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August 2014 • The PCB Design Magazine 57 SPACING IS IRRELEVANT BELoW 270 VoLTS continues This revelation came about through con- sulting contracts I've had with large companies over the past year. All involved working with trace spacing to help stop PCB assemblies from failing. One big surprise that resulted from my research: Based on physics from Paschen charts and voltage arcing formulas, it is impossible to have a self-generating arc at sea level in normal humidity at less than about 330 volts, and, cor- respondingly, less than 270 volts in high-alti- tude applications. So, all the spacing charts and worry about voltage-based line spacing was incomplete without adding these important environmental conditions to the chart. I researched as many spacing charts as I could, and studied the phys- ics of arcs and corona, leakage, dust and mois- ture. My findings changed the way I looked at circuit line spacing. The conclusion was that for 95% of PCBs, the line spacing used has almost nothing to do with the circuit operating volt- age, but almost 100% with environmental con- ditions. Now, I can hear the uproar. "What? No! The IPC charts are the Bible, and we can't think of looking at spacing in any other way." But the Paschen charts and physics show that it is impossible to generate an arc between two sharp points in normal air at sea level with one millionth of an inch spacing while applying 350 volts. I need not remind you that 350 volts is below the majority of PCB voltages used. So, why do we need spacing charts for less than 350 volts if the voltage is not the cause of shorts and arcs? And why do spacing charts show voltages down to 5 volts? My conclusion: Over time, we have learned that specific spacing is required to stop PCBs from failing in operation, but I believe we were seeing voltage as the guilty party when mois- ture, dust, dendrite growth and other environ- mental conditions are the bad guys in town. To relate this to another industry, let's look up 5 volts in our PCB design charts. These charts mandate 50 microns of minimum spacing, yet trillions of integrated circuits run at 5 volts with spacings down to 50 nanometers. What's the difference? It comes down to a sealed package that eliminates all environmental conditions vs. an open PCB with moisture-absorbing epoxy with glass fibers, surfaces exposed to moisture, nasty air quality, dust build-up and surface re- sistance. One good example for not following the standard spacing charts can be found in a fail- ing flexible circuit in a kitchen appliance. In this case, the designer followed the spacing rules and added more spacing for safety. The circuit was a trace of more than 20 volts and a ground trace at .075" spacing between traces with sol- der mask. This circuit was failing in large num- bers, and I was hired to find out why. Research showed it was excessive moisture on the flex causing dendrite growth right under the solder mask, causing shorting between the two traces. The standard spacing charts failed to address higher moisture situations with silver ink. To fix the spacing information problem, we need more spacing variables, different charts for specified conditions, because a single voltage spacing chart with a few simple environmental conditions cannot even come close to properly relating to all situations that PCB designers and engineers may encounter. Shameless plug: My soon-to-be-released e- Book will provide new information about spac- ing requirements and voltage, as well as more new information on PCB design and manu- facturing for increased reliability. It should be available soon at www.dmrpcb.com. Dan and I will have more information in upcoming issues, so stay tuned. PCBDESIGN Bob Tarzwell is a PCB consultant who has spent 50 years in the PCB industry, inventing technol- ogy and building almost every type of PCB. he is the co-owner of DB Publishing, the publisher of the PCB 101 and Quality 101 handbooks. Dan Beaulieu is a well-known in- dustry consultant and co-owner of DB Publishing. his column it's Only Common sense appears Monday mornings in the i-Con- nect 007 Daily newsletter. he can be reached at danbbeaulieu@aol.com. bob and me