Monday, 7 September 2009

Soldering - a "how to"

Here is what I gleaned from extensive reading before I started to solder. I put together everything pertinent that I learnt, in my own words, and went over it afresh before starting on the opto endstop boards. You could call it a tutorial. Unfortunately, it isn't possible to grow a third hand for taking photographs whilst actually soldering, and my photo's aren't perfect anyhow, but they give an idea....

How to....

Check that you have all the right components for the board. It is a good idea to lay them out around the board, each on the respective head/foot/left/right side, to assess sizes and locations of the individual components.

Start with the smallest component on the board.

Firstly, check the component leads for signs of dirt or corrosion (see later), and clean by wiping as necessary. If the component has been shipped stuck onto tape, cut the leads alongside the sticky tape to prevent future issues with glue interfering with the solder joint.

Bend the component leads down to align with the fitting holes in the board. Check which way round the component must be inserted; with some components this doesn't matter, but with others it is critical. Insert the leads, from the top towards the bottom surface of the board, both at the same time, but do not press the component down onto the board, because that may kink and weaken or snap the leads.

For any component that may heat up in use, eg a resistor, leave a small gap so that the component is raised above the board slightly to allow air to circulate and heat to dissipate. For temperature-sensitive components, eg transistors and diodes, where excessive heat from the soldering operation can be harmful, again leave a small gap, so that a small crocodile clip or similar metal item can be attached - during soldering - to act as a heat sink and dissipate excess heat.

Bend the leads outwards away from each other slightly on the underside of the board to hold the component in place.
(This picture shows a ferrite bead sat vertically on the board, but most components will be aligned between their fixing holes, known as pads.)

Cut off the leads a few mm (approx 3 mm) away from the board. Do not cut them off flush with the board. Cutting the leads before soldering prevents disturbing or damaging the finished joint.

Repeat with one or two other small components, working from the centre of a big board outwards.

Now heat up the soldering iron. Clean the tip using a dampened card egg-box (my resourceful Dad's method) or a wetted sponge (commercial method). The tip should be shiny.

Apply the tiniest amount of solder, containing flux, to the iron tip, by just touching the solder against the tip, but all around it, to prevent it oxidising. Now wipe the tip on the damp card/sponge.

If the component leads are lightly corroded, and not shiny themselves, use the soldering iron to heat the leads. Without moving the soldering iron, now touch the solder to the lead on the side away from the soldering iron.
Move the solder wire away, remove the soldering iron and check that there is now a layer of solder around the lead. This process is "tinning". "Tinned" parts will solder together well. Discard the excess solder blob on the iron tip by wiping it on the dampened card/sponge. If too much solder is applied to the part being tinned, use desoldering wick/braid to remove the excess by applying the wick to the coating, and the tip of the soldering iron to the wick, gently pulling the wick along under the iron tip as the wick becomes full of solder. Remove both wick and solder together.

On the underside of the board, use the hot iron tip to heat the lead AND the copper connection together, for just a second or two. Now, without moving the iron tip away, add a tiny amount of solder to the side of the lead by touching the solder against the hot lead and copper connection, followed by quickly adding solder, using a dabbing motion, to the side of the lead away from the iron, again down against the copper connection.
Repeat this movement until sufficient solder has been applied to create a good joint. Take the solder wire away, then immediately remove the soldering iron, to avoid boiling off the flux and creating spikes, without knocking the joint. Do not move the board until the solder joint has set.

Now inspect the joint.
The outline of the leads should be jutting out of the solder joint slightly. As the joint cools, it should have concave sides, should be flush against the copper connection, and pulled up around the component lead. The solder should not "bridge" to neighbouring connections. If using leaded solder, the joint should look very shiny; lead-free solder will look a little duller or grainier, but still shiny.
Check the joint from the top side of the board, too; the appearance here should be the same.

If the solder does not look shiny, or has not flowed around the lead well, re-melt it with the soldering iron slightly hotter, and ALWAYS add a small amount of flux, which may be contained in extra fresh solder, as necessary, so that the joint becomes shiny and complete.

Below is a picture of a row of good joints in the foreground, - this was my fourth board ever, so it isn't perfect,
but the front left solder-joint is perfect, in size, shape and shininess. The brown marks you can see on some of the others are burnt flux, which can be cleaned off with a commercial cleaner or by scraping gently (eg fingernail), but in this case don't compromise the electrical contact.

Here is a close-up picture of the joint on the left in the background; this isn't good, having too much solder. You can see that the solder is convex instead of concave. However, it has made electrical contact and is a viable join.

Here is a picture showing a bad joint; there are 2 large connecting holes in the middle of the board, and you can see that the top one in the picture has shiny solder in it, whereas the bottom one has dull solder. This would be a bad joint if it all looked like this. In this case, the solder on the other side of the board is shiny and I believe there is a sound connection there. Otherwise this would be a candidate for re-doing the solder joint.

Continue with the other joints, checking each as it is completed, before moving on to the next. Re-clean the soldering iron tip as necessary, probably after every few joins.

When the first batch is soldered successfully, continue with another batch of components, moving up in size and outwards on the board.
Finally, add a tiny amount of fresh solder to the tip, to protect it from oxidation in storage, before switching off. Store the soldering iron covered to keep it clean.



  1. Sounds like you've got the hang of it.

    One thing I find really useful when soldering is blue-tack to hold the components in place while soldering them. It's best to keep it of the metal parts of the components and just put it on the component casings otherwise it will melt.

  2. Nice.
    A really good, well-written tutorial. It contains some tips I haven't heard before (like cutting the legs before soldering).

    The only thing that might make it perfect is a picture of a 'good' and 'bad' joint at the end.

    There are plenty of clear instructions for any beginners. Is is worth posting this on the RepRap builder's wiki for any newcomers?

    Thanks for the time and effort you've put into this.

  3. Oh, thanks, Renoir. And thanks for the suggestion of improvements and putting it on the wiki. I am better at writing instructions than I am at doing the soldering! I'm going to insert some extra photographs now and investigate the wiki.

  4. Hi Greenarrow - I didn't know Blu-tak melted! How did you find that out, and were you able to clean it off?!

  5. It doesn't completely melt, it just smells a little and goes a bit like chewing gum so you have to wait for it to cool down before you can get if off again (it returns to normal).

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