SGD | USD

Printed Circuit Boards: So Much Responsibility, So Little Respect

Printed circuit boards are literally the foundation of electronic products and systems. They connect and “wire up” the tens, hundreds, and even thousands of active and passive components with tiny lands (pads) and hair-thin traces, while also providing physical support, mounting tabs, connector arrangements, and more. They are often referred to as PCBs or pc boards, and there was an attempt some years back by the IPC, a key industry standards-setting organization formerly called the Institute for Interconnecting and Packaging Electronic Circuits, to re-name them as printed wiring boards or PWBs; but that name change never caught on.

Certainly, there’s no need to tell this audience about the indispensable role of printed circuit (pc) boards as well as their versatility and capabilities. Yet in many discussions, they are casually considered to be just another simple, albeit essential, no-big-deal passive component; that’s a misleading simplification.

The interesting history of the pc board

It’s been an interesting journey for these boards. When they were initially developed about 50 years ago, many designers considered them to be both necessary and a headache. They were needed to replace the use of point-to-point wiring and soldering by hand, a manual technique which could no longer support the density and manufacturing time needed for products such as color TVs with their 100+ vacuum tubes. In fact, one leading TV vendor of the time boasted that their TVs were hand-made by craftsman rather than using an anonymous circuit board. We know how that marketing story ends.

The first pc boards were single-sided and made of phenolic or Bakelite, instead of our modern glass epoxy composite; had punched rather than drilled holes for the through-hole component and socket leads; and were still soldered by hand (Figure 1). Line widths were on the order of 3 to 6 millimeters (mm).

Figure 1: Basic one-sided, through-hole phenolic boards, similar to this one, were the first widely used iterations of the pc board concept. (Image source:TheEngineeringProjects.com)

The reliability of these early boards was marginal due to delamination of the cladding, tolerance issues, and inconsistencies in soldering. But as they say, failure was not an option, since pc boards offered the only viable approach to dealing with higher component counts, IC packages, smaller components, higher pin counts, and eventually, surface mount components. Today’s pc boards have advanced many orders of magnitude compared to those early ones with respect to every performance and capability parameter.

Interestingly, single-sided phenolic boards are still used in some consumer appliances to hold nearly all their components; top-side wire jumpers are inserted so that a very low-cost, single-sided board can be used (Figure 2).

Figure 2: This phenolic pc board from a 2010 microwave oven contains the power supply (low and high voltage), transformer, power devices, and much of the rest of the circuitry; note the use of top-side jumpers to allow use of a low-cost single-sided board. (Image source: Low Price Mart)

The multi-tasking precision of a pc board

Despite the casual way we often talk about them, today’s pc boards are highly engineered, precision components. They are expected to do so much, much more than just acting as a component carrier and interconnection platform. Among their tasks:

  1. They route power and ground on their exposed layers if it’s a basic two-sided pc board.
  2. In multilayer boards, such as the common four-layer version, one inner layer provides power distribution for one or more rails and the other inner layer provides ground functions; conductive vias (short for vertical interconnect access and never capitalized) connect these layers as needed.
  3. The copper around or near a hot component functions as a heat sink, or as a thermal conduit to route the heat away to a discrete sink.
  4. The pc board copper can be configured to act as an RF transmission line, filter, isolator, or circulator using stripline or microstrip topologies.
  5. The pc board can also be designed to be an antenna, often as a multiband arrangement antenna, rather than a single-band antenna.
  6. RF passive devices—capacitors and inductors—can also be constructed using appropriate copper patterns.
  7. Precisely dimensioned traces can act as low-value resistors (several milliohms) for measuring current flow by the IR drop across the trace.
  8. The copper can also provide a guard ring around sensitive, low-level analog sensor inputs to op amps.
  9. The board’s copper can provide EMC shielding to prevent incident RF from affecting circuits or the complement of attenuating emissions from the board.
  10. Be the push-in receptacle for both stiff and flexible pins which terminate individual wires in a harness.

If that isn’t enough, there’s a new role which has been added to the list: to act as the mating connector for a ribbon-cable IDC (insulation displacement connector) from Würth Elektronik. Instead of the standard mating pair of IDCs, one as male with pin contacts (pins) and the other as female with sockets, the Würth approach uses the board as the mate for the male IDC.

Note that this is not the first time that wires have been plugged directly into a board. For many years, individual solid or flexible pins were pushed into plated holes in a pc board. But these pins cannot be removed without damaging the pin and board, so they were one-time insertion only. In contrast, Würth’s REDFIT IDC SKEDD Connector family can be plugged and unplugged up to ten times using the specified pc board hole sizing and plating, and up to 25 times with relaxed tolerances.

Figure 3: Würth’s REDFIT IDC SKEDD Connector family eliminates the need for an IDC receptacle to mate with the male (pin) IDC and flat cable, thus saving cost, simplifying the BOM, and reducing wire-to-connector transitions and thus potential sources of problems. (Image source: Würth Elektronik)

What’s next for the humble and underappreciated pc board? It looks as if the widely used FR-4 epoxy-glass substrate will no longer be as dominant as it now is. Its inherent characteristics fall short of the stringent demands of multi-gigahertz (GHz) designs, where subtle electrical and materials factors such as dielectric constant, dielectric constant (er), loss factor (tδ), moisture absorption, and others are critical. Also, not only must these numbers match the needs of GHz designs, they must have very low temperature coefficients, or tempcos, which FR-4 does not have. Even the mechanical and dimensional tempcos take on added significance as even minute shifts affect electronic performance at these frequencies.

The next time someone dismisses the pc board as “no big deal,” don’t fall for that attitude or misconception. The success of a project depends as much on that board as it does on any other component. The ability to maximize its functions, produce a multilayer board to incredibly tight specs, load it, and solder it properly, directly impact basic performance, reject/scrap rate, and field reliability.

 

References:

1 – Wikipedia, “FR-4” https://en.wikipedia.org/wiki/FR-4

2 – Wikipedia, “Printed circuit board” https://en.wikipedia.org/wiki/Printed_circuit_board#Materials

3 – Wikipedia, “Via (electronics)” https://en.wikipedia.org/wiki/Via_(electronics)

4 – SEEED Studio, “Printed Circuit Board (PCB) Material Types and Comparison” https://www.seeedstudio.com/blog/2017/03/23/pcb-material/

5 – Al Wright, Epec LLC., “PCB Vias - Everything You Need To Know” https://blog.epectec.com/pcb-vias-everything-you-need-to-know

6 – John W. Schultz, Compass Technology Group, “A New Dielectric Analyzer for Rapid Measurement of Microwave Substrates up to 6 GHz” https://compasstech.com/wp-content/uploads/2019/02/A-New-Dielectric-Analyzer-for-Rapid-Measurement-of-Microwave-Substrates-up-to-6-GHz.pdf

7 – Rogers Corp., “Characterizing Circuit Materials at mmWave Frequencies” https://www.microwavejournal.com/articles/32237-characterizing-circuit-materials-at-mmwave-frequencies?v=preview

8 – Rogers Corp., “Laminate Materials Simultaneously Increase μ and ε, Reducing Antenna Size” https://www.microwavejournal.com/articles/32056-laminate-materials-simultaneously-increase-mu-and-epsilon-reducing-antenna-size

About this author

Image of Bill Schweber

Bill Schweber is an electronics engineer who has written three textbooks on electronic communications systems, as well as hundreds of technical articles, opinion columns, and product features. In past roles, he worked as a technical web-site manager for multiple topic-specific sites for EE Times, as well as both the Executive Editor and Analog Editor at EDN.

At Analog Devices, Inc. (a leading vendor of analog and mixed-signal ICs), Bill was in marketing communications (public relations); as a result, he has been on both sides of the technical PR function, presenting company products, stories, and messages to the media and also as the recipient of these.

Prior to the MarCom role at Analog, Bill was associate editor of their respected technical journal, and also worked in their product marketing and applications engineering groups. Before those roles, Bill was at Instron Corp., doing hands-on analog- and power-circuit design and systems integration for materials-testing machine controls.

He has an MSEE (Univ. of Mass) and BSEE (Columbia Univ.), is a Registered Professional Engineer, and holds an Advanced Class amateur radio license. Bill has also planned, written, and presented on-line courses on a variety of engineering topics, including MOSFET basics, ADC selection, and driving LEDs.

More posts by Bill Schweber