The Physics of Failure: Predicting Reliability in Electronic Components

A lot has been written about Moore’s Law and the potential limitations of semiconductor design and manufacturing in the future. Can we really continue to double the number of transistors on a given-sized die every eighteen months? For the last 30 years, Moore’s Law has held, but, we may be seeing the real limitations to future semiconductor development and to Moore’s Law — severely shortened operational lifetimes of advanced chips. That, in turn, creates massive reliability problems for critical embedded systems in the future.

As we advanced our semiconductor development knowledge and manufacturing processes over the years, the reliability and operational lifetimes of those chips increased to such a level that all semiconductor makers dropped their 883B testing procedures and eliminated their military chip lines and designations. The chips coming off the production line were “good enough” for many critical systems applications, and the military market was too small to justify separate testing and product designations. While the commercial chips were typically rated for 0-50 degrees C operation, some critical applications needed reliable operation over extended temperature ranges (-40 to +125 degrees C). Those system builders took the commercial- grade devices, sent them out to testing labs, and found that a large majority of them would, in fact, operate reliably at those extended temperature ranges.

Be Cool

We did run into some problems in this micron to sub-micron shift. We could operate those small-geometry chips at much lower voltages, but they consumed much more current and produced more heat, requiring better cooling techniques. Leakage currents also became a severe problem, adding to the high current requirements and cooling issues. We all know that heat kills electronic components. The Arrhenius formulas tell us that for every 10 degrees C you decrease the temperature on your electronics, your MTBF (Mean Time Between Failure) will double. We typically used air to cool these devices. Air is a great insulator, but a terrible conductor of heat. With forced-air, you can remove about 1 to 1.5 watts of heat per square inch of PCB area. Sub-micron chips, particularly microprocessors, generate 25 watts of heat per square inch or more.