Column: Embedded design
encountered blanket prohibitions on tracks under surface-mount (SM) parts (when a 0.3mm track passes comfortably under an 0603-sized passive), archaic restrictions on via pad sizes versus hole diameter, and SM pad dimensions set up for wave soldering even when the boards are nitrogen reflowed. Tere are important rules to follow.
Termal reliefs on parts in copper-poured ground planes and prohibiting vias in pads to reduce solder wicking both spring to mind, but otherwise make sure today’s rules relate to today’s methods and not something from thirty years ago.
• Higher tech isn’t always better Your PCB vendor will inevitably offer very fine geometry features, exotic substrates and ever-increasing numbers of layers. These things are valuable in their place, but if you are a lone engineer laying out a first prototype to test an idea in the lab, then it is very unlikely that you will need a sixteen-layer board with buried vias, 0.1mm diameter holes and track widths that you can’t see. An awful lot can be achieved with very ordinary two-layer (or at most four-layer) FR4 substrate boards with entirely pedestrian 0.3mm holes and 0.25mm tracking. Likewise, using the finest-pitched
connectors and the smallest, latest package sizes makes no sense, unless physical dimension restrictions are truly stringent. I have seen simple, un-cluttered boards with 0201-sized pull-up resistors and decoupling capacitors where available space would have allowed easier-to-source 0603 parts, with additional advantages in rework and inspection.
An awful lot can be achieved with very ordinary
two-layer (or at most four-layer) FR4 substrate boards with
entirely pedestrian 0.3mm holes and 0.25mm tracking
• Be flexible The beauty of being responsible for both the circuit design and the layout is that you can often modify the circuit to improve the PCB layout. Dual SO8 op-amps can be split into pairs of SOT23-5 parts. You can change decoupling or bias circuits for equivalents that are easier to configure. SOT23 packages can swap for SOT323, and vice versa, and with RF transistors you often have the same part offered in three- and four-pad packages. Adding ferrite chokes to supply feeds, or very low value series resistors to signal lines, can greatly simplify otherwise knotty layouts, while counter-intuitively swapping a small passive for a larger one can often give a more elegant result.
• Design for the build you are actually doing It might seem that “a PCB is a PCB”, but a board layout intended for a one-off prototype hand assembly can be quite different to a volume production job. A production board is likely to be carefully tailored to the assembly process, and can use parts in packages like BGA or QFN that can’t readily be hand-soldered or inspected by eye. A hand-built prototype is a different
creature. Here you want to be able to access the critical parts of the circuits, to conduct tests and have enough spare board area to address the changes. There is a good argument for including test pads, added indicator LEDs, or even additional break-out connectors. It’s best to use parts that are readily hand- soldered and inspected by eye. Where possible, it’s certainly worth
using SOIC parts over TSSOP, avoiding fine-pitch DFN packages, and laying out 0805 pads but hand-placing 0603 parts on them.
• Relax and enjoy Assuming you are working with a reliable CAD tool, PCB layout is a very enjoyable exercise. It’s a challenge to come up with a layout that elegantly realises your schematic in the space that you have, but it is satisfying in much the same way that a crossword or a jigsaw puzzle is. It’s not called a PCB “artwork” for
nothing.
This column on embedded design prepared by Myk Dormer continues in the next edition of Electronics World
www.electronicsworld.co.uk November 2026 15
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