Paul Cooke – Aerospace Circuit Boards and Their Challenges

Paul Cooke – Aerospace Circuit Boards and Their Challenges post thumbnail image

Aerospace circuit boards and electronics come with their fair share of challenges. Paul Cooke, Director of Field Application Engineering at FTG, answered our questions on special features, reliability, materials, EMI, etc. for the avionics and aerospace industry.

  • An aerospace customer or satellite customer is going to expect their electronics to last 15-20 years with zero failure.
  • The PCB fabricator needs to be set up to build boards with reliable processes.
  • Most space applications are using very heavy copper, much heavier than 2-inch copper.
  • There’s a lot of redundancy built within the aircraft.

0:10 What are the special features avionics and aerospace circuit boards have compared to other applications?

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 15-20 years with zero failure. A lot of my time would be spent working with aerospace and satellite customers on designing, basically, higher reliability into the product itself. So electronics’ longevity and zero failure.

0:43 What can improve the reliability of an aerospace circuit board?

To improve reliability in PCBs, you’d really need to start at the PCB fabrication and design itself. So obviously the first thing the designer needs is to design the board to be reliable. They’re not going to use bleeding edge technology. They’re going to use standard technologies that we know are proven reliable, are not going to fail.

The second thing is, you need to have a PCB facility that, you know, each process is standardized, it’s repeatable, and you have to have the process controls and measurements in that process to ensure that your repeatability is 100% all the time. A lot of our customers would basically come and ask for data from our processes. A lot of satellite customers will ask for DPA-type analysis, basically, destroying forced reproduction to prove you’ve got the right plating thicknesses, etc.

And all the way down to SPC process control analysis for all your different processes. It’s not just one thing, 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.

2:09 When the RoHS initiative started, it was said to not be a well-proven technology for military and aerospace. Has that changed?

The RoHS initiative in aerospace and defense is never going to apply. One, aerospace probably is 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 as well, 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’ll always be HASL.

I mean, we can use different surface finish. We can use ENIG, for instance, but on the assembly process, we’re using leaded materials.

3:09 How do you make sure electronics won’t fail once in space?

Basically, for the environment, the product has to go into a whole set of different testing requirements. A lot of our satellite customers have their own vacuum chambers, and within their vacuum chambers, they are actually temperature cycling the product itself. They have to mimic, basically, zero-gravity in a vacuum, and they basically need to go from -50 to +125. Depending on if it’s a low-orbit satellite or geostationary, it depends on the type of environment it’s going into.

Changing environment

And even on the aerospace side, if you imagine an engine controller on an aircraft, and one of them is based up in Alaska and a similar aircraft’s based in the desert in the Middle East, now you can see the massive temperature changes as well. Then if the aircraft’s moving from one of the environments to another, it’s constantly changing. You know when an aircraft gets up, to 10,000 feet, it’s sitting at -50, so, any exposed electronics has to be able to withstand that temperature. Obviously, elevated temperature when it lands on the ground. It has to cycle through these temperature multiple times per day.

So really, when you’re dealing with that type of customer, they’ll have a whole set of testing loops that you need to jump through to prove, one, the design is reliable, but, two, it can be manufactured to be reliable in these types of environment.

4:48 Generally, in space, electronics face a vacuum kind of scenario. How is the heat removed from the circuit board?

Most space applications are using very heavy copper, much heavier than 2-inch copper. I see a lot of designs with up to 5-inch copper. I see some 20-layer products with 20 layers of 4-inch copper. How do they manage heat in space is basically through dissipation. They use very heavy copper designs.

They’re also doing a lot of thermal vias. They’re using a chassis to dissipate heat as well. Because remember, energy in a satellite is key. You can’t have fan-cooled systems, so they’re basically using the natural properties of the copper itself as part of their heating strategies. But it’s a challenge, and as you know, building very heavy copper boards is a challenge as well. Plus, it’s 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 as well.

6:06 Is polyimide a favorite material for aerospace?

Yes, polyimide’s probably 95% of applications. It depends on the satellite. If it’s a 15-20 year geostationary satellite, it’s going to be polyimide. Some of the lower-orbit satellites that maybe are only useful for two to three years, can use other materials. But people like Rogers are moving into producing materials that are now becoming attractive to the guys that are doing the space application.

6:36 How is the J-STD-001 standard relevant to designers and fabricators?

It’s basically like an enhanced IPC Class 3.

Basically, your release requirements, your quality requirements, minimum plating, etc., are elevated within these specifications. But what I find is, a lot of satellite customers, their own specifications are above and beyond even what IPC Class 3A would be, as well. They’re very stringent on design rules, they’re very stringent on process control, and release, and DPA.

Normally, you might do 100% micro-sectioning for IPC Class 3. Some of these satellite guys are at 200% micro-sections. They just want to make sure everything is exactly built to the way they want it to be. Paperwork is huge as well. So the amount of QC, testing, reporting, micro-section analysis, fares, is much stricter than, for instance, just in aerospace, which is a pretty stringent product as well. But satellite just takes it to one more level. Paperwork’s pretty key to all the satellite people.

8:05 Are most electronics in aerospace or satellites exposed to the elements or are they in a controlled environment?

It depends where it would be in the aircraft. Something like the JetWave program, which is basically Wi-Fi on aircraft, that’s actually I’d say in the fuselage, that sits on a cap sitting on top of the fuselage itself. So, that’s in an exposed environment. The engine is an exposed environment.

So really, the only electronics that are internal would be your entertainment systems, all the galley lighting systems, and also the cockpit’s a controlled environment as well. We actually don’t have a differentiator between internal and external fuselage. The electronics would be built exactly the same. And how they’re encapsulated and controlled is much different as well.

I’d say fuselage, and engine and wing electronics are basically being encapsulated in hermetically-sealed boxes. So the boards themselves aren’t exposed to any elements. But they are exposed to temperature and heat and cold cycling on a daily basis.

8:54 Is there any concern for aerospace electronics facing electromagnetic interference?

Yes. Not as much as, for instance, satellite is concerned about it. But the big challenge would be if you had a lightning strike. There’s a lot of redundancy built in the aircraft. Typically you’ll have more than one system that, basically, you can switch over to. And the way that aerospace works is that they’re not ever both supplied from the same supplier. So, if there’s ever a defect, you don’t want to run the risk of having the same defect on the backup system. There’s a lot of redundancy built within the aircraft.

Engines could die, and the aircraft can still fly on one engine and same in the electronics. If one board gets dying, there’s typically, on the critical systems, they’ll be a backup for that system as well.

9:53 Space travel commercial is becoming a thing. Will the electronics for aircraft carrying human beings be different?

We may be close to people getting into space, but I think they’ll only be there for 30-minutes or an hour. It’s more of a low-orbit and back thing. But, the electronics on, for instance, the space station, is a much different animal to deal with. Even electronics we send out to Mars, like the Mars Rover. There’s a new huge satellite being built that’s actually going up to a million miles away, which will be 100 times more powerful than Hubble, so the planning is intense. And they run through multiple, multiple scenarios on releasing these products into space. So it’s pretty key.

The big challenge

The big challenge is getting electronics from the ground to space. That’s the big challenge. Once it’s in space, if it’s working, things are good.

But there’s a new mentality to move space satellites to modular. They send up little mini robots, and in five years, they’ll be able to launch just a module. And what’ll happen is the robot can go pick the module up, and it’s built by modules. And it’ll up the capability of the satellite because, if you look at the satellite usage, there’s no space left to put any more satellites up. So, all the little windows are all full. What they’re now looking at is putting two satellites in per window.

But the big challenge is power supply. You need power supply to keep it geostationary. If you want close to it, you don’t want to two satellites crashing into each other. So the thinking is, in future satellites, if you have replaceable power supplies, you could extend the life of that satellite to 20, 30 years. And if you could replace the electronics, you’ve now extended the technology within the satellite at the same time. There are thoughts about how they’re actually going to adopt this in to the satellite business.

12:12 What is the source of power supply currently in space electronics?

They use different types of power supplies

Some of it is solar, some of it is chemical. And the other is batteries. So there are different ones.

The problem with the thrusters, it typically needs to be a chemical-type reaction. The thrusters only, it’ll just burn for a couple of seconds, just to keep the satellite in position. More of the power within the electronics themselves is basically solar and battery. It’s using solar to capture energy, recharge batteries, and then basically it can run off battery systems.

But the other problem is, once the chemicals power supply, or source, exhausts itself, you have nothing to reposition your satellite. Because the solar part of it just can keep re-energizing itself forever. But, the chemical part, for the thrusters, etc., that’s the part they need to try and replace. That typically is going to exhaust itself in 10 to 15 years. Even though it only uses a very, very small amount of energy, but it’ll deplete eventually.

13:24 How is the growth in the aerospace industry?

The projected growth for aircraft is, the number of aircraft globally will double in the next 20 years. And that just doesn’t mean aircraft – obviously there’s a huge growth in Asia, China’s demand is going to be massive – but that also is for simulators, for pilots. So the whole industry needs, they’re talking about 900,000 pilots over the next 20 years.

And then in 20 years, we have all the pilots retiring, so there’s just going to be a constant need for pilots. Then you need simulators to train them, and obviously, you need aircraft to put them on. So the number of passenger aircraft’s going to double over the next 20 years.

 

Mil-Spec/Aerospace

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