My method for
soldering SMDs

Original page created on 07/06/2023; last modified on 08/06/2023.

The SMD devices

What is an SMD?

Surface-mounted devices (SMDs) have almost universally replaced traditional through-hole components with pins or wires. As their names suggest, surface-mounted devices are soldered directly to the surface of the board, while through-hole devices have their pins passed through the drilled circuit and soldered to the opposite side.

The advantages of SMDs are numerous: small footprint, higher density, less circuit machining, light weight, etc. The main drawback is the consequence of their small size: they are not designed to be soldered by hand.

Industrial assembly of SMDs

SMD soldering requires a specific heating system, known as reflow, either by infrared or by immersion in a vapour phase bath, the latter process being the best. See Wikipedia for more information.

Some PCB manufacturers also offer component assembly, at an additional cost, because, in addition to the cost of the components, they also have to make a silkscreen or stencil for depositing solder cream on the board. This can be interesting for us hobbyists, but as far as I know, the manufacturer only assembles components that he has in stock. This isn’t a problem for generic components such as resistors, capacitors or diodes. It’s more of a problem for special LEDs.

My hand soldering method

To mount the LEDs on a light strip, I’m reduced to soldering them by hand. Here’s the method I’ve been using for a long time.

What equipment do I need?

You don’t need any special equipment, apart from a fine-tip soldering iron. I recommend using a pair of non-magnetic tweezers to pick up SMDs. Indeed, many components include ferrous elements, and there’s nothing more annoying than seeing them refuse to stay in place, attracted as they are by steel tools.

This tool may seem rather expensive, but it is excellent quality. Since I bought it, I can’t do without it, for a wide variety of uses in model making.

Equipment: soldering iron and tweezers

First step: preparing the soldering “chips”

Soldering requires very small segments of tin, which some people call solder chips.

Unroll about ten centimetres of tin, of the smallest possible diameter, ⌀ 0.6 mm for me. Flatten the wire moderately with a cylinder of some kind. I use an X-Acto knife handle for this.

Flattening the tin wire

Cut the wire into 1 to 2 mm lengths. Accuracy is not important: lengths will be chosen to fit the width of the components to be soldered. At this step, you can realise the importance of flattening the wire, otherwise the sections will roll all over the place and won’t stay in place.

Cutting the tin wire

Click on the photo for a closer look at the chips.

The chips are stored in a small box, better not spill it…

Storing tin chips

Second step: preparing the components

Simply consult the list. Be careful: don’t take them out of their perforated strip packaging until the last moment!

Assortment of components

Assortment, from left to right and from top to bottom: diodes, rectifiers, resistors, LEDs and a through-hole capacitor.

It’s not always clear which direction the diodes and LEDs are mounted.

Components seen up close

In this photo, the direction of the diode (bottom) is indicated by the white bar marking the cathode (the −). The direction of the LED is indicated by a cut corner in the yellow zone, visible here thanks to magnification, but difficult to discern with the naked eye. In this case, it’s best to use a diode tester, which is usually included in digital multimeters. On the right, a resistor clearly marked: here, the value is 100 Ω; 101 means 10 followed by 1 zero. For more details, see this page on marking resistors.

Third step: soldering the LEDs

In most of my circuits, the LEDs are on one side and the other components on the other. So I start with the LEDs. For each one:

Holding blade

In this photo, you can see three notches of different widths. The central one is for the LEDs, the others for 1206 cases (resistors, diodes, etc.).

Soldering

Click on the photo for a closer view.

This has two advantages: firstly, I don’t have to keep turning the strip over and over again. Secondly, and more importantly, it allows me to check that I haven’t made a mistake in direction: it’s very easy to desolder the component if only one end is soldered. Much less so if both are!

Fourth step: soldering the components on the other side

For the other side of the circuit, there’s a little difficulty: the LEDs on the underside prevent the board from lying flat. So I glued two small strips (of a failed epoxy PCB) to a support of the same material, leaving a space between the two where the LEDs will fit. I hold the strip in place with hair clips.

Assembling the opposite side
Montage de la face opposée

Detail - it’s all done by hand!

Assembling the opposite side, detail

The rest is exactly the same as before.

Different aspects of a soldering joint

Without wishing to achieve the perfection of industrial soldering, let’s have a look at what a hand-made solder joint can look like.

Aspects of an SMD solder joint

On the right, the soldering is virtually ideal. On the left, the quality is rather poor: the tin forms small balls. This is probably due to insufficient heating. Contrary to what might be feared, SMDs are fairly robust and can withstand relatively high overheating.

A little exercise, by the way: what is the value of resistor R12?
Here’s the answer.

Fifth step (optional): cleaning the circuit

The most effective way is to use acetone, but your lungs won’t thank you... In fact, given the very small quantity of tin used, there’s very little flux left on the board. But, if you’re a maniac, why not…

Sixth step: checking and testing the circuit

First, do a visual inspection.

For the test, I solder two wires to the pads to be connected to the track, and I supply the strip with a laboratory power supply, setting the current as low as possible (my strips generally work with currents of less than 5 mA), and gradually increasing the voltage from 5 V up to the nominal value of 15 V. If there’s a problem, we’ll see it before we risk component breakdown.

If you don’t own a lab power supply, you can use an old train transformer, putting a resistor of a few hundred ohms in series to avoid unpleasant surprises.

Please note: my recent circuits based on voltage doublers or quadruplers require a DCC power supply to operate. You won’t be able to test them with direct current. You will therefore need to connect them to a DCC source, again using a protection resistor.

Possible causes of malfunction

  1. Inversion. Possible culprits: LEDs? Unlikely, because you’ve tested them before, haven’t you? It could be the other diodes or the rectifier bridges.
  2. Dry soldering, which looks OK, but where the tin has not joined the component to the pad. It often has the shape of a ball. Personally, I find them quite easily by (gently!) twisting the strip, starting at one end. In this way, you can see if the operation appears randomly.
  3. On the opposite, solder in excess that short-circuits under a component or between two tracks. With professional quality circuits, with a solder mask, this incident is very rare: just try bridging two pads with tin!
  4. Confusing components, usually resistors: if you swap a high value and a low one, it won’t work as well!
  5. Cut track: extremely rare. Normally, the manufacturer carries out a continuity test on all circuits as part of the manufacturing process.

In conclusion

I hope you won’t be put off by the apparent complexity of this method. With a bit of practice, it goes fairly quickly and the light strips almost always work first attempt.

Special SMD non-magnetic tweezers
Bernstein ref. 5-078-13
£8.89 (price 2023) at TME

Metal hair clips
50 pcs, length 46 mm
Approx. £5.00 shipping included, (price 2023)
at AliExpress

Marking 183 = 18 followed by 3 zeros.
So 18,000 ohms, or 18 kΩ.