Estimated reading time: 7 minutes

Defense Speak Interpreted: Rad-Hard Electronics

Have you ever seen electronics described as “rad-hard,” or radiation-hardened, and wondered what that meant and how that was done? Did you like me just assume that “rad-hard” and “expensive” were synonymous? Did you think that this was a Defense Department term since they deal with nuclear weapons?

What part of electronics needs to be protected from radiation—chips, packaging, boards, assemblies? What kinds of radiation harm electronics? How is radiation protection accomplished for each type of radiation?

What I thought was a pretty simple technology to keep electronics working in space turns out to be a highly complex problem with multiple causes, intensities, results, and solutions. And that is for electronics not considering radiation damage to humans or terrestrial organisms. I was aware of occasional news about sunspot activity, but I had little idea that this was a sudden increase in solar radiation that would affect satellite/radio communications and reportedly included ATM machines.

The concept of radiation only dates back about 125 years to the discovery of radioactive elements and the imaging properties of radiation that evolved into X-rays. The critical step in the process was that radiation activated phosphors, which emitted visible light and, in turn, exposed silver films or screens. Radiation recognition included:

1. Broken bone X-rays by linking electronic “tube” radiation sources to recording film
2. Massive radiation injuries from the atomic bombs in WW2
3. Scary devices as foot X-rays to show shoe size/fit in the 1950s

This all preceded any recognition of radiation problems for electronics later in the 1960s. By the time electronics shrunk to semiconductor size in the late 1960s, we were ready to add outer space radiation damage effects to the problem list for electronics—especially for orbiting satellites and long-range missiles. Some of the first space missions were to map the intensity of radiation in the Van Allen radiation belt around the earth to prepare for further radiation-hardening and human space flights.

What is the spectrum of radiation that can cause damage to electronics? We need to look at radiation in terms of electron volts in orders of magnitude (Figures 1 and 2).

Figure 1: Relative particle energies and rates of cosmic rays ^[1].

Figure 2: Electromagnetic spectrum ^[2].

This is a huge range with X-rays starting at 10 to 5^th power to cosmic rays at 10 to 15^th power, causing damage to electronics. And worse, the effects of radiation cause different types of damage to electronics—some of which is permanent. Imagine in billiards how the cue ball scatters the rack of 15 balls all around the table upon impact. The scattered balls can be recovered (re-racked) by checking to see if they are replaced properly in the rack.

But imagine a sonic speed cue ball hitting the rack of balls and breaking some while causing others to fly through the walls or ceiling. That is the higher energy collision of gamma or cosmic rays with electronics. The electronics survivability is to allow for this damage is with redundant electronics.

According to one research article ^[3], there are seven sources and damage impacts of radiation that have to be considered in electronics design:

1. “Cosmic rays: They consist of approximately 85% protons, 14% alpha particles, and 1% heavy ions with X-ray radiation.” The atmosphere filters most of them, so they are mainly considered for spacecraft and high-altitude aircraft.
2. “Solar particle events: They come from the sun and consist of a large flux of high-energy protons and heavy ions accompanied by X-ray radiation.” Worse, this varies with solar flares and sunspot activity
3. “Van Allen radiation belts: They include electrons and protons trapped in the earth geomagnetic field. They mainly affect satellites.”
4. “Secondary particles: They result from the interaction of other kinds of radiation with structures around the electronic devices.”
5. “Nuclear reactors: They produce gamma radiation and neutron radiation, which can affect sensor and control circuits in nuclear power plants.”
6. “Nuclear explosions: They produce a short and extremely intense surge through the entire spectrum of electromagnetic radiation, an electromagnetic pulse (EMP), neutron radiation, and a flux of both primary and secondary charged particles.” Remember the “neutron bomb,” which kills all people but leaves infrastructure intact.
7. “Chip packaging materials: They are a kind of insidious source of radiation that was found to cause soft errors in new DRAM chips in the 1970s. Traces of radioactive elements in the chip packaging will produce alpha particles, which discharge occasionally some of the capacitors used to store the DRAM data bits.” Have you heard of “low alpha” lead for assembly?

There are two kinds of radiation damage electronics: electromagnetic emissions and particles. Don’t get me started on whether electrons are actually energy or are “particles.” Just protect electronics from a very energetic electron bombardment. We can see that radiation damage occurs in many places: space (Van Allen belt), nuclear reactors, particle accelerators, radiation weapon systems, isotopes in packaging materials (give up particles), on earth from penetrating cosmic radiation, nuclear radiation weapon systems (neutron bomb).

Radiation-hardening protects electronics through (1) a specialty design, (2) radiation shielding enclosure, and (3) error-correction algorithms that “restore the pool balls to the rack.” Design starts with the utilization of known good chip designs and then beefs them up with more tolerant materials of construction. Since rad-hard electronics needs proven designs, rad-hard chips are seldom leading edge, and once proven in use, rad-hard designs tend to use these workhorse chips over and over. For instance, bipolar ICs are less sensitive to similar doses of radiation damage than CMOS. Total ionization dose (TID) is the term for accumulated damage from relatively constant radiation bombardment and factors into the choice of materials.

Different damage occurs from single events like nuclear explosions. These transient dose events can both completely throw off the function of the semiconductor and permanently damage (fry) the device if radiation intensity is large enough. The damage caused by a high energy single cosmic ray impact is somewhat similar to the transient of a nuclear explosion. The damage topic quickly gets into the functions of semiconductor sources, drains, gates, and other specific terms way beyond my capability to present here.

As sensors are frequently made with semiconductor techniques, they also need to also be rad-hard. And with the promotion of such ideas as the “Space Force,” radiation-hardening is forecast to see greatly increased use.

Again, what can be done (that a layperson can understand) to defeat radiation damage? Shielding with radiation-absorbing metal comes to mind. Semiconductors on insulator (SOI), like silicon oxide structures, are not as sensitive as pure silicon. And newer materials, like silicon carbide or gallium nitride, are coming down in price and promise even better radiation tolerance. Some good choices may come from using S-RAM (static) versus D-RAM (dynamic) memory. Also, error-correcting logic can be incorporated into the chip at the expense of making each die larger and more expensive.

Since you see X-rays on the damage chart, you may ask, “Does X-ray inspection of electronics cause permanent damage”? This is a known factor, and vendors of electronics X-ray inspection equipment limit dose and exposure time to minimize any radiation damage. Electronics damage is the product of both exposure dose and the time exposed to the source. But don’t fear; instead, continue to evaluate solder joints with X-ray. Unless you leave the assembly being exposed over the weekend, you will be okay.

Finally, the U.S. is not the only country work on rad-hard. Here is a recent tidbit I picked up ^[4]:

“Researchers at Peking University, the Chinese Academy of Sciences, and Shanghai Tech University have recently fabricated a radiation-hardened and repairable integrated circuit (IC) based on carbon nanotube transistors with ion gel gates. This IC, first presented in a paper pre-published in Nature Electronics, could be used to build new electronic devices that are more resistant to high-energy radiation.”

You can either be a trained radiation-hardening expert with lifetime employment, or you can trust the rad-hard engineers to do their job and use their electronics in confidence in your designs.

References

1. “Cosmic Ray,” Wikipedia.
2. Horst Frank, “Electromagnetic Spectrum,” Wikimedia Commons.
3. Fa-Xin Yu, Jia-Rui Liu, Zheng-Liang Huang, Hao Luo, and Zhe-Ming Lu, “Overview of Radiation Hardening Techniques for IC Design,” Information Technology Journal, 9: 1068-1080, 2010.
4. Ingrid Fadelli, “Radiation-immune and repairable chips to fabricate durable electronics,” Tech Xplore, October 2, 2020.

Dennis Fritz was a 20-year direct employee of MacDermid Inc. and is retired after 12 years as a senior engineer at (SAIC) supporting the Naval Surface Warfare Center in Crane, Indiana. He was elected to the IPC Hall of Fame in 2012.