Alistair Blaikie - QA Engineer
The ability to solder is an essential skill for anyone involved with electronics. A single bad joint can take hours of fault-finding to isolate, and considerably raise the stress levels of the constructor!
The soldering process enables two metal surfaces to be joined by addition of a molten filler, which "sets" to form a solid connection. In electronic work the filler is a an alloy of 60% Tin and 40% Lead. This alloy is supplied in the form of a wire with cores of flux running through it. The flux is a very aggressive chemical, activated by the heat of the soldering iron. It's job is to strip away all of the grease from the surfaces to be soldered, thus ensuring that the solder will flow properly.
Choosing the Right Equipment
With a soldering iron, the factors to consider are wattage, tip temperature and tip profile. A larger wattage does not mean a higher tip temperature, rather that there will be more heat energy available to heat up physically larger items. Thus a wattage of between 12W and 25W is fine for simple boards and small wires. A good example of this type of fixed temperature iron is the Antex CS18 shown in figure.1.
Although solder will melt at 183°C, a fixed temperature iron will run at about 400°C to ensure that there will always be enough heat to make the joint. Soldering to a PCB pad connected to a power plane or heatsink can cause a large drop in tip temperature each time a joint is made. If several joints are made the temperature may fall below a usable level. The best way to deal with this is to use an iron which can maintain the tip temperature to the optimum for soldering (320 to 380°C). The TCS iron, shown in figure 2, has a sensor in the tip and a control circuit in the handle to achieve this.
Keeping the tip temperature in this range has several advantages:
- tips will last longer;
- fluxing will be more efficient;
- less irritating fume will be given off;
- the risk of damaging nearby components is minimised.
The task of the soldering tip is to transfer the power developed by the heating element into the joint. The quicker the heat transfer, the better the joint, so it is important to consider the profile of the tip. Maximum heat transfer will occur when there is the largest possible surface contact between tip and the joint. As can be seen in figure 3, a fine pointed bit will give a very small contact area. In general terms the cross section of the tip flat should be as large possible given the physical size of the joint.
When working with CMOS devices there is always the risk of causing electrostatic damage to them. To avoid this always ensure that the tip of your iron is securely earthed. This will always be the case with mains powered irons, but care is needed with low-voltage irons that need no safety earth.
In the vast majority of cases the standard electronics solders mentioned above will be suitable. A word of warning though: they contain rosin based fluxes, the fumes from which can cause respiratory irritation. Always work in a well ventilated area and if you suffer from any chest problems consider buying some rosin-free solder. Finally, burns to both limbs and to workbench can be avoided by using a stand for your iron, as in figure 4.
Figure
4 ANTEX Soldering iron stand
The Right Technique
The state of iron's tip is often neglected. Solder applied to the tip
should flow quickly across the whole area and stick to it. This is good
"wetting", the tip needs to be kept in this state when not in use, particularly
if it is to be switched on for long periods. Should a tip be difficult
to wet then it is likely that a layer of hard oxide has built up. This
can be removed by the use of a compound such as the Multicore Tip Tinner
and Cleaner. These products should be used sparingly as they consist of
a concentrated flux that will reduce tip life.
Cleanliness is everything, most soldering problems are due to the fact
that solder will not wet to a dirty surface. To complement the cleaning
action of the flux it is good practice to wipe the PCB surface with a
small amount of solvent, and to avoid touching it with bare fingers.
For best results when making a joint follow the following sequence:
- Wipe the tip on a damp sponge and wet it with a small amount of solder;
- Apply the hot iron to one side of the joint and then feed in solder from the other;
- Allow the flux to work on the surfaces and the solder to flow across the whole joint, this should take no more than a couple of seconds;
- Remove the solder, then the iron;
- Allow the joint to cool and solidify without any disturbance.
If
a joint is taking too long to make it is likely that the iron being used
does not have enough power, or the joint surface is too dirty.
Visual Inspection
The best device for detecting faulty soldered joints is the human eye.
The shape of a joint, as seen in figure 5, will tell all that you need
to know.
Figure 5 Anatomy of a good solder joint
- The solder needs to completely wet the component lead and the pad on the solder side of the PCB. A consistent, concave, meniscus should be formed with an angle of 40 and 70° from the horizontal.
- If any of the components have moved during solidification of the joint this will show up as a dull, grainy, finish. This type of joint will be mechanically weak and unreliable.
- Applying the iron for too long will cause spikes of solder to form which may bridge to other connections, causing a short circuit.
- Black areas on the joint and signs of poor wetting indicate a "dry" joint. This is caused by dirt in the joint area and will result in an unacceptably high resistance.
- If the profile of the joint is convex, rather than concave, then there is too much solder in place.
Conclusions
By careful choice of soldering tools, and by following the simple principles outlined in this article, your chances of finishing with a working PCB are greatly improved.
If you have any further soldering queries, try the FAQ section of this website.