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18 Dec 2019, 16:51

Building a Modular Synthesiser Part 4: Soldering Surface Mount Devices

Learning to solder surface mount components and building a Synthesiser EQ module.

A couple of months ago I had the pleasure of attending the What’s New In Electronics show with the DesignSpark team, to show off my Red Tin. One of the features of the show was a soldering competition and just for a fleeting moment, I thought, “I’m pretty good at soldering, I might have a go at that” — before I then realised it was soldering Surface Mount Devices (SMD), something I had absolutely no experience of. I had also encountered Surface Mount Technology (SMT), as it is also known when I was searching around for suitable self build modules for my synthesiser, so I thought soldering these typically much smaller devices would be a useful skill to acquire.

What is Surface Mount Technology?


Surface mount technology, originally called "planar mounting", emerged in the 1960s with IBM doing most of the development and by the 1980s had become the most common method for producing electronic circuits. It differs from the “through-hole” technology that preceded it (and is still common for small scale production and prototyping) in that the components are mounted or placed directly onto the surface of printed circuit boards (PCBs), rather than fitting components with wire leads into holes in the circuit board. Its development has been driven partly by the desire for miniaturisation; an SMT component is usually a lot smaller than its through-hole counterpart because it has either smaller leads or no leads at all, and since the components are often designed to be handled by machines rather than humans, some limitations of scale are removed. In addition, SMT allows for components to be soldered to both sides of a PCB, which also saves space.

The use of SMT also speeds up the production process and facilitates the automation of circuit production through the use of pick and place machines. The downside is that, due to the tiny size of the components and the fact that they are often packed closely together on a PCB, the risk of mistakes increases.

SMDs come in a variety of sizes varying from 0201 (0.25 mm × 0.125 mm) to 7451 (7.4 mm × 5.1 mm). I assume because the standards were developed in the US, imperial sizing is more commonly used than metric, so the above sizes translate as 008004 to 2920. For hand soldering projects the most common size for resistors and capacitors is 1206 (3216 Metric).

Practise makes perfect

I thought it best to have a practise before I attempted a synthesiser module, so I ordered a couple of exercise boards (178-3497) (528-0403) .

Along with my trusty soldering iron (045-6018) with its finest tip (0.4mm) and good quality solder, I decided that I was going to also need the following:

I retrieved a selection of components to use that our pick-and-place machine had failed to place on its last run. A quick Internet search brought up loads of guides and tutorials, including this one on DesignSpark from a former colleague about reworking using a hot air gun. The PDF guide for putting together my synthesiser EQ module also has lots of useful tips.

I got on OK with the practice board, following advice to use plenty of flux. For the small components, 1201 resistors and capacitors, I fixed a small amount of solder to one pad, then positioned the part and, holding it in place with the tweezers, applied solder to the other pad.

For the Integrated Circuits (IC) I followed a similar technique fixing one corner first and checking it was correctly lined up, before soldering the corner diagonally opposite and then the rest of the legs. It is almost inevitable that there will be some shorts — solder that goes across 2 or more legs of an IC — and this can be cleaned up using the solder wick.

This is done by applying the wick to the excess solder, heating it with the soldering iron and then removing the wick and the iron together. If you remove the iron and then try to move the wick you will probably find it is soldered in place! If this does happen, do not try to just pull it free as this may damage your PCB. Simply put a small amount of solder on the tip of your iron, reapply it to the wick, the solder will become molten and you will be able to remove it.

One thing that I have not seen mentioned, but I discovered for myself: remember to hold the wick by its container and not the wick itself, as it gets very hot very quickly! Which, of course, is its “raison d'être”, but that does not stop it from burning your fingers!

Now I was happy I was not going to make a mess of things, I could start on my module.

Building the EQ module


The EQ module came as a PCB and front panel. I put together a BOM for the other necessary parts, a link to which you will find at the end of this article. The information on this small module claimed it was a good introduction to SMT soldering – and it is not wrong.

As mentioned previously, the comprehensive build guide was very helpful and, as it suggested, I started by soldering the resistors. Once they were in place I cleaned off any flux residue with IPA and a cotton bud to make it easier to spot any mistakes, then examined my work under a magnifying lamp.

Everything looked OK and so I carried on by soldering the capacitors, again cleaning up and examining them before continuing.

The diodes and resettable fuses were a bit more fiddly to get lined up.

Once that was done it was time for the ICs, which went on trouble-free — no shorts to wick off!

It was now time to add the 2 though hole capacitors and the power connector, before turning the board over and adding the sockets and potentiometers.

The potentiometers intended for the module have 7mm bushes, unlike mine which were 9mm, so I needed to drill the holes in the faceplate to accommodate my larger pots.

I also needed to snap off the small fixing lug with some snips so the pots would sit flush against the back of the panel. Once this was done I fitted the panel before soldering sockets and pots in place to make sure they were all lined up correctly.

All that remained then was to fit the module in my case with its partners and test it out...

The module has 2 EQ circuits. The upper 2 jacks are the input & output for the EQ controlled by the treble & bass knobs. The lower 2 sockets are the input & output for the tilt EQ. With the tilt knob fully counter-clockwise it boosts bass and cuts treble. Fully clockwise it cuts bass and boosts treble.

Conclusion and coming up

Soldering this project proved to be less daunting than I imagined, although it did remind me that I need to book an appointment at the opticians! I will not be put off now by synthesiser kits that have surface mount components. Having said that, I am back to a through-hole build for my next module which is my most ambitious task yet, comprising a Voltage Controlled Oscillator (VCO), Voltage Controlled Filter (VCF) and an Envelope Generator all crammed into a 26hp Eurorack module.

Downloads

I currently look after production at AB Open. I have a background in the arts, environmental conservation and IT support. In my spare time I do a bit of DJing and I like making things.

18 Dec 2019, 16:51

Comments

January 7, 2020 09:04

An appropriately small solder tip and solder is a must with surface mount components. Good luck when encountering 0603 sized components. If you use a pin vise and small drill bit you can hand drill a pilot hole to utilize the fixing lug on potentiometers. It also comes in handy when installing 10 turn potentiometer dials which have an indexing bump.

0 Votes

January 1, 2020 13:39

Very many thanks for the brilliant practical intro to SMT soldering!
Just one question: why use solder wick to remove solder shorts between legs? why not use a solder pump (solder sucker)?

0 Votes

January 2, 2020 15:23

@geek45 I find the solder wick easier to use and more effective for removing the right amount of solder. As Boss says above the pump feels a bit big and clumsy - but each to their own, I'd say use what ever method works for you

January 2, 2020 15:23

@geek45 sorry to jump in... I use the solder pump and wick. I find the wick much better for the finer pitch parts and use a bit of flux on the offending solder bridge. The wick is directly on the leads and pads so seems to wick the bridge away easily often leaving enough solder for the required joint (probably not enough for a pro inspection!). The pump is a bit on the large size for the small pads and then you have the soldering iron bit in the area so it just gets more fiddly, but it does work when done carefully.