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How to Solder
First a few safety precautions:
Never touch the element or tip of the soldering iron.
They are very hot (about 400°C) and will give you a nasty burn.
Take great care to avoid touching the mains flex with the tip of the iron.
The iron should have a heatproof flex for extra protection. An ordinary plastic
flex will melt immediately if touched by a hot iron and there is a serious
risk of burns and electric shock.
Always return the soldering iron to its stand when not in use.
Never put it down on your workbench, even for a moment!
Work in a well-ventilated area.
The smoke formed as you melt solder is mostly from the flux and quite irritating.
Avoid breathing it by keeping you head to the side of, not above, your work.
Wash your hands after using solder.
Traditional solder contains lead which is a poisonous metal.
Place the soldering iron in its stand and plug in.
The iron will take a few minutes to
reach its operating temperature of about 400°C.
Dampen the sponge in the stand.
The best way to do this is to lift it out the stand and hold it under a cold tap for
a moment, then squeeze to remove excess water. It should be damp, not dripping wet.
Wait a few minutes for the soldering iron to warm up.
You can check if it is ready by trying to melt a little solder on the tip.
Wipe the tip of the iron on the damp sponge.
This will clean the tip.
Melt a little solder on the tip of the iron.
This is called 'tinning' and it will help the heat to flow from the iron's tip
to the joint. It only needs to be done when you plug in the iron, and occasionally
while soldering if you need to wipe the tip clean on the sponge.
You are now ready to start soldering:
Hold the soldering iron like a pen, near the base of the handle
(imagine you are going to write your name). Remember to never touch the hot element or tip.
Touch the soldering iron onto the joint to be made.
Make sure it touches both the component lead and the track. Hold the tip there
for a few seconds and...
Feed a little solder onto the joint.
It should flow smoothly onto the lead and track to form a volcano shape as shown
in the diagram. Apply the solder to the joint, not the iron.
Remove the solder, then the iron, while keeping the joint still.
Allow the joint a few seconds to cool before you move the circuit board.
Inspect the joint closely.
It should look shiny and have a 'volcano' shape. If not, you will need to reheat it
and feed in a little more solder. This time ensure that both the lead and track
are heated fully before applying solder.
Some components, such as transistors, can be damaged by heat when soldering so if you are
not an expert it is wise to use a heat sink clipped to the lead between the joint and the
component body. You can buy a special tool, but a standard crocodile clip (without a plastic cover)
works just as well and is cheaper.
The heat sink works by taking some of the heat being supplied by the soldering iron and this
helps to prevent the component's temperature increasing too much.
It is very tempting to start soldering components onto the circuit board straight away,
but please take time to identify all the parts first. Sticking them onto a sheet of scrap paper
and labelling each one is worthwhile and you are less likely to make a mistake if you do this.
Some ICs are static sensitive and will be supplied in antistatic packaging -
leave these ICs in their packaging until you need them, then earth your hands by touching a metal
water pipe or window frame before handling the ICs.
Stick the components onto paper using sticky tape.
Identify each component and write its name or value beside it.
Add the labels (R1, R2, C1 etc.) used in the project diagram too if necessary.
Capacitor values can be a little more difficult, the various labelling systems are
explained on the capacitors page.
Some components require special care when soldering.
Many must be placed the correct way round and a few can be easily damaged by the heat from soldering.
The table shows advice for the various components and a suggested order to put them
on the board. Generally it is best to start with the smallest parts but for stripboard
it is helpful to start with the IC holder(s) as a reference point for other parts.
Wire links between points on the board can be made with plastic-coated single core wire which will need stripping,
or tinned copper wire if the link won't touch other parts. Tinned copper wire looks just like solder but you can
feel the difference, it is stiffer than solder (and it won't melt).
Wires to parts off the board need to be flexible so use plastic-coated stranded wire for these,
a popular type is 7/0.2mm wire (7 strands of 0.2 mm diameter wire).
Single core wire is unsuitable because it snaps when repeatedly flexed.
Solder for electronics use contains tiny cores of flux, like the wires inside a mains flex.
The flux is corrosive, like an acid, and it cleans the metal surfaces as the solder melts.
This is why you must melt the solder actually on the joint, not on the iron tip. Without
flux most joints would fail because metals quickly oxidise and the solder itself will not
flow properly onto a dirty, oxidised, metal surface.
At some stage you will probably need to desolder a joint to remove or re-position a
wire or component. There are two ways to remove the solder:
1. With a desoldering pump
Also known as a 'solder sucker'. It is best to use one with an ESD (electrostatic discharge)
nozzle to protect some ICs which can be damaged by static electricity.
Set the pump by pushing the spring-loaded plunger down until it locks.
Apply both the pump nozzle and the tip of your soldering iron to the joint.
Wait a second or two for the solder to melt.
Then press the button on the pump to release the plunger and suck the
molten solder into the tool.
Repeat if necessary to remove as much solder as possible.
The pump will need emptying occasionally by unscrewing the nozzle.
After removing most of the solder from the joint(s) you may be able to remove the
wire or component lead straight away (allow a few seconds for it to cool).
If the joint will not come apart easily apply your soldering iron to melt the
remaining traces of solder at the same time as pulling the joint apart, taking
care to avoid burning yourself.
First Aid for Burns
Most burns from soldering are likely to be minor and treatment is simple:
Immediately cool the affected area under gently running cold water. Keep the burn in the cold water for at least 5 minutes (15 minutes is recommended).
If ice is readily available this can be helpful too, but do not delay the initial
cooling with cold water.
Do not apply any creams or ointments. The burn will heal better without them. A dry dressing, such as a clean handkerchief,
may be applied if you wish to protect the area from dirt.
Seek medical attention if the burn covers an area bigger than your hand.
To reduce the risk of burns:
Always return your soldering iron to its stand immediately after use.
Allow joints and components a minute or so to cool down before you touch them.
Never touch the element or tip of a soldering iron unless you are certain it is cold.
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