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.
Dr. Jennings’ work contributed greatly to the industry and was the basis of much progress. He was a giant in his time.
He like many other historical industry “greats” will not be recognized for his accomplishment and contribution unless a major outside campaign is waged in his behalf.
…and that in itself may serve to provide the justifed recognition – even without a small plaque from the trade association.
Peer recognition can be the highest honor.
He was just not popular enough nor close enough to those that make the final decisons regardingthe granting of recognitions.
Mike Beutow was one of the folks that helped keep the effort of creating IPC-2152 moving along through some inspirational writing. Thanks Mike. For the sake of clarity, which is the purpose behind 2152, the technical paper by Dr. Jennings was not the source behind the original charts. I thought it was, as a result of some of my first inquiries, but it turns out that the original charts go back to work done by folks at the National Bureau of Standards around 1955.
We used IPC-TP-117 when we ran the first set of experiments. That was before finding IPC-TM-650 and the method that was used for the remaining tests used to create the new charts in 2152. The work that Jennings reported in IPC-TP-117 was all performed for short time durations of 30 seconds. The current was applied to the trace for 30 seconds and the traces didn’t have time to come to a steady state temperature. That had us going for a while. I met a fellow co-worker of Dr. Jennings after I presented a technical paper at an IPC conference and had a good chat with him. He sent me a box of papers that he had in his garage that Dr. Jennings had left to him. Dr. Jennings had since passed away at that time. In the box was another technical paper that Dr. Jennings had written, but not published. The paper was on steady state results from further current carrying capacity testing that he had performed. It was one of the first sets of data that we were able to compare against that showed a good comparison with the new data.
Mike is too kind. Here’s a guy who left a great job and risked his career and finances in order to do something on behalf of the industry. All I did was try to call attention to him.
(Thanks for the correction on Dr. Jennings, btw.)