How to Read a Circuit Board: What All Those Little Markings Mean

#electronics #hardware #beginners #pcb

"What do the PCB markings mean?" is, by a wide margin, the most-viewed PCB question I could find on Electrical Engineering Stack Exchange. That makes sense. Every board you'll ever repair, modify or reverse-engineer is covered in small white letters and symbols, and nobody explains them because the people who know them learned them by osmosis.

There are really only three kinds of marking: what a part is, which way round it goes, and facts about the board itself.

What a part is: reference designators

Every part gets a label: a letter or two for its type and a number, like R12 or U3. The same label appears in the schematic and the bill of materials, which is where the actual value lives. The letters follow common conventions:

1 designator, 2 pin-1 mark, 3 square pin-1 pad, 4 diode cathode, 5 LED cathode, 6 capacitor plus, 7 input polarity, 8 header pin names, 9 test point, 10 revision, 11 date code (year and week), 12 94V-0, 13 fiducial.

R resistor, C capacitor, L inductor, FB ferrite bead
D diode (LEDs are often D too), Q transistor
U (or IC) integrated circuit, Y or X crystal or oscillator
J and P connectors, SW switch, K relay, F fuse, T transformer
TP test point, JP jumper, BT battery, FID fiducial

The numbers say nothing about value or importance. They're assigned when the design is annotated, often left to right across the schematic, so R1 and R2 can be on opposite corners of the board.

Which way round it goes

Some parts only work one way. The board shows which, and the part carries a matching mark.

Chips: a dot, notch or cut corner marks pin 1, matching a dot or notch on the chip.
Through-hole parts: the square pad is pin 1; the rest are round.
Diodes: a bar on the outline marks the cathode, matching the band on the diode.
Electrolytic capacitors: the board marks the positive pad with a plus sign or shades the negative half; the stripe on the can is the negative lead.

And the one that catches people: tantalum capacitors are marked the other way. On a tantalum, the bar is the positive end. Fit one backwards and it can fail violently. Trust the board's plus sign, not just the stripe.

Facts about the board

REV B, V1.2: the design revision. Check it before trusting a schematic you found online.
A four-digit number like 2641: usually a date code, year and week of manufacture, though the order varies.
94V-0: the UL 94 flammability rating of the board material; V-0 is the strictest of the vertical-burn ratings.
Small copper dots in the corners: fiducials, targets for the pick-and-place machine's camera.
A random alphanumeric string: some low-cost fabs print their order number on the silkscreen unless you tell them where to put it.

Markings on the parts themselves

SMD resistors usually carry a code: 103 is 10 × 10³ = 10 kΩ, 4701 is 4.7 kΩ. Small SMD ceramic capacitors usually carry nothing at all, so you measure them. Tiny transistors and chips carry a two- or three-character marking code instead of a part number; search for the code plus the package name ("SOT-23") to find candidates.

Using all this

When I'm tracing an unfamiliar board, I start at the connectors (J) and the power parts (a U with an L and some large Cs around it is usually a regulator) and follow the copper. Test points are there so a probe can reach important nets, and they're often labelled with the net name. A burned part with an unreadable body still has its reference designator next to it, which tells you its type, and its neighbours usually tell you its job.

The full guide, with an annotated board diagram and the complete designator table, is on pcbeditor.com.

What's the strangest marking you've found on a board? I'd like to add the odd ones to the guide.