Technology continues to advance with solutions that demand more and more performance in terms of energy efficiency and data processing. Kuprion, Inc., has patented a copper-filled via technology that addresses the heat problem at the board level.
In an interview with EE Times, Kuprion president & founder Alfred Zinn pointed out that thermal management of PCBs is very important, as improving heat transfer can increase mean time between failures (MTBF) while at the same time increase the performance of heat-generating components.
Zinn said Kuprion’s Copper Filled Thermal Vias technology is an easy and cost-effective method that, placed directly under a component that needs to be cooled, opens a second heat dissipation duct doubling the cooling rate.
PCB thermal management
The heat generated by integrated circuits poses great challenges, especially given today’s higher speeds, smaller board surface areas, and many devices populated on PCBs. These demands call for solutions to effectively dissipate the heat and ensure the performance and lifetime of the entire system.
Proper thermal management is necessary to keep each component within safe temperature limits. The junction temperature should never exceed the limit indicated in the manufacturer’s datasheet (generally between +125 °C and +175 °C for silicon-based devices). The heat generated by each component is transferred to the outside through the package and the connection pins. In recent years, electronic component manufacturers have built increasingly thermo-compatible packages. Even with these package advances, heat dissipation becomes increasingly complicated as the size of integrated circuits continues to shrink.
The growing popularity of wide bandgap semiconductors (WBG), such as gallium nitride (GaN) and silicon carbide (SiC), has enabled components to achieve higher operating temperatures and power outputs than silicon-based technology. However, this has not eliminated the need for careful thermal management to evenly distribute the heat generated to avoid the formation of dangerous ‘hot-spots’ and to minimize power losses.
The two main techniques used to improve PCB thermal management are creating large ground planes and the insertion of thermal vias. Thermal vias are used to transfer heat from one layer to another layer on the same board. Their function is to direct heat from the hottest spots on the board to other layers.
FR4 does not offer high thermal conductivity and current Electroplating and Cu Coin techniques are limited by the slow manufacturing process and the requirement for heat sinks that can be bulky (Figure 1).
Copper filled via
Lead-free and RoHs-compliant, Kuprion’s copper formulation is a flowable, engineered copper paste—a dense and sticky form of copper, which is safe to handle in air. The company’s Copper Filled Thermal Vias provide an efficient heat dissipation path directly incorporated into PCBs with placement directly under a surface-mounted IC ‘heat source’ that allows direct surface mount bonding for maximum heat transfer using Kuprion’s surface mount copper material.
Kuprion’s Copper Thermal Via paste is capable of filling vias of at least 5 mm in diameter. When fused, the copper paste converts to solid copper without melting, which provides thermal conductivities in the range of approximately 110-180 W m-1K-1 (CTE, coefficient of thermal expansion, adjusted) and up to 290 W m-1K-1 for microvias (up to 25mil in diameter).
“Our technology provides multiple advantages: since the paste does not go through a liquid stage during fusion, it does not cause wicking practically eliminating short circuits and allows for placing contacts very close together for maximum I/O density. It also ensures a significant reduction in thermal resistance by direct bonding components to the thermal via, facilitating efficient heat dissipation not only on the top surface but through the PCB to the backside. You can also place these interconnects very close together and is stable to temperatures in excess of 500 °C,” said Zinn.
Kuprion’s Copper Thermal Vias offer low-temperature processing (235°C) with high-temperature operation (>300 °C). The material does not degrade or migrate under bias and/or temperature and offers high thermal conductivity about 110-290 W/mK depending on the specific CTE dialed in. Other features are: Thermal shock stability of 1000 cycles (-30 °C to +200 °C), and Hot shear strength (260°C / 20 sec) similar to r.t shear strength.
Kuprion’s thermal via copper technology leverages the CTE (Coefficient of Thermal Expansion) tuning capability (5-17 ppm range) of its Engineered Copper Material to mitigate CTE mismatches that can otherwise lead to warping and other reliability issues when using standard copper coin technology.
“Right now, the focus is more on the PCB level, but it’s device-level applications. One technique that’s being considered a lot is the development of using copper pillars instead of ball grid areas that get to very high density interconnects at the package level, so you can have small integrated systems that can be put on a circuit and have much denser, much smaller systems. The maximum we’ve measured so far for our thermal vias is 290 watts per meter Kelvin,” said Zinn.
“High power operation can lead to high junction and component temperatures and that’s where thermal expansion causes reliability issues. Because we can tailor the thermal expansion of our material with certain additives over a wide range, we were able to match it to semiconductor materials like Si, GaN and SiC, and we are able to replace the typical tungsten-copper heat sinks and heat spreaders that are used to avoid those problems. Now, with our copper, you have something that is much lighter, nearly equivalent to a quarter density, and this is of particular interest for aerospace applications, for example. It’s also much easier to machine because the material is softer,” says Zinn.
He added, “for high power, especially high current, you need very good conductivity, and low resistance so that your system doesn’t heat up and you can just carry high power densities. Another property of materials to consider when you have these high currents is migration. And currently, sintered silver is used more and more. The disadvantage of silver is that it readily migrates under increasing fields and is prone to making little dendrites that can lead to shorts, which the copper doesn’t do.”
Kuprion is currently expanding on its reliability testing such as thermal shock and vibration while establishing the large-scale production process to implement it with orders being accepted this quarter. Kuprion’s Copper Filled Thermal Vias can find applications for advanced high-power applications such as 5G transceivers/power amplifiers, industrial LEDs, graphics cards, data servers, routers, and automotive lighting.
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