In this third article in a four-part series, I look at how generating electronic printed circuit board (PCB) layouts has evolved since the mid-1980s (and likely earlier).
Part I: Electronics Manufacturing Review
Part II: Schematics – Then and Now
Part III: PCB Design – Then and Now
Part IV: Electronics Assembly – Then and Now
Circuit designers make the schematic and bill of materials (BOM) that create the concept of how electronics will work. PCB designers turn that dream into a physical reality. In addition to connecting the components, they also must have a strong understanding of the board capacitance, trace impedance and heat transfer just to name a few. I was one of the engineers who was naive enough to think once I had a circuit designed, all I needed to do was to connect the components with wires or traces. That’s when it didn’t work and that became my first lesson in focus on what I know and leave the PCB layout to the experts.
A typical process to design a circuit card looks like this:
- Start with the circuit schematic (a graphical representation of an electrical circuit) and BOM
- Be certain mechanical requirements are known, including board size, mounting hole locations, connector locations, height requirements and any thermal transfer needs
- Place the parts where they need to be
- Route the signals (aka connect the traces/wires)
- Opimize the PCB design based on experience
In the 1980’s when I started in the electronics industry, this was a completely manual process. Today, circuit design is automated with CAD software, revolutionizing the process.
Vintage PCB Design
PCB designers were also referred to simply as drafters, but the reality is they were artists creating electronics masterpieces. Many of the integrated circuits (IC) and passives (resistors, capacitors and inductors) used were very similar in package size. The PCB designer had to manually decide how all of these components could be placed, then connected, per the schematic. Some short traces could just be connected from point A to point B. Some traces could route around others by taking a different path. Other traces were impossible to route on a single layer without shorting to other traces. Routing these signals would need to jump to another layer to avoid crossing other signals, then possibly back to the original layer to complete the connection. One thing that helped a little was that components were not as complex decades ago. Even so, there would be times when it became obvious that the original concept was not going to work and needed to be modified or even scrapped – and go back to the drawing board (so to speak).
These designs were hand-drawn on translucent velum, which could be stacked to see how the layers lined up while designing and could be adjusted if needed. Imagine how complicated even one single IC could get after the initial design was complete. Depending on where the new circuit needed to be, a major overhaul of the design may be required.
Modern PCB Design
Computer-aided PCB layout packages are now available that automate the manual process described above.
Prep
Starting with the BOM, a library of all the parts is created that defines the footprints of the components. A footprint is basically where the pads need to be to solder the component to it. The PCB designer creates the outline of the board. Once the library is complete, the schematic and library are loaded. These are placed in a work area on the screen.
Parts placement
The PCB designer then places the critical components on the board in locations that satisfy the mechanical requirements, which include:
- Connector locations
- Mounting hole locations
- Tall components placed in acceptable areas
Depending on the board, the remaining components may be auto-placed or placed in specific areas, especially in the case of RF circuit design. This step really helps efficiency and accuracy. Once the parts are placed, a review is performed with the engineer to identify any changes required.
Routing
From a non-PCB designer’s point of view, this step is amazing. When the schematic is loaded, all the connections are made. The PCB design software keeps these connections and auto-routes the traces. Controlled impedance pairs or differential pairs can also be specified for required circuits.
Optimization
Even with all the tools provided, the PCB designer experience is what really matters. The tools are good, but they don’t replace the knowledge of what needs to be done. Some examples are:
- Ensuring the silkscreens are readable
- Ensuring areas that need heat taken away have a path for it
- Ensuring pads do not connect to such a huge ground plane to make them too difficult to solder
- Exposing test points for testability
- Ensuring the board is manufacturable (DFM)
The PCB layout tools of the last several decades have simplified design complexity and improved efficiency of design, as well as keeping the integrity of the schematic intact.
Bob DiDonato
Engineering Program Manager
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