Soldering

What is it? 

Soldering is a way of joining metal parts together, by allowing a molten metal alloy, called solder, to flow around them.  When the solder cools and solidifies, it provides a good electrical connection between the parts, with a little mechanical strength.

Why do it? 

You may wish to design and build some small electronic circuits to act as sensors for your robot.  You can try out your ideas using breadboard, but you cannot use breadboard on your robot - it is too big and heavy, and the connections are not secure enough to be reliable on a robot.  So you will have to build your circuits by soldering the components onto small circuit boards.

Safety

Traditional solder is an alloy of tin and lead, usually in a 60:40 ratio.  It melts at about 200°C.  The electronics industry is moving to lead-free solder, usually also an alloy of tin, with a higher melting point.

The solder you will use is in the form of a thin wire, which also contains a small amount of flux.  The flux melts at a lower temperature than the solder, and helps the solder to flow over the metal parts to be joined.  Some of the flux burns or boils away while you are soldering.

You will also be using heat-shrink tube - a plastic tubing which shrinks to a much smaller diameter when it is heated.  You will use a hot-air gun to heat this tubing.

Hazards

The business end of the soldering iron, which you use to heat the parts to be joined, may be at 300°C or more.  It can cause serious burns if it comes in contact with your skin.  Take care with the soldering iron, and always place it in its stand when you are not using it.  Never leave it on the bench or balanced on any other object.

The hot soldering iron will melt most plastic material very quickly, often producing toxic fumes.  Do not solder anywhere near your robot or your computer - take your circuit board to one of the marked soldering stations in the lab.  Keep loose clothing and hair away from the soldering iron also.

Take particular care with cables carrying mains electricity - the soldering iron can melt through the plastic insulation almost instantly, causing an electric shock hazard and an electrical burn hazard.  Keep all cables away from the soldering area, and away from the stand for the soldering iron.

Molten solder at 200°C will burn your skin, but that will probably heal.  If it splashes into your eye, it will do serious, irreversible damage.  Wear eye protection while soldering.

The soldering process produces smoke and fumes which are harmful.  Work in a well ventilated area, and keep your face beside the work, not above it.  Minimise smoke and fumes by keeping the soldering iron clean - wipe it on a damp sponge before you put it back in its stand.

The materials used in solder are toxic.  Wash your hands carefully when you finish soldering.  Do not eat or drink in the laboratory.

The hot-air gun may look a bit like a hair-dryer, but it produces very hot air, at about 400°C at the nozzle.  This will burn exposed skin and melt plastic just like the soldering iron.  The hot air cools fairly quickly as it moves away from the nozzle and mixes with colder air, but you must take care where you point the hot-air gun, and think about what is behind the parts you are trying to heat.  

The nozzle of the hot-air gun remains hot after you switch it off - it can still burn skin and melt plastic.  Be careful where you put it down when you have finished using it.

First Aid

If you do burn your skin, you must cool the affected area as soon as possible.  The best method is to hold it under cold running water for at least 5 minutes.

There is a sink in the office at one end of the laboratory.  There is a drinking water fountain in the corridor near the other end of the laboratory.

Always call a member of staff immediately if there is any accident or injury in the laboratory.

How to Solder

Making good solder joints is harder than it looks.  It will take a bit of practice to get good at it.  However, it is worth the effort - bad solder joints can appear to work for a while, but they are very weak and will leave you with an unreliable robot.

Preparation

Solder has to melt and flow over the parts to be joined in less than a second, making good electrical contact with both parts.  This will only happen if everything is clean.

Use new components if possible - avoid the resistor which has already been pushed into several breadboards.  Try to avoid handling the component leads and the copper side of the strip-board, to keep them clean.  If necessary, remove any dirt or corrosion using sandpaper.

You must also keep the tip of the soldering iron clean.  Before every joint, wipe the tip of the iron on a damp sponge to remove any old solder or dirt.  If the iron has been idle for a while, melt a few mm of solder wire onto the tip, then wipe it off on the sponge.  This will help to remove any dirt and old solder from the tip of the iron.

Soldering on strip-board

Strip-board is a thin sheet of insulating board, with parallel copper strips or tracks on one side, and lots of holes.  To build a circuit, you place components on the plain side of the board, with their leads passing through the holes.  You then solder the component leads to the copper tracks on the other side.  The finished circuit might look like the one shown below:

Step-by-step

Fit the component on the plain side of the strip-board, with its leads passing through the holes.
Bend the leads outwards slightly on the copper side, to hold the component in place.  Use a vice (or an assistant) to hold the circuit board steady while you solder.
Wipe the tip of the soldering iron on the sponge.   The sponge should be damp, so that it does not burn, but not so wet that it cools the soldering iron too much.
Apply a tiny amount of solder to the tip of the iron - this helps the flow of heat from the iron to the parts to be joined.
Press the tip of the soldering iron against the component lead and the track at the same time - it is important to heat both of them evenly.
After a second, touch the end of the solder wire against the opposite side of the joint - NOT against the soldering iron.  The parts should be hot enough to melt the solder, and it should flow all over the joint almost instantly.
Remove the solder wire and then remove the soldering iron.  Wipe the iron on the sponge and put it back in its stand. Leave the joint to cool.  Do not blow on it to cool it, and do not move it until the solder has solidified.
Cut off any surplus wire from the component lead - be careful where the cut piece flies.

The good, the bad and the ugly

A good solder joint should look smooth and shiny.  The solder should flow out a little along the track and the component lead, giving a cone-shaped joint.
A dry joint - the solder is dull and brittle.  This can happen if the joint is cooled too fast, or moved while it is cooling.
If the track is not heated properly, the solder will not flow along it, and will form a blob.
This joint does not have enough solder, and it did not flow very well along the track - it will be very weak.
This joint has too much solder - it has spilled over onto the next track, possibly causing a short-circuit.  As the second track had not been heated, the solder did not flow out along it, and remains in a blob.
A spiky joint is a sign that the solder was heated for too long, and all the flux has been burnt away.

De-soldering

A bad solder joint will be brittle and un-reliable.  Trying to re-heat it or add more solder will only make it worse.  The only solution is to remove all the solder and start again.

The de-soldering pump, or solder-sucker, works like a syringe in reverse, and sucks molten solder away from the joint. To arm it, press down the plunger - it should lock into place.
Use the soldering iron to melt the solder which you want to remove. Put the nozzle of the solder-sucker close to the molten solder, and press the release button.

Empty the solder-sucker by re-arming it, with the nozzle pointing downwards, over a waste bin.  The solder will usually come out in very small particles, so take care not to breathe them in or get them on your clothes.

If this did not remove all the solder, you can repeat the process.  However, if you spend too long heating the joint you will probably damage the component.  Too much heat can also damage the strip-board, causing tracks to separate from the board.