Practical methods to fix a bad solder joint

23rd February 2021
Alex Lynn

In this month’s issue of Hakko Tech Tips, Hakko is going to explore a number of aspects of soldering, largely covering the pain point of fixing a bad solder joint, we have all been there and hopefully our tips can help. To mix it up a little, we will also look at metals that can’t be soldered and examine the differences between soldering and sweating, as well as welding and soldering.

To put it simply, PCBs are unforgiving and will fail or short circuit if there are any soldering errors. No pressure! In electronics manufacturing we all strive to achieve perfectly soldered joints but every industry is susceptible to making the occasional error. It’s not the defect produced but how you deal with it that really counts.

Being resourceful and taking quick remedial action to rectify mistakes as quickly and efficiently as possible is key to maintaining production schedules and avoiding errors in future. Let’s begin by kickstarting proceedings in our five-point format:

1) Are there any practical methods to fix a poor solder joint?

Additionally, there should be wetting visible for at least 180o, up to a maximum of 270o within the circumference of the hole. Usually, the rework of an unacceptable solder joint would involve the application of flux and re-application of the heat from a soldering iron.

This method can also be used to remove excess amounts of solder as the soldering iron will attract excess material onto its tip. If a joint is proving particularly stubborn and reaching the required standard is proving difficult, it is sometimes advisable to start again, i.e. remove all, or as much solder as possible from the joint, then re-flux and re-solder using a flux-cored solder wire.

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, the majority of poor solder joints can be reworked to attain a satisfactory standard.

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 to 330 degrees around the circumference of the lead. The outline of the pad and lead should still be clearly visible. On the component side, 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 to 270 degrees within the hole circumference.

It is worth noting that when using a soldering iron, heavy oxides can build up on your solder tip. One of the most important things to do for effective soldering, is to maintain a clean tip to ensure the correct 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 extend tip life. 

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 wire in a convenient holder so you can easily work the tip to remove the vast majority of the oxides. The brass wire can be replaced after extensive use.

2) What is the difference between soldering and sweating?

Essentially soldering and sweating are the same thing, but sweating is a term more widely used in the plumbing/construction industry, for example, production of a soldered joint on a copper pipe. Sweating metals together refers to soldering with a torch or in an oven.

Two metals, which can be dissimilar, are joined by a solder made of an alloy that has a lower melting temperature than the joined metals. Capillary action, or wicking, draws the solder alloy into spaces between the two joined metals, which makes a strong joint when cooled.

3) Are there metals that cannot be soldered?

Yes, but the solderability depends upon the type of solder alloy being considered. Noble metals, e.g. Gold and Silver have high solderability but they may produce brittle solder joints. Copper, bronze, brass and lead also solder well but, due to their high thermal conductivity, require flux application as they oxidise so quickly during the application of the high heat required to facilitate soldering.

Examples of metals that are very difficult to solder to would be titanium, chromium, tantalum and cast iron.

4) What are some of the more unusual uses of soldering irons that do not necessarily spring to mind – i.e. away from the classic industrial settings?

Obviously, the primary use for soldering irons is in traditional electronic and electrical applications. However, their use is not confined to just this. By selecting the correct model and style of tip they may be used in many other applications, i.e. Wiring, Stained Glass and Mosaics, Jewellery, Pyrography/Woodburning and Plastic Welding.

5) What is the difference between welding and soldering?

The main difference between welding and soldering is the melting process. In soldering, the metals to be joined are heated to a temperature whereby the solder alloy can be melted to produce a bond between the two substrates. In welding the two base metals would be heated until molten allowing them to fuse together to form the joint required.

We welcome feedback from our customers and potential new customers, so please do talk to one of our team if you have any questions. Look out for next month’s Hakko Tech Tips where we will be providing more advice on how to get the best performance from your Hakko equipment.

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