Design challenges for PCBs in the space industry
Designers of PCBs for the space industry know that, as Star Trek said, space really is the final frontier. Overcoming the hurdles in designing circuit boards capable of functioning flawlessly from blast off and beyond presents some interesting challenges and solutions.
Let's take a look at them now...
The problems faced by space capable PCBs
There are two basic problems that PCBs in the space industry need to overcome: leaving the atmosphere intact, and surviving in the most extreme environment known to man once they arrive in the vacuum of space.
PCBs will have to deal with:
• Vibration - NASA's Spacecraft Vibration Lab tests in a frequency range of 5-2000 Hz, and its Acoustic Lab up to 165 dB, which gives you some indication of the forces expected during a launch procedure.
• Radiation - single event effects (SEE) caused by single high-energy particles leaving an ion trail can disrupt electronics in a number of ways, leading to potential memory errors and short-circuits.
• Outgassing in a vacuum - materials used in standard PCBs are stable in an atmosphere, but once placed in a vacuum they might outgas, which can lead to component damage and weakening of the substrate.
• Temperature extremes - once outside our atmosphere, circuit boards will be exposed to the background temperature of -270 Celsius, but may also feel the full force of the sun in orbit - for example, the Moon's surface is around -200°C at night and +200°C during the day.
• Tin whiskers - pure tin (along with zinc and cadmium) cannot be used in a vacuum due to the potential for crystalline structures (whiskers) to form that conduct electrical current and short circuits.
In addition to these environmental factors, space-bound PCBs must adhere to tight weight restrictions because every additional gram requires more fuel, and leaving the atmosphere is expensive... so, how does the electronics industry answer these PCB design challenges and the environmental demands?
Designing PCBs for space travel
As with every task the electronics industry is given, the answers to these challenges must be cost-effective and robust, so let's go through them one-by-one:
• Vibration - NASA used to solve this problem by simply making substrates thicker, but since the introduction of flexible PCBs it's often turned to polyamides like Kapton® instead, because they bend rather than break under vibrational stress. However, they still use common glass-fibre and phenolic resins where appropriate, but alongside substances like Kevlar and Teflon that offer favourable strength-to-weight ratios.
• Radiation - there's no foolproof way to protect against the effects of radiation – NASA uses a multi-pronged approach with some lead shielding; smaller, less vulnerable components; antifuse technology to create permanent conductive routes between transistors, and effective back-up systems in case of failure.
• Outgassing - this is controlled with the use of PTFE resin substrates that are reinforced with glass fibre.
• Temperature - ceramic boards are often used to protect against temperature extremes – especially where there’s no airflow to assist cooling, and heat must be radiated out into space.
• Tin whiskers - the addition of approximately 3% lead to the solder prevents tin whiskers.
As with many PCB solutions, the answers are often elegant and simple, while demonstrating that if humanity is to continue in our quest to 'boldly go where no one has gone before' it will be largely because of the pivotal role of PCBs in the space industry, and their very considered and clever design.