Do you ever look up at the moon and wonder what mysteries lie beyond? Every once in awhile, while I’m gawking at the moon, a moving star catches my eye. The International Space Station (ISS) is occasionally visible from Earth and is a constant reminder of humankind’s achievements in space. SpaceX recently added another accomplishment to the list, by resupplying the ISS with a previously used capsule. SpaceX’s advancements in reusable spacecraft are making space more commercially accessible. As space launches become more common, PCB designers are going to have to prepare their boards for the rigors of space and the lift off to get up there. Quick temperature changes and rapid acceleration will make designers think more about the mechanical and material properties of their boards.
Dragon Capsule Docking: Part 2
You may remember back in 2012 when SpaceX became the first private company to dock with the ISS. That first docking was a major achievement for SpaceX, but they didn’t just stop there. On June 5, 2017, they resupplied the station again, but this time with a previously used Dragon capsule. SpaceX has been on a recycling mission and has been landing and reusing its rockets, but this is their first time reusing a capsule. Their next milestone will be reusing both a rocket and a capsule for a resupply mission.
The reason why SpaceX is pushing so hard for multi-use spacecraft is simple, money. Their goal is to make spaceflight more affordable. It costs much less to refurbish a rocket or pod than to build a new one. As launches become less expensive we can expect to see more of them. SpaceX’s long term goal of reaching Mars will also mean more space missions. They’re planning on sending multi-use rockets on round trips between Earth and Mars. The average PCB designer doesn’t usually think much about space, but with interplanetary colonization on the map, it’s about time to start.
Rapid thermal cycling can spell doom for your PCB.
Normally when I think of temperatures in space I think “cold.” Surprisingly, though, space can be quite hot. Direct sunlight really turns up the heat when there’s no atmosphere to absorb its energy. As spacecraft orbit planets or rotate they will be alternately shaded and exposed to the sun, causing huge temperature differentials. This thermal cycling can change a PCB’s electrical characteristics and reduce its lifetime.
When designing a high-performance board, I know you’re thinking about what materials to use and their dielectric constants (Dk). One problem with space is that its oscillating temperatures can change your material’s Dk. This happens because temperature reorients the polarized molecules that make up a dielectric. Depending on the temperature, your substrates Dk may rise or fall. Obviously, this is not going to help your PCB. So if you’re designing a PCB for a rocket, you’ll need to make sure that it can maintain its Dk in every temperature.
Temperature has some more obvious effects as well. You probably think about the coefficient of thermal expansion (CTE) of your materials when designing. As your board goes from direct sunlight to full shade, it will experience a huge temperature change, causing it to expand and contract. This accordion effect will break down your board and cause it to fail if you don’t choose the right materials. Space is a wild place, so make sure your materials are flexible enough to handle whatever temperatures come their way.
As far as materials go, Dk and CTE are going to be two of your main concerns. You’re going to need to select materials with Dks that remain fairly constant over a wide range of temperatures. On the temperature side you want materials that can take the heat without breaking.
Launch vibrations are not good vibrations. Editorial credit: Nadezda Murmakova / Shutterstock.com
Speaking of flexibility, your boards may also need to be mechanically flexible. The most dangerous part of a space mission for a PCB is the launch. During launch, spacecraft experience huge forces. The acceleration and vibrations involved shake up astronauts and are sure to rattle your boards as well.
During blast off the primary engines produce waves that can cause problems for many different parts of the ship. This shaking can be particularly destructive for PCBs. Flexibility is a good way to counter vibrations. Rigid flex PCBs are generally lighter than traditional PCBs, making them less prone to vibration. A light circuit can save precious weight on a spacecraft as well. Flexible circuits by nature are shock averse, but still need to be designed for specific categories of vibration. When designed properly flexible and rigid flex PCBs work well. These kinds of boards are already being used in aerospace applications from helicopters to satellites.
While space travel isn’t exactly a rarity these days, it’s sure to become more common as time moves on. Companies like SpaceX are making huge strides in the commercial space industry. As SpaceX continues to hit their milestones we’ll start designing a lot more PCBs for space exploration. Two major things to consider when designing PCBs for space are thermal cycling and vibrations. The large temperature swings in space can cause your PCB to fail from thermal expansion, or can reduce its functionality by changing your substrates Dk. Vibration will simply rip your board to shreds. However, if you start using rigid flex PCBs you can mitigate vibration effects and ensure your board makes it to the great unknown.
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