Farewell, Dr. Jennings

Charles Jennings isn’t a household name – even to electronics engineers – but most every person who designs or builds circuit boards for a living owes a small debt of gratitude to him.

For it was Jennings, a Sandia National Labs chemist, who in the early 1970s conducted the research that led to the current carrying capacity charts that have been a staple of electronics design ever since.

As I wrote on the TechNet ListServ more than a dozen years ago:

Jennings’ report describes a series of tests to establish electrical properties. The results includes voltage holdoff, current carrying capacity and insulation resistance for two-sided bare, coated, and encapsulated boards.

Average breakdown voltage (V) followed the relationship

V = 3.1 S(superscript)0.51,

where separation (S) ranged from 0.25 to 1.5 mm.

(This is for bare boards at ambient conditions.)

Current carrying capacity of conductors was evaluated by temperature rise between conductors generated with step increases in current. Variations in temperature rise between conductors with the same nominal or design width were correlated with meaured

differences in conductor cross-sectional areas. Resistances calculated from conductor lengths and cross sectional areas were within 10 percent of the measured values.

The boards tested were fabricated using a panel plate and solder dip or plate and liquid level process. A few measurements were made on boards fabricated using a pattern plating process with thin clad laminate.

Testing was “frequently” extended until functional failure to obtain a better understanding of the failure mode.

IPC published the Jennings’ paper, “Electrical Properties of Printed Wiring Boards,” as IPC-TP-117 in September 1976.

The results of the study indicated that conductor spacing recommendations in MIL-STD-275 were very conservative and could be reduced. Yet the mil spec’s commercial equivalent, IPC-D-275 (known famously as Table 3-4), and later IPC-2211, all pulled from the original work.

Since then, several companies, including AMP, Hughes and Lockheed-Martin, tried to duplicate the measurements. Generally speaking, they discovered certain holes in the findings.

On several occasions, I have talked with Mike Jouppi, the heady and creative Colorado engineer who has been wrestling with Table 3-4 for a decade. It looks like Mike has won. This week, IPC published IPC-2152, which finally replaces IPC-2221’s conductor sizing charts. Now, almost 40 years later, it is only a matter of time until Dr. Jennings’ landmark work will finally be laid to rest. But let’s not let the inevitable progress of technology to sand over a truly remarkable history of utility, one from which we have all benefitted.

P.S. IPC has sold tens of thousands of copies of IPC-D-275 and its successors, all based on Dr. Jennings’ famous study. Yet Jennings never made it into the IPC Hall of Fame. Go figure.