Two boards can be built from the exact same schematic, and one works perfectly while the other fails EMI testing, resets randomly, or has almost no wireless range.
The layout is where the real physics happens, and small placement and routing decisions can make or break your entire product.
I'm counting down 11 PCB layout rules you should never break, ending with the one that's the most invisible and the most expensive to fix, and for every single one I'll tell you what to do instead.
Rule #11 - NEVER Use Via-in-Pad Without Knowing the Cost
Via-in-pad means placing a via directly inside a component's solder pad instead of next to it.
It's tempting because it saves space, and for dense parts like fine-pitch BGAs it's sometimes the only option.
But there are two costs most people don't see coming.
First, during assembly the molten solder wicks down into the open via like a straw, which starves the joint and leaves you with weak or even open connections.
Second, the fix for that is having your board fab fill each via with epoxy and plate copper over the top, and that process adds real money to every single board you ever build.
So unless you truly need it, place vias next to the pad and connect them with a short trace.
And if your design does require via-in-pad, specify filled and capped vias in your fab notes, and budget for that extra cost from day one.
Rule #10 - NEVER Design a Board Without Test Points
A board with no test points can't be tested in production.
That means you're either shipping units that were never verified, or you're paying someone to probe every single board by hand.
Factories test boards using a fixture with spring-loaded pogo pins, and those pins need dedicated copper pads to land on.
If you didn't put those pads in your layout, there's nowhere for the pins to go, and adding them later means a new board revision.
The good news is test points are basically free, since they're just small exposed copper circles placed on your signals and power rails.
Add them to every power rail, every communication bus, and anything you'd need for programming or debugging the board.
Test points are also a lifesaver during debugging, especially with leadless packages like QFNs and BGAs where the pins are hidden underneath the chip and there's nothing to probe.
Rule #9 - NEVER Skip a Thermal Path for Hot Components
A voltage regulator that works fine for 10 minutes and then browns out is commonly a layout problem, not a component problem.
Many regulators and other hot components dissipate their heat through an exposed pad on the bottom of the package called a power pad.
That pad needs somewhere for the heat to go, and on most boards that means an array of thermal vias connecting it down to a large copper pour on another layer.
And yes, those thermal vias are technically via-in-pad, but a power pad is the one place where it's normally acceptable, since a little solder wicking there mostly causes small voids instead of failed joints.
Skip those vias, and the heat just piles up until the part hits thermal shutdown, and then your product resets over and over out in the field.
So follow the land pattern in the datasheet exactly, including the recommended via array, and give the part as much copper area as you can spare.
Copper is free, and it's the cheapest heatsink you'll ever get.
Rule #8 - NEVER Run High-Speed Traces Near the Board Edge
High-speed traces routed near the edge of the board are a classic way to fail EMI testing.
Near the edge, the electric fields around a trace are no longer fully contained between the trace and its ground plane.
Part of the field fringes out past the edge of the board, and that fringing field radiates like a little antenna.
Everything will look fine on your bench, but the antennas at the test lab will pick it up immediately, and every failed EMI scan costs you money and weeks of schedule.
The fix is to keep fast signals like clocks, USB, and memory buses away from the board edge by at least 4 to 5 times the distance between the trace and the ground plane below it, which on most boards works out to around 1 mm or more, and to route them on inner layers when you can.
Rule #7 - NEVER Leave the Board Without Stitching Vias
Stitching vias are the rows of ground vias that tie your ground planes together across the board.
Without them, ground currents on different layers can't take the shortest path back to their source, and the gaps between your planes start acting like slots that leak radiation.
Stitching vias along the board edges also form a fence that helps contain noise inside the board.
I recently reviewed a board that looked clean everywhere else but had almost no stitching vias, and we caught it right before the fab order went out.
Adding them cost nothing, and it meaningfully improved that product's odds of passing FCC testing on the first try.
So sprinkle ground stitching vias generously across the board, especially along the edges, around any high-speed routing, and around any RF circuits or antenna feedlines.
Rule #6 - NEVER Ignore the Switching Regulator Hot Loop
Every switching regulator has a hot loop, which is the current path where current starts and stops in just a few nanoseconds.
That fast-changing current makes the loop behave like a tiny transmitting antenna, and the bigger the loop area, the more noise it sprays across your board.
Chip makers like TI and Analog Devices publish recommended layouts in their datasheets, and those layouts exist specifically to keep this loop tiny.
Place the input capacitor as close to the regulator as physically possible, keep the loop on a single layer, and copy the datasheet layout instead of improvising your own.
Rule #5 - NEVER Route Differential Pairs Like Ordinary Traces
Differential pairs like USB, Ethernet, and HDMI carry equal and opposite signals on two traces, and the receiver only reads the difference between them.
This makes them extremely immune to noise, because interference hits both traces equally and cancels out at the receiver, but only when the layout is done correctly.
For that to work, both traces need matched lengths, a controlled impedance, an unbroken reference plane underneath, and no stubs hanging off the lines.
Get any of those wrong and the signals arrive skewed or reflected, which turns into random errors that are brutal to debug.
Your PCB software has built-in tools for routing differential pairs with matched lengths, so use them.
And ask your board fab for a stackup with the impedance control you need, since they'll gladly tell you the exact trace width and spacing to use.
Rule #4 - NEVER Starve a Power Trace or Via
A power trace that's too thin drops voltage under load, heats up, and in the worst case burns right off the board.
The same goes for vias, since a single small via might only handle an amp or so before it becomes the bottleneck.
Product creators often route power using the same default trace width as their signals, and everything seems fine at low current during early testing.
Then the product hits a high-load condition, the supply rail sags, and the microcontroller resets at the worst possible moment.
Trace width calculators are free and take seconds to use, so size every power trace for its actual current with plenty of margin.
For higher currents, use copper pours instead of traces, and always place multiple vias in parallel wherever power changes layers.
Rule #3 - NEVER Put Copper Under the Antenna
Copper placed under an antenna will kill your wireless range.
An antenna works by coupling energy into the space around it, and a ground plane or any copper sitting underneath detunes the antenna and absorbs the energy before it ever leaves the board.
Every antenna and every pre-certified wireless module has a keepout zone defined in its datasheet, which is an area where you're not allowed to put copper, traces, or components on any layer.
The same goes for metal mounting screws and standoffs, so keep those away from the antenna too, since metal that close will detune it just like copper does.
This one also has a certification angle, because a pre-certified module is only certified when it's used the way the datasheet specifies.
Violate the keepout, and the module in your product no longer performs as certified, which can come back to bite you during compliance testing.
Look at any dev board for a wireless module and you'll find a bare region around the antenna, and you should copy that keepout exactly in your own layout.
Rule #2 - NEVER Place Decoupling Caps Far From the Pin
Decoupling capacitors only work when they're close to the power pin they're protecting.
A decoupling cap acts like a tiny local energy tank that supplies the chip during sudden current spikes, faster than the main power supply can respond.
Every mm of trace between the cap and the pin adds inductance, and inductance resists exactly those fast current changes.
Put the cap too far away, and you've built a filter that blocks the cap from doing its one job, so the chip's supply dips and you get random glitches and resets.
Place the smallest cap closest to the pin, connect it with short wide traces, and drop the ground vias right at the cap's pads.
Rule #1 - NEVER Route Signals Across a Plane Split
Routing a signal across a split in your ground or power plane is the most invisible and most expensive mistake on this entire list.
Every signal current has to return to its source, and at high frequencies that return current flows in the plane directly underneath the trace.
When the trace crosses a split, the return current can't follow it, so it detours all the way around the gap.
That detour creates a huge loop that radiates noise, couples interference into every nearby circuit, and is a leading cause of both EMI failures and mystery glitches nobody can reproduce.
And nothing about it looks wrong, since the trace appears perfectly normal and every net checks out in your design software.
To be clear, plane splits themselves are sometimes the right choice, like separating a sensitive analog section, and the rule is about what you route over them.
So before you order boards, check every trace against the planes beneath it, on every layer, and reroute anything that crosses a gap.
Talk soon,
John
P.S. If you need help applying these PCB layout rules to your own designs, then you can get help from me and other experts inside the Hardware Academy.
P.P.S. If you missed last week's video, here's 7 Protection Circuits You Should NEVER Ship a Product Without.