How to Prepare Your PCB for Outgassing in Ultra-High Vacuum Systems
You’ve heard the phrase, “rain on your parade” right? When I was in high school I played saxophone in the marching band. Well, one night we had a competition and it ended up drizzling on us 2 miles into the 4-mile march. We had to run for cover. All that hard work ruined by a little rain. On Earth, condensation just makes us seek shelter, but in low pressure, it can spell doom for your experiments. You might not think about condensation in the vacuum of space, but many space endeavors undergo complications from a little vapor released in a process called “outgassing.” This is a phenomenon that occurs in high vacuum environments and can hinder or ruin nearby instruments. If you’re designing PCBs for ultra-high vacuum (UHV), you need to figure out how to reduce PCB outgassing. In an ideal world we would be able to eliminate outgassing, but since nothing is perfect it’s important to have mitigation strategies to deal with it when it occurs. While there are many things to consider (solder mask, conformal coating, thermal pad, ect), let's take a look at what exactly outgassing is first.
What is Outgassing and Why Does It Matter?
When I went to college I switched from the saxophone to the bassoon. If you don’t know what that is, you’re not alone. You also may not have heard much about what is pcb outgassing (offgassing). Offgassing occurs when gassed trapped in materials are sucked out in UHV. This can be a big problem in boards because they’re often made of a variety of porous composites. These permeable materials often trap air and other vapors inside them during the manufacturing process. Outgassing can have a considerable effect on space missions. When the is exposed to the vacuum of space the vapors are drawn out and can condense on nearby surfaces.
The problem with outgassing is that it produces condensate that can both interfere with optical instruments and contaminate measurements. This is particularly relevant for space probes designed to measure cometary atmospheres, where outgassing can be larger than the cometary signature.
Even relatively familiar vapors, like water, can interfere with missions. These kinds of risks can severely inhibit experiments and space missions. That’s why NASA gives tips on outgassing reduction in workshops and has even developed guidelines for testing materials for outgassing. Even if you follow those guidelines you Printed Board will still experience some level of outgassing. So you need to focus not only on a components limitation but also on mitigation.
Lenses and other optical equipment can be contaminated by outgassing
After that event, it took days for our instruments to dry. We couldn’t just throw them in an oven to cook out all the moisture. Fortunately, that’s exactly how you can acquire low outgassing in your PCBs. You should also choose materials that have good outgassing characteristics.
Manufacturers bake their printed circuit board for one purpose that serves a variety of ends; to remove moisture. Water trapped in circuit board can vaporize and outgas during solder reflow, causing defects. That same moisture, or vapor, is what causes outgassing in space. In order to reduce the risk of outgassing in the void, you’ll need to not only bake your but do it in a vacuum. This will help ensure there is as little residual moisture as possible.
Choosing the right printed circuit board thermal resistance materials is key when trying to reduce outgassing. Make sure you build your circuit board with elements that are designed for the job, like Rogers Corporation’s RT/duroid series. Some flexible circuits materials, like Kapton, also have good outgassing characteristics.
Start your experiment on the right foot, by choosing materials made for vacuum. Then talk to your manufacturer about using a vacuum prebake process to remove as much moisture as possible from your boards.
Make a path for gas to follow
You may not be able to avoid going out in a storm, but an umbrella or raincoat can keep you from getting too wet. Even if you use the best PCB material and fabrication processes, chances are your circuit board will still experience outgassing. You should have some mitigation strategies in place to keep your PCB working when things don’t go as planned. The effects of outgassing can be lessened by venting vapor away from instruments and using heating elements.
Outgassing is especially problematic for lenses and other optical instruments. So, if you think your PCB is going to release vapor, direct it away from sensitive surfaces. You can design a path for any vented gases to follow and send them into space, or somewhere they can’t do any harm.
If you’re not sure you can keep condensate away from any delicate apparatus, you can use heating elements to burn off any films or ice layers. If your printed circuit board thermal conductivity is on a spacecraft, you could also turn affected surfaces towards the sun.
Outgassing can be a major pain, but it doesn’t have to ruin your parade. If you use the right materials and then vacuum pre-bake them, you can minimize the chances of outgassing. Then, if you’re still worried about mission critical elements, you still have some options. Make a path of least resistance to direct gases where you want them to go, or use heat to clear any affected surfaces.
Designing for UHV can be difficult, but designing your circuits doesn’t have to be. Altium Designer® is some of the best professional PCB design software and will have you singing in the rain. Its excellent array of tools will help you build a PCB that can operate on the ground or above it.
Have more questions about outgassing? Call an expert at Altium Designer.