Ten years ago I began my first job designing gaming motherboards. I had a lot to learn, particularly in the area of thermal performance. Before long, it became second nature to add heat sinks, cooling fans, and other equipment that I knew my PCB’s would need to thrive. Just when I thought I had heat mitigation down to an art, a friend told me about how electric vehicles (EV) are taking heat mitigation to the next level. Power regulators, electric motors, and engine control units are all EV components that put out large heat totals. Unlike gaming motherboards, the conventional heat mitigation methods, which require bulky SMT components and fans, are not an option for these small boards. Talking with my friend left me intrigued, so I asked him how the designers on his team deal with it all. If you can’t use heat sinks and fans, then what can you use?
Conventional heat sinks like this one just can’t fit in the tight spaces in EV projects.
The Ceramic PCB: The Best Thermal Performance Money Can Buy
For EV hardware designers, it means taking a whole new perspective. The PCB itself needs to play into the thermal equation. Specifically, the substrate of the PCB must be a strongly heat conducting material. Boards that effectively wick away heat from components can get away without fans and use much smaller heat sinks. Bearing this in mind, the designer will probably discover ceramic PCB’s as their best option. Compared to standard “FR-4” resin boards, ceramics are up to 170 times more thermally conductive and can handle operating temperatures up to 350C. Not only that, they are significantly stronger and more dielectric than resin boards. For EV designers, they’ve become the substrate of choice for power converter boards, which is a crucial component in EV’s. These boards are designed to convert the DC voltage from the vehicle’s batteries into the high-frequency AC voltage that the drive motors use. This challenging application generates substantial heat and will fry boards that can’t hold up in the long-term. They’re also used in charging circuits and voltage regulators for low-voltage equipment, applications with similar needs to power converters. Not surprisingly, ceramic boards are a standout choice for the EV PCB designer.
Before seeing use in EV’s, ceramic substrates were a favorite for computer processors and IC’s, like the one shown.
Best of Both Worlds: The Glass-Reinforced PTFE PCB
Now ceramic PCB’s sound great, right? Well, that’s not the whole story. While they are strong performers from a thermal viewpoint, they are terrible at keeping costs down. Ceramic PCB’s are manufactured using an advanced punching and machining process. In contrast, conventional boards use a layered silkscreening process that scales well and is very cost-effective. As my designer friend told me, he’d love to use ceramics for everything in his work - “it would certainly make my job easier,” he said with a laugh. Unfortunately, it’s just not feasible, which is why ceramics are only used for the most critical EV components. This brings us to glass-reinforced Polytetrafluorethylene (PTFE) boards, which EV manufacturers have been gravitating to for safety, control, and lighting PCB’s. So-called “thermal enhanced” PTFE boards are a particular favorite for LED headlight units, which output densely concentrated heat but don’t warrant a ceramic board. Like resin boards, the use a cheaper, layered manufacturing process but have much better thermal stability. In fact, with operating temperatures up to 200C, these boards are able to cope with some pretty demanding applications. It’s no wonder that PTFE is the same material used on nonstick frying pans for the past half century.
In addition to high heat PCB’s PTFE provides the non-stick coating for most everyday frying pans.
These are two popular options for the EV PCB designer, but they’re not the only two. Thanks to emerging “hybrid” substrate designs, it’s likely we’ll be seeing many more options in the near future. When considering a possible board type, my designer friend stressed two key values to pay attention to: the glass transition temperature and the thermal conductivity coefficient.
The glass transition temperature: The temperature where the board substrate stops behaving like a solid and starts becoming viscous. I wouldn’t call it melting per se, but you get the idea. It’s an absolute temperature ceiling that should be compensated for with a sensible safety factor.
The thermal conductivity coefficient: This value tells you how well the board “absorbs” and distributes heat. The higher this figure, the better the board will be at pulling heat away from IC’s and other components.
Pay attention to these two values to be sure that your board is able to conduct the necessary amount of heat without deforming. and your next EV project won’t suffer from underspecified PCB’s.
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