How to solder

Soldering also has to be learned by practice and can not be learned out of a book. The key factors affecting the quality of a soldered joint are Cleanliness, Temperature, Time, and Coverage - each is covered in detail below:


Firstly, and without exception, all parts - including the iron tip itself - must be clean and free from contamination. Solder just will not "take" to dirty parts! Old components or copper board can be notoriously difficult to solder, because of the layer of oxidation which builds up on the surface of the leads. This repels the molten solder and this will soon be evident because the solder will "bead" into globules, going everywhere except where you need it. Dirt is the enemy of a good quality soldered joint!

Therefore it is absolutely essential that you ensure that parts are free from grease, oxidation and other contamination. In the case of old resistors or capacitors, for example, where the leads have started to oxidise, use a small hand-held file or perhaps scrape a knife blade or rub a fine emery cloth over them to reveal fresh metal underneath. Stripboard and copper printed circuit board will generally oxidise after a few months, especially if it has been fingerprinted, and the copper strips can be cleaned using an abrasive rubber block, like an aggressive eraser, to reveal fresh shiny copper underneath.

Another side effect of having dirty surfaces is the tendency for people to want to apply more heat in an attempt to "force the solder to take". This will often do more harm than good because it may not be possible to burn off any contaminants anyway, and the component may be overheated. In the case of semiconductors, temperature is quite critical and they may be harmed by applying such excessive heat.

Before using the iron to make a joint, it should be "tinned" (coated with solder) by applying a few millimetres of solder, then wiped on a damp sponge preparing it for use: you should always do this immediately with a new bit, anyway. Personally, I always re-apply a very small amount of solder again, mainly to improve the thermal contact between the iron and the joint, so that the solder will flow more quickly and easily.

Normal electronics grade solder is usually 60% lead - 40% tin or 40/60, and it already contains cores of "flux" which helps the molten solder to flow more easily over the joint. Flux removes oxides which arise during heating, and is seen as a brown fluid bubbling away on the joint. Acid fluxes (e.g. as used by plumbers) should never be necessary in normal electronics applications.


Another step to successful soldering is to ensure that the temperature of all the parts is raised to roughly the same level before applying solder. Imagine, for instance, trying to solder a resistor into place on a printed circuit board: it's far better to heat both the copper p.c.b. and the resistor lead at the same time before applying solder, so that the solder will flow much more readily over the joint. Heating one part but not the other is far less satisfactory joint, so strive to ensure that the iron is in contact with all the components first, before touching the solder to it. The melting point of most solder is in the region of 188°C (370°F) and the iron tip temperature is typically 330-350°C (626°-662°F).


Next, the joint should be heated with the bit for just the right amount of time - during which a short length of solder is applied to the joint. Do not use the iron to carry molten solder over to the joint! Excessive time will damage the component and perhaps the circuit board copper foil too! Heat the joint with the tip of the iron, then continue heating whilst applying solder, then remove the iron and allow the joint to cool. This should take only a few seconds, with experience. The heating period depends on the temperature of your iron and size of the joint - and larger parts need more heat than smaller ones - but some parts (semiconductor diodes, transistors and i.c.s), are sensitive to heat and should not be heated for more than a few seconds. Novices sometimes buy a small clip-on heat-shunt, which resembles a pair of aluminium tweezers. In the case of, say, a transistor, the shunt is attached to one of the leads near to the transistor's body. Any excess heat then diverts up the heat shunt instead of into the transistor junction, thereby saving the device from over-heating. Beginners find them reassuring until they've gained more experience.


The final key to a successful solder joint is to apply an appropriate amount of solder. Too much solder is an unnecessary waste and may cause short circuits with adjacent joints. Too little and it may not support the component properly, or may not fully form a working joint. How much to apply, only really comes with practice. A few millimetres is enough for an "average" joint.

A quality solder joint uses minimal solder - just enough to "fill" the area between the elements. You shouldn't need to cover the connection with a ball of solder. A quality connection will appear smooth and shiny as opposed to dull as the solder flows into the gaps. Your soldering iron should appear bright right before it is used. To prepare it (I recommend using a soldering station with a chrome plated tip) clean it with a damp sponge immediately before use

Soldering Rules

Soldering is actually quite simple if you observe a few rules:

1. Keep the solder tip clean (hopefully you have purchased a chrome plated tip). Immediately before you use it wipe it clean. After use place it in the holder - the solder from the solderingoperation should keep the tip tinned.

2. It is also imperative that the elements being soldered be relatively clean. (You should be getting the idea by now that cleanliness is a good thing.) Rosin based solder has powerful chemicals that when heated do a pretty good job of cutting through component and pc board oxides. But this takes time and heat. And the longer you are required to heat the junction to dissolve oxides the more the risk that you will physically damage the pc board foil or melt the insulation on the wire. (By the way if you use teflon insulation this latter problem is eliminated.)

3. Place the items being soldered in a relatively stable position such that once competed there is no possibility of jarring or relative movement between the soldered elements

4. Now, for eyelet connections using your needle nose pliers make a good mechanical connection. For printed circuit boards, simply bend the leads with a slight radius and inset into the board.

5. Hold the component in place so it remains undisturbed. Apply heat to the junction with the bright shiny tip of the iron.

6. After a few second the junction temperature will rise and you can then apply solder to the other side of the junction (not the solder iron). It should melt immediately and flow smoothly into the junction. You will note that the solder seems to fuse with the two elements.

7. Allow this to cool undisturbed.

8. Examine the joint - it should appear clean and shiny - not dull or pitted.

OK, that's it. The process is actually much simpler that it sounds. If you are new to this, take the time to practice on some scrap cables, pc boards and terminals until you feel confident.

On printed circuit boards, and especially neat high density IC components, it is very common to accidentally form a solder bridge. This is when a connection of solder forms between two adjacent pins. Many hours have been dedicated to troubleshooting and many components destroyed as well as a result of this "minor" error. I would suggest that at the end of each component connection you stop to visually inspect each connection. Look for both dry joints and bridges, and don't be shy about using a magnifying glass to examine each connection in detail. This is a valuable investment with a payback in both trouble free start up and long term reliability.