dimensions and metallized surfaces it turns out to be a solid thermal solution for unique problems where it’s difficult to get to heat sinks without disrupting electrical performance.