In this second article in a four-part series, I will take a look at how generating electronic schematic diagrams and bill of materials (BOMs) have 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
Designing a circuit involves determining what electronic components need to be connected to achieve the desired operation of the circuit. The diagram illustrating how the electronics are connected is called the schematic diagram. The document showing which components and sometimes other reference materials is called the bill of material (BOM). The layout of these components and wire or trace routing of this will be covered in the next installment of this series (Part III – PCB Design – Then and Now).
A typical process to design a circuit is:
- Start with the input and output requirements of the circuit board
- Block diagram the elements required
- Break down the blocks into electrical components to achieve the goal (develop a schematic)
- Choose components that will work in the schematic (develop the BOM)
- Finalize the design, adjusting the schematic and BOM as required
While these basic design elements have remained the same, the schematic design process and BOM generation have changed dramatically over the last several decades.
Vintage Schematic Design
To create a schematic in the mid-1980s, an engineer would typically hand-draw everything. This included the logic gates, resistors, capacitors and any other components required. Connectors would be rectangular boxes with pin numbers indicated. Wires or traces would be black hand-drawn lines connecting the required components. To avoid confusion, whenever a line crossed over another line, a semi-circle would sometimes be used to emphasize that the wires were not connected. Dots were used to indicate if wires are connected, but these sometimes were hard to see on copies of the schematic. Computer-generated schematics were not available yet. By the time an engineer was finished with the schematic, modifications would have been squeezed in where possible and several re-draws had probably been made. Electrical engineers tried to keep as much of the schematic as possible on a single page for convenience. When they needed to go off the page, they made sure the signal had a name and often referred to the grid space that it went to on the other page. Once this part was complete, that hand-drawn mess would be handed over to a drafter to make it readable and consistent for other engineers to be able to read. The drafter would draw the schematic on B-size (11” x 17”) vellum paper, which was a translucent thick material. If simple modifications were needed, the pencil marks used to create the schematic could be cleanly erased and changed. It had to be handled with care so as not to crease it or smudge it. Once this was complete the engineer checked it for accuracy and then approved it. This vellum schematic was carefully stored under physical configuration control and copies were made as needed to provide to engineers. This entire process could take several weeks to months depending on the complexity of the circuit.
Modifying Schematics
Whenever engineering changes were needed, a standard way of doing it was to mark deletions in YELLOW and additions in RED on a copy of the schematic. This often became hard to follow, so it was a good idea to also provide what it should look like by cutting and pasting lines and symbols together using a copy machine to provide to the drafter. This cut down on interpretation errors and often saved weeks in making the changes. If changes were significant, the entire schematic may have been re-drawn.
Vintage BOM Generation
Computers and especially the internet did not exist yet in the mid-1980s. Components were all chosen from data books. The company I worked for at the time had a huge inventory of axial resistors and capacitors with part numbers assigned to them, so I just needed to pick out which ones I needed. I needed data books for the integrated circuits (ICs) which I borrowed from the old-timers. Fortunately, almost everything I needed was transistor-transistor-logic (TTL), so one data book covered most of them. It was very expensive to purchase, but a local distributor, AVNET, sponsored engineer days where they had free data books for the taking on one Saturday every year – usually last year’s print but good enough for us. My friends and I would go down and pick out about 10-20 books we needed every year. Since we did not have the internet to search for parts, we would need to thumb through the data book to pick what we needed, although we started to memorize the part numbers (I still remember that a 7404 was an inverter and that the 5404 was the military version of that part.) This may seem archaic compared to today, and it was, but the IC choices were basic and a lot more limited than today, so it wasn’t too bad.
Modern Schematic Design
In the latter part of the 1980s, I was able to start using this new schematic capture tool called ORCAD. It was new, clunky, and often froze up causing me to lose my work, so I quickly learned the importance of saving often. There was a learning curve, so when we needed something fast, we reverted to hand-drawing schematics. We also tried to adapt the way we were drawing schematics to the new computer-generated schematics, which was the wrong way to use it. Fortunately, schematic capture software evolved rapidly, and better ways of working with it followed. It quickly became obvious that several significant improvements were available:
- The schematic was stored digitally, so the danger of damaging the only official copy no longer existed
- Modifications did not require re-draws – just slide components over as needed, add or delete components and connect or disconnect wires as needed
- Netlists became a thing, which allowed someone to quickly see what should be connected
- Keeping everything on a minimum of pages was no longer needed, so circuits could be broken up by function making them easier to understand
- Checks could be performed to make sure everything was connected
- The data could be imported into PCB design software, making that process easier
- Schematics are typically exported to PDF searchable documents, making it easy to search for signal names and components
- Part numbers, especially for ICs are typically placed on schematics making them more useful
Modern BOM Generation
Fortunately, with the advancement of the internet, several tools became available online. The major chipmakers all have online sites with data sheets available for all their components. The distributors have amazing parametric search tools to help in choosing parts. They also link to the manufacturer datasheets. I personally use both Digikey and Mouser extensively for this purpose, although there are others. There are a lot more components available than decades ago and in many cases, a single IC can provide the same function that previously took several discrete ICs. IC datasheets also typically provide reference schematics that assist in designing the circuit, although these need to be verified for the specific circuit. Once a BOM is generated, there are several tools that assist in keeping track of obsolescence and ordering parts, whereas before that was all a manual process.
These changes in schematic capture and BOM generation have allowed, as well as the advancement in ICs, for much more efficient circuit design and troubleshooting, enabling a much faster product design cycle.
To learn more about how ACDi can partner with you on your next electronics manufacturing project, visit www.acdi.com.
Bob DiDonato
Engineering Program Manager