Avoid bad solder joints and prepare for soldering

In many cases, however, boards can fail or short circuit due to soldering mistakes. We all aim to achieve the ultimate soldering process to reduce assembly imperfections in electronics manufacturing, however, all industries are susceptible to errors from time to time. Whilst mistakes unfortunately do happen, being resourceful and resolving mistakes as quickly and efficiently as possible is key to maintaining production schedules and avoiding errors in future.

In this edition of Hakko Tech Tips, Hakko has examined some examples of bad solder joints and explore how to avoid certain pitfalls. It will also look more closely at how to adequately prepare for your soldering process to ensure it runs as seamlessly as possible and help you achieve maximum output from your Hakko tips and equipment.

What is a solder starved joint?

As suggested by the name, a solder starved joint is one which simply doesn’t contain enough solder. It may be adequate to provide a good electrical connection but will have very little mechanical strength. None the less, insufficient solder weakens the joint and later on, may develop stress cracks and ultimately cause failure.

This may manifest itself as insufficient hole fill on Plated Through PCB’s, poor solder fillet or unsatisfactory wetting of the pad/component lead. To rectify a starving joint and improve long-term reliability, simply re-heat and add more solder to strengthen the joint.

What are the potential consequences of untrimmed leads?

If leads are not cut to length during assembly they can easily promote bridging between adjacent pads or component leads but could also cause a short circuit when the PCB/unit is in operation, leading to poor reliability and expensive field failures.

Untrimmed leads are easy to spot as they stick out more obviously and are easy to rectify by trimming them to an appropriate size for a standard joint. Simply use the same solder trimming tool to reduce the height of the leads until the solder joint reaches the same height as other joints on the board.

I understand that most bad solder joints can be repaired with patience, are there any that simply cannot be repaired?

Usually, suspect solder joints can be reworked by re-heating and applying more solder, effectively re-touched. In some cases, it may be necessary to start again by removing the ‘old’ solder utilising any approved method and re-applying new.

If there is any existing damage to the PCB, such as raised tracks and/or pads, serious consideration should be given to establish if a repair is economically and technically viable. However, most bad solder joints can be repaired, even overheated joints, which can be challenging to repair, are capable of being remedied.

What are the key steps to preparation before commencing work on a soldering task?

Successful soldering is heavily dependent on the quality of components being soldered, so it is important to always use components with little or no oxidation and zero contamination, e.g. Greases, Oils, etc. Also, ensure that the correct size and shape of solder tip is selected for the application.

You should also select a tip that has the highest thermal mass possible as this will help maintain the tip temperature and allow lower soldering temperatures to be used. Regardless of solder alloy the lowest tip temperature possible should be used although this is going to be determined by the application and heatsink effect of components and PCB.

Solder Joints need to be created quickly and neatly otherwise damage to components and/or faulty joints can occur. Firstly, you need to strip the wire to prepare it for soldering. Removing insulation from fine wires is a difficult task. The traditional way to remove the insulation material surrounding the wire would be to use mechanical strippers or even wire cutters, however, the huge downside of this is the potential damage that can affect the wiring.

If executed incorrectly, damaged wire cores and/or cut strands present serious reliability issues that can result in broken connections caused by vibration, expansion, contraction, and other external factors. To overcome all of the common problems associated with using hand tools, HAKKO developed a world-class range of thermal wire strippers, including the brand new FT-802, which removes insulation cleanly without damaging core wires and competently meet the demands for precise quality from the military, medical and aerospace industries.

When using a soldering iron, heavy oxides can build up on your soldering tip. One of the most important things to do for effective soldering, is to always maintain a clean tip to ensure the proper amount of heat is transferred to the area that you are working on and also to ensure effective wetting of solder to the tip.

Regularly maintaining your soldering tip will also save you replacing them so frequently.  If the oxidisation on the tip is not too extreme, maintenance can be simply carried out by using HAKKO’s 599B. The 599B is made up of brass wires in a convenient holder so you can easily work the tip to remove most of the oxides. The brass can also be replaced after substantial use.

For re-tinning soldering tips and removing oxidisation, the HAKKO FS-100 chemical paste is an essential workstation companion. Heat the oxidised tip to 350°C, then tin the soldering tip, clean off the FS-100 paste with any HAKKO tip cleaning sponge or the HAKKO 599B cleaning wire and check if the tip surface wets or not. This process will need to be repeated twice or more until the tip surface wets. Re-tinning is now completed but to prevent oxidisation, it is advisable to apply sufficient solder to the tip when soldering work is completed.

Can you describe the perfect solder joint or name some key characteristics?

The perfect solder joint should exhibit excellent wetting of the component lead and extend to the extremities of the PCB pad. On the solder side of the PCB, the solder fillet should be concave in shape and extend for at least 270 degrees around circumference.

The outline of the pad and lead should still be clearly visible. There should be no holes in the solder which should fill at least 75% of a plated through hole with wetting visible for at least 180 degrees within the hole circumference (Component Side).