Every system and circuit design is an exercise in balancing trade-offs in size, weight, power, reliability, performance, and many other parameters. After all, that’s a lot of what engineering design is all about, working on that combination of hard numbers and analysis based on experience and judgment to assess, balance, and execute trade-offs.
In some cases, one or a few goals may have by far the most critical weighting, so other goals may be sacrificed to some degree to satisfy those that are most important. In other cases, weighing and balancing trade-offs and choices is a more reflective and iterative process: “should we give up 5% of runtime if it improves sensor accuracy by 15%? ” which is often difficult to quantify and harder to answer. Of course, many design goals are just that: lofty goals where some must be achieved while others are ones the design strives for but are not absolute “must haves”.
Among the must-haves are the many EMC, efficiency and safety mandates that various regulatory and standards bodies (government and industry) have put in place. Unlike performance targets where there are give-and-takes, many of these mandates are absolute: either you meet them or the design will not be approved and certified. They have little “wiggle room” or areas of compromise except for the specific approach and tactics you choose to respond to some of them.
I thought about this problem as I was finishing the last part of a ten part series”Evaluation of EMC emissions and grounding techniques on 1 and 2 layer PCBs with power converterspublished by In Compliance (this last part contains links to the previous nine parts). The series detailed the many performance and mandate issues associated with an EMC acceptable design as well as the somewhat related field considerations (Figure 1).
Figure 1 A printed circuit board should be viewed and evaluated simultaneously from its thermal, power distribution, and EMC domains, as well as other perspectives. (Image sources: Electronic Concepts and Engineering, Inc.; Open Airbus Cockpit; ResearchGate).
Reading this series of articles was both exhilarating and discouraging. It was the first because it showed all that was understood and explained by the three authors with regard to theory, practice, measurements, etc. With this information, designers should have a better chance of designing a configuration for power, grounding, and DC/DC switching regulators that meets system performance needs and passes certification. So far, so good.
But the article also challenged me: it made me even more aware of the many expectations placed on design and, by extension, on the design team. There are so many best practices and often conflicting guidelines to adhere to, and so many compromises where doing something right also has a negative effect. Some of these imperatives are defined by the laws of physics and Maxwell’s equations, while others are due to well-intentioned regulatory compliance standards. In many cases, you need a compliance expert to guide you to, through, and beyond the thick thicket of standards.
The article quoted above was for relatively simple one- and two-sided PC boards, but many designs are now on four-, eight-, and more-layer boards. In some ways, the availability of additional layers makes it easier to meet EMC and floor requirements, as there are more degrees of freedom for floor planes and other good things; in other respects, their presence complicates the design because there are many more paths for signal routing and current flow, emission sources, and emission-sensitive detection points.
We’re asking for a lot of circuit and board designs these days, of course. We regularly have modestly sized boards handling tens and hundreds of amps, which inherently leads to IR drop and connection resistance issues. Also, almost all of that power current is converted into heat, so there are thermal issues in dissipating all that heat to that magical, mystical place called “far away.”
At some point, I’m afraid we’ll run out of room for the runners. The multiple requirements (DC electricity, signal integrity, EMC, thermal, insulation, creepage/clearance) placed on the circuit board design will either produce a zero assembly or the need for a severe compromise on levels power, circuit density, thermal density, EMC performance, size… it’s a long list.
Admittedly, thinking we’ve reached the end of what we can deliver and can’t go any further is nothing new in engineering. Somehow we find a way to surpass it with new materials, techniques, components and other innovations. After all, it’s the story of Moore’s “Law” (a brilliant, prescient guess, but not a law, sorry) at every major knot. Perhaps the engineering imperative is a bit like Samuel Beckett wrote at the end of his 1953 novel “The Unnamable”: “…you must go on. I can’t go on. I Or it could be what Samuel C. Florman was referring to by the title of his book “The Existential Pleasures of Engineering.”
Yet, at some point, radical new approaches are needed. Right now I don’t see such breakthroughs, nor do I see designers stopping their attempt to fit more components and more functions at higher frequencies and dissipation on these PCBs . Perhaps there is a three-way fork in the road to the circuit board: a path leads to a dead end and we stop; the middle way is that of slow, steady and gradual progress; and the final route is some kind of somewhat revolutionary approach such as a switch to fully integrated optics which is low power and also has no EMC issues.
What is your sense of the map design situation? Are we asymptotically approaching a limit to what we can achieve with technology? Will the way forward be one of steady progress made possible by a series of small steps? Or will some breakthrough technology that we don’t yet clearly see change the whole picture and enable significant advances, much like the transition from hand-placed components and hand-wired circuits to the pick- and-place and PC cards?
Bill Schweber is an EE who has written three manuals, hundreds of technical articles, opinion columns, and product specs.