SMT Soldering Notes

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Soldering SMT components at home is perfectly achievable with some patience, but you need a little bit more equipment than for a through-hole project.

Contents

Temperature controlled soldering station

It doesn't need to be expensive; once you have one you'll wonder how you ever managed with a standard 25W iron. I solder with mine set to 260*C which is the rated soldering temperature for many chips. With fux and a clean board, this is plenty hot enough.

Fine solder

Use 0.7mm or even thinner. Get a good quality 60/40 (tin/lead) solder; lead-free soldering is harder and needs higher temperatures.

Flux

This is absolutely essential, and again once you've used it you will wonder how you ever soldered any circuit board without it. One of the best types is EDSYN FL-22. There are many videos showing how to solder SMT by hand, but this video by Dangerous Prototypes (actually a review of different fluxes) shows the process in great detail.

Note: Have your workspace well ventilated. For example, use a small fan to gently blow flux and soldering fumes away from you, and have an extractor fan nearby.

Solder braid

This (and flux) are the key to soldering fine-pitched SMT devices. It's clearly impossible to solder each pin of an SMT chip seperately, so the trick is to solder the chip without worrying about using too much, then use the wick to draw off all the excess resulting in a professional looking finish.

Pointed-tipped tweezers

Used to place components and hold them in place whilst soldering.

Magnifying Glass

Verifying the joints by the naked eye just isn't possible, especially before the board is cleaned. A simple magnifying glass is essential.

Cleaning Fluids

Because of the flux, it is essential to properly clean the board after soldering. But anyway, all boards should be cleaned really, and it will give it the professional look with nice bright and shiny solder.

There are plenty of commercial flux removers available, isopropynol (isopropyl alcohol, 99%) or the derivative acetone and a toothbrush work well enough. Exercise some caution with acetone as it can melt some plastics.

  • Isopropynol is no longer available in UK highstreet chemists, but is in plentiful supply (and cheap) online (especially eBay)
  • Nail varnish remover often is no longer acetone based, but acetone is also available cheaply online and can still be bought in highstreet chemists.

To clean to boards, let the assembled board rest in the tray, just submerged, for a few minutes, then use the toothbrush to gently work out remaining residue between chip pins and any other crevaces. Avoid submerging components not rated for such - in particular check the datasheets of electrolytic capacitors and switches to ensure they can be submersed. When using parts that cannot be submersed, fit them last and clean the board thoroughly before hand, then clean around those parts using cotton bud and isopropynol once fitted.

There are a number of options for disposal of used (dirty) fluid. For example it can be saved up in another (labelled!) jar, and taken to a local checmical collection point in some places, or it can be left to evaporate outside. Be aware it is VERY flamable and treat it accordingly.

See Also