My first PCB arrived from the fab house looking perfect. Professional solder mask. Clean silkscreen. Proper hole sizes. I populated it with components, powered it on, and watched a trace literally burn off the board.
Turns out, 8 mil traces can't handle 3 amps. Who knew? Not me. I'd used the default trace width from my PCB software because I didn't know trace width mattered for anything except fitting between pads.
Three board revisions and $340 later, I learned which PCB specs actually matter and which ones are just defaults you can ignore. The difference isn't obvious until you've burned through a few prototypes.
When Defaults Become Dangerous
June 2024. I'm designing my first custom PCB for a motor controller project. Arduino-based. Two DC motors. Simple H-bridge driver. Power supply section. Nothing exotic.
I watch six YouTube tutorials on KiCad. Read four beginner guides. They all say the same thing: "Check your design rules." So I do. Default trace width: 8 mils. Default clearance: 8 mils. Default via size: 0.8mm drill, 1.4mm pad. Everything looks reasonable.
I export Gerbers. Upload to JLCPCB. $23 for five boards with shipping. Three weeks later, they arrive. They look amazing. Like real products.
I solder on my Arduino headers, motor drivers, terminal blocks for power. Check continuity with my multimeter. Everything connects properly. Time to test.
Power supply: 12V, 5A capable. Motors: rated for 2A peak each. I've got headroom. Should be fine. I connect the first motor. Press the button to run it forward.
The motor spins for maybe two seconds. Then I smell burning plastic. The trace between my power supply and motor driver is glowing. Actually glowing orange for a brief moment before it vaporizes.
I yank the power cord. The trace is gone. Just... gone. A gap in the copper where a connection used to be. The board is ruined. $23 and three weeks, destroyed in two seconds.
The Over-Correction Problem
I'm not making that mistake twice. I open KiCad. Find the trace width calculator. 3 amps continuous, 1 oz copper, 10°C temperature rise. Calculator says I need 40 mils minimum. I round up to 50 mils. Safe.
But I don't stop there. If wider is better for power traces, wider must be better for everything, right? I make ALL my traces 50 mils. Signal lines. Ground connections. Everything. Why risk it?
I also read that smaller vias have higher resistance. So I make all my vias huge. 1.5mm drill, 2.5mm pad. Maximum current capacity. Maximum reliability.
My second board arrives. It works. The motor traces don't burn. Power flows properly. Victory.
But the board is enormous. It's 150mm x 100mm for a circuit that should fit in half that space. The giant traces forced everything apart. The massive vias ate up routing space. I had to add a ground plane on the back just to fit all the connections.
Worse, my signal lines are too wide. My Arduino's I2C communication to a sensor keeps glitching. Random errors. Intermittent failures. I spend a week debugging before I realize: my 50 mil I2C traces have too much capacitance. They're supposed to be thin. The spec I chose to be "safe" is causing new problems.
The Spec Sheet That Changed Everything
My coworker Luis does PCB design professionally. He sees my second board. Laughs. Not mean. Just... honest.
"You're treating every trace like a power line," he says. "Look at what each trace actually does."
He shows me his process. Not "make everything as good as possible." Categorize first. Match specs to function second.
Power traces carrying multiple amps? Wide copper. 30-50 mils depending on current. Maybe 2 oz copper if pushing serious power.
Signal lines running between digital chips? 8-12 mils is fine. They carry microamps. Width doesn't matter for current. It matters for impedance and capacitance.
Ground and supply planes? Fill the space. Make them big. Low impedance matters more than precise width.
High-speed signals like USB or SPI? Controlled impedance routing. Width is calculated based on the PCB stackup and target impedance. Not arbitrary.
He pulls up a cheat sheet. Project categories and their critical specs. Motor controllers need fat power traces. Sensor boards don't. RF designs need controlled impedance. Digital logic boards don't care as much.
That's when it clicked. PCB specs aren't universal requirements. They're project-specific tools. Using the wrong specs for your project type wastes space, money, and causes mysterious failures.
The Framework That Prevents Failures
Stop using default settings for everything. Stop over-engineering everything. Match your specifications to what your board actually does.
Low Power Digital Projects: Ignore Most Specs
Building a sensor node? Microcontroller reading temperature? LED blinker? Low power project under 500mA total?
Trace width: Use defaults (8-10 mils). Current carrying capacity doesn't matter at milliamp levels. Route for convenience, not current.
Copper weight: 1 oz is fine. The default. Don't pay extra for 2 oz copper you'll never stress.
Via size: Standard 0.8mm drill, 1.4mm pad works. You're not pushing current through vias. They're just connections.
Clearance: 8-10 mils between traces is plenty. You're not running high voltage. Default spacing prevents shorts without wasting space.
Critical spec: None, really. These projects are forgiving. Focus on getting traces connected correctly. The exact specifications don't matter much because you're operating well within limits.
Why this works: Most hobby projects fall in this category. Defaults exist for these use cases. Fighting them wastes time and space. Save your mental energy for actual circuit problems, not imaginary current capacity issues.
Motor Controllers and Power Electronics: Trace Width Dominates
Driving motors? Switching high current? LED strips with dozens of LEDs? Power above 2 amps?
Trace width: Calculate it. Use online calculators. 3A needs about 40-50 mils with 1 oz copper. 5A needs 80-100 mils. Don't guess. Do the math. Undersizing by 50% will burn traces. Oversizing by 2x wastes board space but works fine.
Copper weight: Consider 2 oz copper for sustained high current. Costs a bit more ($5-10 extra per batch). Lets you use narrower traces. Easier routing on constrained boards.
Via size: Larger for current paths. 1.0-1.2mm drill minimum for power vias. Multiple vias in parallel if carrying serious current. One via isn't enough for 5A paths.
Clearance: Stay with 8-10 mils unless you're running over 50V. High current doesn't need extra clearance. High voltage does.
Critical spec: Trace width for power paths. Calculate for 10°C temperature rise maximum. This is the spec that burns boards. Get it wrong and your project literally fails in flames.
Why this works: Current creates heat. Heat destroys traces. The math isn't optional. Calculate, don't estimate. Your motors don't care about other specs, but they absolutely care about having enough copper to move their current.
High Speed Digital: Impedance Trumps Everything
USB communication? SPI at high clock speeds? HDMI? DDR memory? Signals above 10MHz?
Trace width: Calculated for controlled impedance, not current. 50 ohm traces might be 10-15 mils depending on your PCB stackup. 90 ohm differential pairs might be 8 mils with 8 mil spacing. Use impedance calculators, not current calculators.
Copper weight: 1 oz standard. Impedance calculations assume this. Changing it changes your impedance. Stick with standard unless you have a specific reason.
Via size: Minimize via count on high speed signals. Each via adds capacitance and inductance. Use smallest practical vias (0.3-0.4mm drill) to reduce impedance discontinuities.
Clearance: Matters for crosstalk. Space high speed differential pairs away from other traces. 3x trace width minimum clearance to nearby signals. Prevents coupling.
Critical spec: Controlled impedance routing. USB won't work reliably if your differential pair impedance is wrong. DDR memory will have errors. Calculate stackup, match impedance, verify with manufacturer capabilities.
Why this works: High frequency signals reflect off impedance mismatches. Wrong impedance creates signal integrity problems that look like mysterious intermittent failures. The current is low, but the frequencies make electrical properties critical.
RF and Wireless: Manufacturing Tolerances Matter
WiFi module? Bluetooth antenna? LoRa radio? Any PCB with RF frequencies above 100MHz?
Trace width: Impedance controlled. 50 ohms typically for antenna feeds. But manufacturing tolerances matter more than usual. Tighter tolerances cost more but improve RF performance.
Copper weight: 1 oz standard. Heavier copper changes impedance and adds losses at high frequency.
Via size: Controlled. Ground vias should be small and numerous. Create RF ground paths without adding inductance. 0.3-0.4mm drills, close spacing around RF sections.
Clearance: Critical around antenna traces. Keep ground planes at specific distances calculated for your antenna design. Too close or too far changes antenna characteristics.
Critical spec: Manufacturing tolerances. RF is sensitive to variation. Pay for tighter tolerances (±0.5 mil trace width, ±0.3 mil etching) if your design is marginal. Saves debugging why your WiFi range is terrible.
Why this works: RF is black magic until you realize it's just really sensitive to physical dimensions. Small variations in trace width or spacing change antenna performance significantly. Tighter manufacturing control means more predictable RF behavior.
The Specs That Rarely Matter
Some specs get obsessed over but almost never cause real problems for hobby projects:
Solder mask color? Cosmetic only. Green is cheapest. Pick whatever you want. Doesn't affect electrical performance at all.
Silkscreen resolution? Unless you're putting tiny text, standard resolution works fine. You're not printing phone numbers in 6 point font.
Board thickness? 1.6mm is standard. Use it. Thinner or thicker costs more and complicates everything unless you have a specific mechanical requirement.
Surface finish? HASL (Hot Air Solder Leveling) works for everything except fine pitch components. It's the cheapest. Use it until you need something better.
Why this works: These specs affect manufacturing cost, appearance, or edge cases. They don't affect whether your circuit works. Save mental energy for specs that matter, ignore specs that don't.
The One Rule That Covers Everything
When in doubt, use the calculator instead of guessing. Trace width calculators are free. Impedance calculators are free. Via calculators are free. They take 30 seconds to use and prevent expensive failures.
The cost of using a calculator: 30 seconds. The cost of not using one: $23 and three weeks every time you guess wrong.
That's not even close.
What Type of Board Are You Building?
Look at your project. Not at default settings. Not at what seems "safe." At what current flows through which traces and what frequencies your signals use.
Low power digital? Defaults work. Stop worrying.
High current motor control? Calculate trace widths. This is non-negotiable.
High speed signals? Calculate impedance. Check with manufacturer stackup specs.
RF wireless? Get tighter tolerances. Invest in controlled manufacturing.
The right PCB specs aren't the "best" specs. They're the specs that match what your board does. Everything else is wasted effort, wasted space, or wasted money.
Which category does your next board fall into?