A Practical Guide for Component Polarity by PCBgogo

KEY DEFINITION  PCB polarity is the match between a component's required terminal orientation and its PCB footprint. For a capacitor, check positive and negative; for a diode or LED, check anode and cathode; for a chip, check pin 1 and the pin map. The safest method is to compare the exact part datasheet, body marking, footprint pads, and assembly drawing before power is applied.

What Is PCB Polarity

PCB polarity tells an assembler which terminal belongs on which pad. A plus sign beside a capacitor pad, a cathode bar beside a diode, and a dot beside an IC footprint communicate different things. The mark is useful only when it agrees with the circuit and the actual manufacturer part number.

Some two terminal parts, including ordinary resistors and most ceramic capacitors, work in either direction. Others can malfunction or fail when reversed. A chip or connector may lack simple positive and negative ends but still has only one correct orientation. That is why component polarity checks should cover both electrical polarity and the full pin map.

What It Means for an Electronic Component to Be Polarized

A polarized electronic component needs its designated terminals connected to the intended circuit nodes. Electrolytic capacitors have positive and negative terminals. Diodes and LEDs have anodes and cathodes. Reversing a simple indicator LED often prevents light; reversing an aluminum electrolytic capacitor can cause heating or venting. The exact result depends on voltage and part construction.

Orientation sensitive packages are a related group. An IC's pin 1, a MOSFET's gate, and a connector's keyed pin must be mapped correctly, even though a plus or minus mark alone cannot describe them. Use the terminal names that appear in the datasheet instead of treating every asymmetric package as a two terminal polarized part.

Common Polarized Components and How to Identify Each One

The groups below combine each common part with the clue an assembler can check on the component and the matching mark needed on the board. Body colors, dots, and stripes vary by manufacturer, so confirm the exact part number before using a visual clue as a rule.

Capacitors

1 Radial and axial aluminum electrolytic capacitors

• Component clue: A sleeve stripe commonly identifies the negative lead. On a new radial part, the longer lead often identifies positive, but cut leads remove that clue.

• Board match: Put + beside the positive pad. The striped side of the capacitor faces the opposite pad.

2 Surface mount aluminum electrolytic capacitors

• Component clue: The can or insulating base normally carries a negative side mark. Compare the printed mark with the manufacturer's top view drawing.

• Board match: Mark the positive pad with + and show the can's negative side in the assembly drawing if the footprint will hide the silkscreen.

3 Solid polymer aluminum capacitors

• Component clue: These aluminum based parts are polarized, but their case styles differ. Read the polarity legend on the exact package drawing instead of borrowing a convention from a nearby tantalum part.

• Board match: Label the positive pad and verify the marked body side against the datasheet.

4 Molded tantalum capacitors

• Component clue: The stripe or bar commonly marks positive, unlike the negative sleeve stripe on many aluminum cans. KEMET's T496 tantalum datasheet identifies its positive terminal with a stripe and sometimes a beveled edge.

• Board match: Place the stripe at the + pad. If the case marking is unfamiliar, confirm the manufacturer's package diagram.

5 Polymer tantalum capacitors

• Component clue: These are polarized tantalum parts with a polymer cathode. Their marked end commonly identifies positive, but the precise symbol depends on the series.

• Board match: Put the documented positive end on the + pad. Never use an aluminum can's stripe convention for this package.

6 Supercapacitor cells

• Component clue: A supercapacitor cell has defined positive and negative terminals. Look for printed + and minus symbols or the terminal diagram on the product drawing.

• Board match: Label both pads and check the cell's rated voltage before energizing the board.

7 Supercapacitor modules

• Component clue: A multi cell module exposes external + and minus connections and may include balancing circuitry. The internal cell arrangement is not an external polarity clue.

• Board match: Label both external terminals and check the complete module pin map.

Most ordinary ceramic and film capacitors are nonpolar. A specially labeled bipolar electrolytic is an exception to the usual electrolytic rule; identify it by its part number, not by the C reference designator.

Light Emitting Diodes

1 Through hole LEDs

• Component clue: A new leaded LED often has a shorter cathode lead and a flat on the cathode side of the lens. Lead length becomes unreliable after trimming.

• Board match: Label the cathode pad K or show a diode symbol with its bar at the cathode. Confirm both clues against the part drawing.

2 Surface mount LEDs

• Component clue: Depending on the package, a notch, corner cut, printed dot, underside pad shape, or colored mark may indicate a terminal. A green mark or molded arrow is not a universal cathode rule.

• Board match: Put A and K beside the correct pads, or show the exact package orientation on the assembly drawing. Check whether the datasheet drawing is a top or bottom view.

3 Infrared LEDs

• Component clue: These emit light outside the visible range and still have anode and cathode terminals. Use the manufacturer's pin drawing because a quick visual power test cannot confirm emission by eye.

• Board match: Mark A and K just as for a visible LED, then verify the footprint pad numbers.

4 RGB LEDs

• Component clue: A multi color package may have a common anode or common cathode plus separate red, green, and blue pins.

• Board match: Mark pin 1 and map every color pad. A single cathode line cannot describe the whole package.

5 LED displays

• Component clue: Seven segment and matrix displays have digit, segment, or row and column pins. The common pins can be anodes or cathodes.

• Board match: Mark pin 1 and verify the full manufacturer pin table before placement.

Diodes and Related Devices

1 Rectifier diodes

• Component clue: Many axial and molded rectifiers put a band at the cathode. TI's 1N4001 teaching datasheet identifies the lead near its gray band as the cathode.

• Board match: Place the body band at the silkscreen cathode bar or K mark. Check the schematic before deciding which circuit node the cathode should reach.

2 Schottky diodes

• Component clue: Two terminal Schottky parts usually identify their cathode with a band or package mark, but small SMD packages may use a dot or code.

• Board match: Match the datasheet's cathode pad to K on the board; do not guess from the printed part code.

3 Zener diodes

• Component clue: A cathode band is common. A Zener may operate in reverse breakdown in its intended circuit, so the terminal name matters more than a generic current direction arrow.

• Board match: Put K at the footprint cathode and confirm the net against the schematic.

4 Unidirectional TVS diodes

• Component clue: These protection parts have defined anode and cathode terminals, often indicated by a band. Read the actual symbol because a bidirectional TVS is a different device.

• Board match: Show K or the full diode symbol. For a bidirectional two terminal TVS, follow its datasheet rather than adding a misleading polarity mark.

5 Photodiodes

• Component clue: Package marks and lead lengths vary. Many circuits reverse bias the photodiode, which makes anode and cathode identification especially important.

• Board match: Label A and K and verify the pin numbers on the manufacturer's drawing.

6 Bridge rectifiers

• Component clue: A bridge package has four terminals, usually two AC inputs plus DC + and minus outputs. Read the molded symbols or package drawing.

• Board match: Label all four pads. One cathode bar cannot represent a bridge's complete pin map.

Integrated Circuits and Other Direction Sensitive Packages

These parts need orientation checks rather than a simple positive and negative label. Their pin map can still cause the same costly assembly error as a reversed capacitor.

1 SOIC and SOP packages

• Component clue: A notch, molded dot, laser mark, or chamfer may locate pin 1. SOIC and SOP leads run along two sides, so use the top view package drawing to count pins correctly.

• Board match: Place a clear pin 1 dot near pad 1. A line or color stripe alone can be ambiguous without the part drawing.

2 QFP packages

• Component clue: Leads extend from all four sides. A top side dot, corner feature, or chamfer commonly identifies pin 1.

• Board match: Put an unambiguous pin 1 marker at the correct corner and keep it visible outside the package outline.

3 QFN packages

• Component clue: The contacts are mainly underneath, and a top side dot or chamfer often marks pin 1. Exposed thermal pads add another pad mapping check.

• Board match: Mark pin 1 outside the package outline and compare the top view drawing with the footprint.

4 BGA packages

• Component clue: The package drawing identifies ball A1, often with a corner dot or other molded feature. Bottom view drawings reverse the apparent position.

• Board match: Show A1 on the assembly drawing and verify rotation using the same viewing side as the placement file.

5 Transistors

• Component clue: MOSFET gate, drain, and source, or bipolar base, collector, and emitter, depend on the exact part and package.

• Board match: Match every pad number to the datasheet pin table. Use pin 1 or named pins where a two terminal polarity mark would mislead.

6 Voltage regulators

• Component clue: Input, output, ground, and enable pins vary by regulator. A metal tab is not universally ground.

• Board match: Check every footprint pad number against the exact part's pin table.

7 Optocouplers

• Component clue: An optocoupler has an LED input with anode and cathode plus an output side pin map.

• Board match: Show pin 1 and check both the LED input and output pads against the datasheet.

Batteries Connectors and Power Modules

1 Battery cells

• Component clue: Use the printed + and minus marks on the cell body, not its shape alone.

• Board match: Label both supply contacts and check installed cell direction in the mechanical drawing.

2 Battery holders

• Component clue: A spring often touches the negative end, but holder designs differ. Read the molded or printed symbols.

• Board match: Check the holder footprint pin numbers against the intended cell orientation.

3 DC power modules

• Component clue: A module may expose input +, input minus, output pins, enable, and ground. Read its labeled pinout from the actual viewing side.

• Board match: Number or name every pad. Do not replace a multi pin map with a single + sign.

4 LED modules

• Component clue: A prebuilt lighting module may have supply + and minus plus control or color channels.

• Board match: Label every connector pad and verify the module's input voltage and wiring diagram.

5 Polarized relays

• Component clue: A relay with an internal flyback diode assigns coil + and minus. An ordinary unprotected coil may not have that requirement.

• Board match: Mark both coil pads and verify the relay's internal circuit diagram.

6 Keyed power connectors

• Component clue: A latch or key indicates mating orientation, but the pin numbering may be shown from the cable or board side.

• Board match: Mark pin 1 and critical power nets. Compare the connector drawing with the mating cable before release.

How to Verify an Unclear Polarity Mark

When a body mark and board mark seem to disagree, trace the part rather than guessing from a familiar shape.

1. Read the exact manufacturer part number and the package drawing's top or bottom view label.

2. Match terminal names to footprint pad numbers, then check those pads against the schematic nets.

3. Compare the physical body cue with the PCB silkscreen and assembly drawing from the same viewing side.

4. Stop before power up if the records conflict. Diode mode can help test an unconnected diode or LED, but surrounding circuitry can distort an in circuit reading.

Examples of Polarity Marks in a PCB Layout

The following examples show how a concise board mark can be checked against the actual part. Put marks beside pads where they remain readable after placement; use an enlarged assembly drawing when space is limited.

For a 12 V input board, a reviewer can trace D1's cathode from the schematic to its footprint K pad, then confirm that the band's orientation matches the assembly drawing. The same review checks C1's positive net against its + pad while the can's negative stripe faces away from that mark. This catches a reversed footprint mapping that silkscreen inspection alone can miss.

The placement file, bill of materials, and drawing must use matching reference designators. PCBgogo's assembly file guidance specifically requests cathode or anode and pin 1 marks on the silkscreen or a separate assembly drawing, plus matching placement data. Providing all three lets the assembler resolve a questionable orientation before soldering.

Summary

Good PCB polarity markings name the correct terminal for each part: + and minus for DC storage and power, A and K for diodes and LEDs, and pin 1 or full pad numbers for multi pin packages. Check the exact datasheet, body cue, footprint mapping, and assembly drawing together before releasing a board for assembly.

Frequently Asked Questions

Is every capacitor polarized?

No. Ordinary ceramic and film capacitors are generally nonpolar. Aluminum electrolytic, tantalum, polymer tantalum, and supercapacitor parts normally have defined positive and negative terminals, subject to the exact product design.

Does a stripe always mark the negative terminal?

No. An aluminum electrolytic sleeve stripe commonly marks negative, while a molded tantalum stripe commonly marks positive. Read the exact part's package drawing when a stripe is unfamiliar.

Which end of a diode matches the PCB cathode bar?

The physical cathode end matches the footprint's cathode bar or K mark. A band is common on many two terminal diodes, but small packages can use other markings.

Are ICs polarized components?

ICs are usually described as orientation sensitive rather than simply positive and negative. Their pin 1 and complete pin map must still match the PCB footprint.

Can silkscreen replace an assembly drawing?

It can for a simple, spacious board if every mark remains clear. A separate drawing is safer for hidden marks, bottom side parts, dense layouts, or connectors whose viewing direction is easy to misread.