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PCBD-Jan2015

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44 The PCB Design Magazine • January 2015 Cosmic rays create eight times more soft errors in ICs with 40nm transistors than those with 130nm. As transistors shrink, the amount of charge required to incite the circuit is reduced. This trend is also evident in the drive for more efficient chips that run at low voltages; circuits running at 0.5V have twice the rate of soft er- rors as those running at 0.7V. Not surprisingly, the space industry began designing its craft with these problems in mind way back in the Apollo mission days. Thin sheets of gold foil, less than 0.15mm thick, were used in space programs as a radiation shield. The lunar modules of the Apollo flights were shrouded in foil. Unfortunately, this thin gold foil offers no obstruction to the extremely short wavelength of cosmic rays. However, it may have offered the astronauts some protection from solar radiation, but provided little shield- ing for sensitive electronics. More recently, ma- terials that have high hydrogen content, such as polyethylene, have been used to reduce ra- diation to a greater extent than metals, such as aluminum. Hydrogen atoms are good at absorb- ing and dispersing radiation. Demron, a mate- rial said to have radiation protection similar to that of lead shielding while being lightweight and flexible, is also being trialed. Spacecraft de- beyond design ELECTROMAgNETIC SuSCEPTIBILITY continues Figure 1: Transistor size vs. microprocessor error rate (source: oracle). Figure 2: Gold foil acts as a radiation shield on the Apollo lunar module (courtesy of World Gold Council).

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