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PCBD-Jan2015

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46 The PCB Design Magazine • January 2015 craft, military aircraft, nuclear power stations, and nuclear weapons. In order to ensure the proper operation of such systems, manufactur- ers of integrated circuits and sensors, intended for the military or aerospace markets, employ various methods of radiation hardening. Radi- ation-hardened (RAD HARD) components are based on their non-hardened equivalents, with some design and manufacturing variations to reduce the susceptibility to radiation damage. Radiation-hardening techniques: • Hardened chips are often manufactured on insulating substrates instead of the usual semiconductor wafers. Silicon on insula- tor (SOI) and silicon on sapphire (SOS) are commonly used. While normal commer- cial-grade chips can withstand between 5 and 10 krad, space-grade SOI and SOS chips can survive doses many orders of magnitude greater. • Bipolar integrated circuits generally have higher radiation tolerance than CMOS cir- cuits. The low-power Schottky (LS) 5400 series can withstand 1,000 krad, and many ECL devices can withstand 10,000 krad. • Magnetoresistive RAM, or MRAM, is consid- ered a likely candidate to provide radiation hardened, rewritable, non-volatile conduc- tor memory. Physical principles and early tests suggest that MRAM is not susceptible to ionization-induced data loss. • Shielding the package against radioactivity, to reduce exposure of the bare device. • Capacitor-based DRAM is often replaced by more rugged (but larger, and more expen- sive) SRAM. • Shielding the chips themselves by use of depleted boron in the borophosphosilicate glass passivation layer protecting the chips, as boron-10 readily captures neutrons. So, it seems that PCBs cannot be totally shielded against the impact of cosmic rays. However, we can take precautions to avoid in- terference from shorter-wavelength radiation that is more prevalent in our environment. 1. Route high-speed signals between the planes, fanout close to the driver (200mil) dropping to an inner plane and route back up to the load again with a short fanout. This will help shield the sensitive signals. 2. Keep critical signals away (200mil) from the board edges. 3. Avoid short stubs as these are more com- patible with shorter wavelengths. 4. Use differential pairs for high-speed signal routing as their equal and opposite polar- ity rejects common-mode noise. 5. Use higher operating voltage components with a larger die size where possible. Embedding signals between the planes re- duces susceptibility to radiation, as well as pro- viding ESD protection. In doing so, not only do we prevent noise from being radiated but we also reduce the possibility of being affected by an external source. So next time your PC blue screens, your cruise control locks up or your 747 suddenly decides to do a death dive—it may just be a ran- dom glitch from cosmic rays or a solar flare. Points to Remember • Noise sources range from low frequency to high frequency, RF and microwave radia- tion and can be generated by nearly any electrical appliance or device. • Cosmic rays can wreak havoc in electronic systems in a number of ways. – One of the most common is called "a single event upset," in which cosmic rays ionize atoms in a semiconductor, releasing a burst of electrons. – A hard error or "a single event burnout" is more serious in which components are damaged or destroyed by a sudden short- circuit caused by the burst of electrons. • Cosmic rays create eight times more soft errors in ICs with 40nm transistors than those with 130nm. • Circuits running at 0.5V have twice the rate of soft errors as those running at 0.7V. • Materials that have high hydrogen con- tent, such as polyethylene, have been used to reduce radiation to a greater extent than metals, such as aluminum. • Demron, a material said to have radiation protection similar to that of lead shielding, ELECTROMAgNETIC SuSCEPTIBILITY continues beyond design

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