Four important lessons gleaned over a three-decade design career.

There’s an old saying among test pilots: “Any landing that you can walk away from is a good landing.” They also know that there are old pilots, there are bold pilots, but no old, bold pilots – or so the saying goes. If you want to hang around as a PCB designer, you can only hope to walk away from your mistakes with your career intact. So, this is a chance to learn from my mistakes from 35 years of design work.

Going all the way back to the ’90s finds me in my first PCB design role. I had just taken an internal transfer to the commercial side of the business after a couple of years of feeding from the government trough. My manager on the mil-spec side, Merrill, was a father of a dozen children and was an all-around nice guy, perhaps a bit of a pushover.

Before applying for the transfer, I wanted to talk with Merrill, so I came up behind him and asked if he wanted to go to Armadillo Willy’s, a local barbecue place, for lunch. I didn’t see that he had a sandwich in his hand and was about to take the first bite. Instead, the sandwich hit the desk with a thunk, and we were off to the restaurant. Such was his dedication to his people.

Accounting for the assembly process will put your design on the fast track.

Thought-provoking questions keep coming my way, and then it’s down the old rabbit hole. So it goes something like this: “How do we integrate so many different parts in such a small PCB area?” The answer is a little deeper than the geometry of Tetris, but that’s a good illustration of packing the available space. This, of course, starts with the CAD symbol library and manifests in the assembly yields at the factory. We have to connect those dots.

Once the PCB logic is sufficiently captured, placement studies can start. Pay attention to the spacing and orientation of components. The interrelationships of neighboring parts can affect the solderability of the overall PCB. The assemblers like to see a consistent rotation of the components and an even distribution across the board.

It’s unlikely that every device on the board will be able to meet that preference. The electrical performance is going to take priority in several cases, particularly with analog designs. That said, you can still pick an orientation that suits most components. The similarity will inform the manufacturing engineer how the board should travel along the placement machinery and guide the soldering process.

The outline of a PCB can serve as more than a simple perimeter.

The perimeter of a PCB defines the extent of whatever electronics have to be packaged therein. The outline can also serve other functions.

Printed circuit boards come in many shapes and sizes. The first thing the outline gives us is the resulting routable area. The positional variation of each layer in the stackup requires us to compensate with a little pullback of the metal from the edge.

These days, pulling the metal back from the edge by 8 mils (0.2mm) is sufficient for most fabricators. I went to a PCB conference walking from booth to booth and asked all the fabricators what their minimum pull back from the edge would be for production quantities. A few of them, call it 20%, said they could plate the board to within 5 mils (0.127mm) of the edge. In a special case, we used lasers to define the edge and had metal just 2 mils (0.05mm) away.

The next increment is to plate right to the edge and wrap copper around to the other side. Edge plating is used in cases where we want to create a more complete Faraday cage around a circuit. It’s also possible to pass voltage and ground from the top to the bottom around the edge of the board or even using a slot within the outline of a board.

Whether in wire or trace form, keep copper thickness in mind for your design.

Once upon a time, about eight decades back, we didn’t have printed circuit boards. We had copper wires that came in various diameters. Carrying a larger amount of current requires conductors of a larger diameter. These various diameters were identified by the American Wire Gauge (AWG) where smaller numbers indicated thicker wires. There is a metric equivalent where the opposite is true – a higher number for a thicker conductor. Set that aside for this discussion.

Heavy gauge wire for power. A 12- to 14-gauge wire is about the diameter of a cooked spaghetti noodle (~2mm) and is commonly found in power cords for smaller electronics such as a table lamp or a fan. An electric dryer running on 220V will require something between 10- and 6-gauge wire depending on the amperage of the appliance. Again, smaller numbers refer to larger cross-sections.

For the sake of flexibility, these thicker wires are typically constructed of several smaller wires twisted like a candy cane prior to adding the insulation coating. The coating itself is not part of the gauge, only the conductor matters in that regard.