Seb's Synth-DIY Pages

Digi-Mod - Surface mount soldering

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This is a brief guide to surface mount soldering by hand.  It is intended to enable those without any surface mount soldering experience to be able to assemble a 4xD or Digi-Mod PCB.

What you will need

Soldering iron One with temperature control is strongly recommended.
Fine soldering iron bit (e.g. 1mm) I use one that's this shape and 1mm across the tip:

(this is a good shape for 'drag-soldering' fine pitch ICs)
Soldering iron sponge If you're not already in the habit of working with a clean bit, you'll want to be for this!
Good lighting Kind of obvious, but very helpful.
Fine tweezers For placing small parts.
Pointed implement Useful for pushing small parts into place and holding them there while soldering.  I'd recommend something like this cheap Soldering Aid Set which has both a straight and a bent pointed thing.
Powerful magnifying glass For final inspection of solder joints.  Personally I use the scanning lens salvaged from an old fax machine, which has a really high magnification.
Very fine no-clean solder (28swg/0.38mm or finer) The "no-clean" is important.  An alternative would be to use organic flux and wash thoroughly and immediately after soldering, but no-clean is just easier!
No-clean flux gel or a no-clean flux pen The gel is preferable to the liquid flux that you get in pens as it is a bit tacky and helps to hold the components in place.  The gel can be applied with a small screwdriver or a matchstick.
Fine no-clean desoldering braid For removing solder bridges between IC pins.  Also note the "no-clean" - some desoldering braids contain rosin flux.

Construction order

The 4xD and Digi-Mod circuit boards are quite densely packed so you definitely want to solder all the surface mount parts first otherwise the bigger thru-hole parts will get in the way.  It doesn't really matter which order you solder the surface mount components, although depending on the shape of your soldering iron bit you may find it easier to solder the ICs before the surrounding capacitors and resistors.

Iron temperature

I tend to work with quite a hot iron temperature (around 375°C) which makes things quicker and is needed for the fine pitch IC soldering technique below to work well.  The actual temperature setting will vary from iron to iron, but it'll probably be somewhere between 330°C/626°F and 380°C/716°F (although with some cheap temperature controlled irons the temperature setting may not be very accurate, so try higher if it's not working well).  Most modern components are quite resilient, being designed to cope with automated soldering and lead-free solder, but even so when working at these higher temperatures try not to leave the iron in contact for too long.

Soldering 0805 chip resistors and capacitors

Soldering SOT-23 transistors

Soldering medium pitch ICs (i.e. the 1.27mm pitch SRAM ICs)

Soldering fine pitch ICs (i.e. the rest of the SMT ICs)

The results

This is a bit of the 4xD, my first SMT soldering project (other than the odd bit of repair work) and I was pretty happy with the results.  It just goes to show that it's not as difficult or scary as one might think!

Further reading

A search on google will find a number of other surface mount soldering tutorials.  Here are a couple: