Aviation and aerospace is a hot topic right now. The number of aircraft is likely to double in the next 20 years and space is slowly becoming accessible to tourists. And SpaceX will fly you across the globe in less than one hour pretty soon.
This means that the number of circuit boards for these applications has already started to increase. Facing a huge demand, designers have to learn the specific rules to design a board for aviation and aerospace. And manufacturers have to be capable of meeting their customers’ needs.
During PCB West, Sierra met with Paul Cooke, Director of Field Application Engineering for an aviation and aerospace company. Expert in this field, he told us about the top must-knows that designers should keep in mind when designing for aviation and aerospace:
Reliability is the key
For space and aerospace, the primary goal is to build reliable products. An aerospace customer or satellite customer is going to expect their electronics to last 15 to 20 years with zero failure. So electronics’ longevity and zero failure are key.
To improve reliability in circuit boards, you really need to start at the PCB fabrication and design itself. The first thing the designer needs to do is to design the board to be reliable. Don’t use bleeding edge technology. Use standard technologies that we know are proven reliable, are not going to fail.
Use heavy copper
Most space applications use very heavy copper, much heavier than 2-oz copper. I see a lot of designs with up to 5-oz copper. I see some 20-layer products with 20 layers of 4-oz copper. How they manage heat in space is basically through dissipation. They use very heavy copper designs, do a lot of thermal vias, and use the chassis to dissipate heat as well. Because remember, energy in a satellite is key. You can’t have fan-cooled systems, so you need to use the natural properties of the copper itself as part of your heating and cooling strategies. But it is a challenge, and as you know, building very heavy copper boards is not easy. Plus, it is going to be in a material, like a polyimide, which is a much more difficult resin system to work with, so it brings its own challenges to meet the requirements.
Polyimide is probably 95% of applications. It depends on the satellite. If it is a 15 to 20-year geostationary satellite, it is going to require polyimide. Some of the lower-orbit satellites that maybe are only useful for two to three years, can use other materials. Companies, like Rogers, are moving into producing materials that are now becoming attractive to the customers who are doing the space application.
There is a lot of redundancy built in an aircraft to make sure everything will be alright in case of a lightning strike, for instance. Typically, there is more than one system so you can switch over to another one in case of a failure. And the way that aerospace works is that they are not ever both supplied from the same supplier. If there is ever a defect, you don’t want to run the risk of having the same defect on the backup system. There is a lot of redundancy built within the aircraft.
Engines could die, and the aircraft can still fly on one engine. Same in the electronics. If one board fails, there is typically a backup for that system as well.
Choose a standardized process manufacturer
When you choose a manufacturer to build your circuit boards for aviation and aerospace, make sure each process is standardized, repeatable. You have to have the process controls and measurements to ensure that the repeatability is 100% all the time. Customers can ask for data from the fabricator’s processes. A lot of satellite customers will ask for DPA-type analysis, basically, destroying good product to prove you have the right plating thicknesses, etc., all the way down to SPC process control analysis for all your different processes.
The design has to be designed to be reliable, and the fabricator needs to be set up to build it to have reliable processes as well.
Forget about RoHS
The RoHS initiative in defense, aviation and aerospace is never going to apply. One reason is that aerospace is probably never going to move away from leaded HASL. The risk is really too high. The risk of moving to a different surface finish, moving to a different SMT, increased temperatures, is really adding additional potential risk to aerospace products. I deal with most of the aerospace customers on a regular basis, and there is absolutely nothing on the horizon to look at having a RoHS type initiative within aerospace. It will always be HASL.
You can use different surface finish. You can use ENIG, for instance, but on the assembly process, you need to use leaded materials.
In an aircraft, most of the electronics, like the engines, sit in an exposed environment. The only electronics that are internal would be your entertainment systems, all the galley lighting systems, etc. The cockpit is a controlled environment as well. However, there is no differentiator between internal and external fuselage. You build the electronics the exact same way. They have to be encapsulated and controlled.
Fuselage, engine, and wing electronics are basically encapsulated in hermetically-sealed boxes. So the boards themselves are not exposed to any elements. But they are exposed to temperature and heat and cold cycling on a daily basis.
Testing, testing and more testing
Electronics have to go into a whole set of different testing requirements to make sure they won’t fail. A lot of satellite customers have their own vacuum chambers in which they can temperature cycle their products. They have to mimic things like zero-gravity in a vacuum, and they need to go from -50 to +125 degrees, depending on whether they are working on a low-orbit or geostationary satellite.
On the aviation side, if you imagine an engine controller on an aircraft, and one of them is based up in Alaska and a similar aircraft is based in the desert in the Middle East, now you can see the massive temperature changes. You need to make sure that the aircraft can handle it when it moves from one of these environments to another. When an aircraft gets up to 10,000 feet, it is sitting at -50 degrees, which means any exposed electronics has to be able to withstand that temperature. And obviously, it will face an elevated temperature when it lands on the ground. Aircraft have to cycle through these temperatures multiple times per day.
So when you are building a circuit board for the aviation and aerospace industry, it comes with a whole set of testing loops that you need to jump through to prove the design is reliable. You need to be able to manufacture a reliable design for these types of environment.
Don’t be afraid of paperwork
IPC 6012DS is a well-known document that Military and Space requires. It is basically an enhanced IPC Class 3. Your release requirements, your quality requirements, minimum plating, etc., are elevated within these specifications.
Paperwork is huge in aviation and aerospace. The amount of quality control, testing, reporting, micro-section analysis, FAIRS, etc. is very strict. It is even stricter for satellite customers who just take it to one more level. Normally, you might do 100% micro-sectioning for IPC Class 3. Some satellite customers are at 200% micro-sections. They just want to make sure everything is exactly built to the way they want it to be and their own specifications are above and beyond even what IPC Class 3A would be. They are very stringent on design rules, they are very stringent on process control, and release, etc.