How to spot soldering defects and desolder

Soldering defects can be fatal, which makes it vital to work with upmost care and be able to desolder the faulty joints when necessary. Here is our advice when working with soldering devices.

Detecting soldering defects

Improperly soldered compounds can cause electrical failures and circuit interruptions. When higher current flows through a broken joint, flying sparks can occur. Thus, most electronics manufacturers undertake great effort to identify faulty joints. During the assembly process, such defects are usually detected by inspection systems, which often use X-ray technology.

After the circuit boards have been installed in the devices, detecting soldering defects and cold-soldered joints becomes difficult, and often requires whole circuit boards to be dismantled. In order to save time and effort, a special endoscope, such as Mobile Scope by Ersa, can be used to inspect hidden joints.

Preventing soldering defects

Most defects can be avoided by observing the following basics and principles for electronic soldering:

• The temperature of the soldering iron tip shouldn´t be too low, otherwise the solder will not melt fully.
• The soldering temperature shouldn´t be too high either, so as not to damage the components.
• Avoid putting too much mechanical pressure onto the iron tip; this can cause mechanical tensions and the fine iron tip to deform.
• The soldering iron tip must be carefully pre-tinned for the solder to spread evenly along the PCB track.
• Use only pure solder and never re-use solder splash.
• Alloys containing low amounts of tin can also result in defective joints.
• Keep the iron tip as clean as possible and avoid contact with plastic, resin, silicone and fat.
• Brush off any solder remnants from the iron tip – not with a dry sponge but with a damp one.
• Keep the direct working area dust-free to prevent dust particles from being trapped in the soldering tin.

Desoldering and re-working

If you accidentally applied too much solder on the joint, thus hot wiring a nearby joint, excess filler has to be removed. This is when you can make use of desoldering pumps and rework stations. These desoldering tools can also be used when replacing defective circuit board components.

Handheld desoldering devices

For occasional desoldering operations, pen-shaped desoldering pumps are the tool of choice. Known as solder suckers, these pumps can be operated singlehandedly and are equipped with a spring-loaded piston and a button trigger to create suction. The solder sucked into the chamber has to be discharged manually, mostly with the help of a removable tip.

Depending on the tip size, desoldering pumps can be applied to SMD pads as well. However, if the area to be desoldered is larger, additional use of a desoldering braid often proves helpful. This finely braided wick is usually made of copper and is between 1-2.5 mm wide, depending on the kind. Some desoldering pumps are equipped with metal casings, while others have a plastic shaft. The design and the length of the tip are also important.  The conductive carbon fibre tips, for example, are heat-resistant only up to 200⁰C, whereas ceramic tips can endure much higher temperatures. Desoldering pumps are typically 20mm wide in diameter, with chamber volume ranging from 8-11cm³.      

Rework stations

For professional use, desoldering stations or rework stations combining soldering and desoldering devices are recommended. These stations are equipped with a soldering head with an integrated sucker vent and with a special vacuum pump. The desoldering irons have an additional electric cable and an air hose connected to the station. Advanced soldering/desoldering stations have two or more separate adjustment controls and connectors, so that both the soldering head and a desoldering device can be employed simultaneously, and both are on stand-by for immediate use.

Solder variants from reichelt