HOW-TO GUIDE
How to Solder a Small Surface Mount Diode by Hand
By the OhmSavvy Research Team ·
Small parts, hot tips
- Work on an unpowered board only, and discharge any large capacitors before touching it.
- Tiny parts can flick off tweezers. Wear safety glasses and work over a light-colored mat.
- Ventilate flux fumes, park the iron in its stand, and wash your hands after using leaded solder.
Why polarity comes first with SMD diodes
A resistor works either way round; a diode does not. The general handling of two-terminal chip parts, tinning, tacking and avoiding tombstones, is covered in how to solder SMD resistors without hot air. This guide is about the extra step that matters for a surface mount diode: getting the cathode to the correct pad, and proving it with a meter before and after the joint is made.
Common SMD diode packages and their cathode bands
| Package | Approx. size incl. terminals | Typical parts | Cathode marking |
|---|---|---|---|
| SOD-523 | 1.6 × 0.8 mm | Small-signal, ESD | Band; often hard to see |
| SOD-323 | 2.5 × 1.3 mm | Small-signal, Schottky, Zener | Band at one end |
| SOD-123 | 3.7 × 1.6 mm | Small-signal, Zener, Schottky | Band at one end |
| SMA (DO-214AC) | 5.2 × 2.6 mm | 1 A rectifiers, Schottky, TVS | Band on the molded body |
| SMB (DO-214AA) | 5.4 × 3.6 mm | Higher-current rectifiers, TVS | Band on the molded body |
Sizes are typical figures; dimensions vary by manufacturer, so check the datasheet. The band is the cathode on standard rectifier, signal, Schottky and Zener diodes. Bidirectional TVS diodes have no polarity and may carry no band at all.
Read the board's silkscreen marks
Boards mark the cathode in several ways: a thick bar at one end of the outline, the diode symbol with its triangle pointing to a bar, a letter K, or a band drawn on the outline. Some boards use a square pad or a dot, but conventions differ, so do not assume. If the marking is unclear, photograph the old part before removing it, compare with an identical diode elsewhere on the board, or trace the pad to the circuit it serves.
1Check polarity in diode mode before fitting
Set the meter to diode mode and hold the loose part in tweezers or on tape. Red probe on one end, black on the other, then reverse. The forward reading appears with red on the anode and black on the cathode, which should be the banded end. A typical silicon signal diode reads around 0.5 V to 0.7 V forward; a Schottky lower, often 0.15 V to 0.45 V. Reversed, expect OL.
A Zener above your meter's test voltage also shows OL in reverse. Full interpretation is in how to test a diode with a multimeter.
2Choose the tip, flux and temperature
For SOD-323 and SOD-123, a fine conical or a 1 mm bevel tip reaches the terminal without touching the body. SMA and SMB parts have more metal and often sit on larger copper, so a 1.6 mm to 2 mm chisel transfers heat faster. Typical setpoints are 300 °C to 340 °C leaded and 330 °C to 370 °C lead-free; see the tip types guide. Use flux paste or a flux pen and 0.5 mm solder.
3Tack one pad
Flux both pads and put a small bead of solder on one. Pick the diode up with fine tweezers, band facing the cathode mark, and slide it against the tinned pad while you touch that pad with the tip. The solder melts and grabs the terminal. Lift the iron and keep the tweezers still for a second. Check that the part sits flat and centred, and that the band is at the right end.
4Solder the second pad
Touch the tip to the second pad and terminal together and feed a short length of solder into the gap. A small fillet rising up the terminal is the target. Then return to the first pad with a touch of flux and reflow it so it gets a proper fillet too.
01
Read marks
Band and silkscreen
02
Meter it
Diode mode, loose part
03
Tack
One pad, check band
04
Finish
Second pad, reflow first
05
Re-test
Diode mode on the pads
5Check polarity again after soldering
With the board unpowered, put the meter across the two pads, red on the anode side. You should see a forward reading close to the loose-part figure, and a high reading or OL reversed. Other components can pull the reverse figure down, so if both directions read low, compare with an identical diode elsewhere on the board before you condemn the joint.
Inspect the joints under magnification
Each terminal should have a smooth fillet that wets both the terminal and the pad. A ball of solder sitting on top usually means a cold joint; a pad with bare copper showing needs a little more solder. On close-pitch boards, check nothing has bridged to a nearby pad; the solder bridge guide covers clearing one.
Removing a wrongly fitted diode
Stations from our soldering reviews
85W digital station (70W iron) with ESD-safe construction.
Soldering iron and hot air gun in one station, for SMD rework. Check the input voltage for your region.
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Frequently asked questions
Which side of an SMD diode is the cathode?
The end with the band. On most surface mount diodes a light or contrasting stripe is printed across one end of the body, and that end is the cathode. If the marking is worn or missing, use your meter's diode mode: the reading appears when the red probe is on the anode and the black probe on the cathode.
What happens if I solder an SMD diode backwards?
It depends on its job. A reversed rectifier or protection diode may short a supply rail, blow a fuse or overheat. A reversed reverse-polarity protection diode can block power entirely, and a backwards Zener behaves like an ordinary forward diode, clamping at well under a volt. Often nothing works at all. Remove it, check the pads, and refit it the right way round.
Can I test an SMD diode while it is soldered on the board?
Yes, as a first check. Put the meter in diode mode across the two pads with the board unpowered. A forward reading in one direction and a much higher reading or OL in the other is a good sign. Parallel parts can lower the reverse reading, so a doubtful result means lifting one end and testing it alone.
Do I need hot air to solder a SOD-323 diode?
No. SOD-323 and SOD-123 diodes have two exposed terminals that a fine iron tip reaches easily, so the tack-one-pad method works well. Hot air helps more for removal, or for parts with a pad underneath the body. Tweezers, flux and good magnification matter more than hot air for fitting.
What does the marking code on a small SMD diode mean?
Small packages have room for only a few characters, so manufacturers print a short code rather than the full part number. The same code can mean different parts from different makers. Look it up in the datasheet of the part you expect, or in the board's parts list, rather than relying on a generic code table.
