Electronics design for space applications is a subject I’ve always been passionate about, and understanding the reasons particles can damage circuitry has always fascinated me. In this article, we’ll discuss how a Beta or Gamma particle can create—for a short period of time (usually referred as SEE or Single Event Effect)—a large current in your design which could lead to permanent damage if ignored. We’ll discuss a technique I have used in my design analysis to estimate the current peak width once the Linear Energy Transfer (usually a design parameter) is known. This technique cannot always be applied, but once you understand the mechanism which creates unwanted peak currents, you have all the tools needed to design Radiation tolerant electronics.
What are we going to learn?
- How to prevent damage from high energy particles hitting the base or gate of your electronics.
- What is a Single Event Effect
- Why Aluminium helps to protect electronic circuits.
- How particles interact with metal and silicon materials
This is a complex subject which can’t be fully described in an article. When I started writing about this subject, I wanted to put it all into a single article, but then I realized I had to cut too many key concepts, so I decided to divide it into two parts. In this article, we’ll see the very basic mechanism for Single Event Effects. In the next article, we’ll see how to use Altium/SPICE for analyzing similar scenarios and for use with design reports. Keep in mind that we are merely going to scratch the tip of a huge iceberg. Also, feel free to ask me questions in the comments section.
In the writing of this article, I’ll assume you are not an expert in physics or microelectronics—we will lose some detail, but we’ll get the concept.
OK, let’s stop talking and let’s start our investigation.
When a satellite is sent to space, it’s irradiated by different types of particles. Some of these particles are ionized (e.g. they have a charge as the Alpha+ and Beta- particles), while others do not have a charge (e.g. photons: Gamma Ray, X-ray).
Since ionized particles have lower energy than gamma and x-ray particles, a good portion of them may get absorbed when they hit a layer of metal like aluminium or gold. The particles that are not absorbed by the aluminium (usually beta, gamma and x-ray) may hit your electronic components, and a portion of this may create an unwanted behavior called a Single Event Effect (SEE) See figure 1.
Figure 1. How different kinds of particles get absorbed from different kind of materials
From figure 1 we can see that heavy ion and electrons may get absorbed by the aluminium. Particles with higher energy instead will pass through, and a sub portion of those may hit sensitive parts of an electronics causing a single event effect.
A short note: In the past I have seen aluminium plates fitted above components (in the attempt to protect them.) While this may seem like a good idea for LEO Satellites (LEO = Low Earth Orbit), it also creates new challenges (see Figure 2). Let’s see the issue of this solution:
Figure 2. When a beta particle hits the plate, it may get trapped, and this can generate a large voltage that, when above a certain level, will discharge over the chip we are trying to protect, destroying it instead.
The solution shown in figure 2 has two issues
The first issue is that when ionized particles get trapped in the aluminium plate, over a long period of time the charge will grow and will create a strong electric field (kV/m) that may discharge on a pin of the chip we are trying to protect. So, when using this solution, I would create a discharge path between the aluminium plate and the GND as well. There are several ways to do this. One possible way is to solder a wire between the plate and the a PCB pad connected to GND (through a resistor).
The second issue is that the aluminium plate will make the chip heavier and this may create issues when performing a vibration test. So be careful when you do this! Check everything!
It is now time to describe how a particle may create a large current in our designs. Let’s consider the example in figures 3 and 4. (I’m using BJT, the same issues is with MOS transistors)…
Figure 3. Normal conditions when Vin = 0V, the NPN is “OFF”, the PNP is “ON”, and we have a relative low current (load dependent).
In figure 3, we can see a totem pole working as it should… e.g. we apply a low (high) voltage at its input and the output goes high (low), but what is going to happen when a particle with sufficient energy hits the base of the NPN? To understand what may happen, let’s have a look at figure 4. Let’s assume the Vin is still 0V and a particle hits the NPNs base. If the current generated from the impact of the particle is sufficiently high, then a current will be generated in the collector and there will be a very large current crossing the totem pole, which may cause destruction of the totem pole itself.
Figure 4. When a particle hits the base of an NPN, it may turn the NPN "ON" as well. This will create a large current that may destroy your circuit.
So… we now know (one of the many) mechanisms that may damage your electronics.
What are we going to learn next?
What is LET (Linear Energy Transfer)
How to estimate the total charge injected in the base, once the LET is known
How to estimate the current variation and the duration with Altium.
Would you like to find out more about how Altium can help you with your next PCB design? Talk to an expert at Altium and explore how to design aerospace PCBs in Altium Designer’s® unified environment.