Let’s talk soldering

5th August 2021
Joe Bush

There is something truly satisfying about achieving the perfect solder joint. However, what exactly does the perfect solder joint even look like? In this month’s edition of Hakko Tech Tips, we will touch on how to create the perfect solder joint as well as exploring various aspects of soldering, it’s different uses, how best to fix a bad solder joint, metals that can and can’t be soldered as well as differences between soldering, sweating and welding.  

Soldering is a big responsibility. Get it right and you can rest assured that your work has near eliminated any assembly imperfections during manufacturing and will ensure many years of service ahead, however, if you make an error, then the chances of you feeling the pinch are more than likely.  Let’s kickstart proceedings in our five-point format:

1. So what does the perfect solder joint look like?

The perfect solder joint will be clean, shiny and uses just the right amount of solder. It exhibits excellent wetting of the component lead and extends 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).

2. Are there any practical methods to fix a bad solder joint?

Good solder joint formation is a pre-requisite in electronics manufacturing. Poorly executed solder joints can significantly reduce the reliability of a device and may cause catastrophic and/or intermittent faults both of which can lead to embarrassing and expensive field failures. Whilst an immediate failure of a joint is hard enough to contend with, it can at least be quickly addressed and remedied

If there is 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 repaired.

Reworking a defective solder joint usually involves 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 any 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, for instance, removing all of the solder, or much as is possible, from the joint, re-flux and re-solder using a flux-cored solder wire.

3. 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?

Understandably, the primary use for soldering irons is within the realms of traditional electronic and electrical applications. However, their use is not confined to just this. By selecting the correct model and style of tip, soldering irons may be used in many other applications, such as wiring, stained glass and mosaics, jewellery, pyrography/woodburning and plastic welding. In addition to being a prerequisite for professionals in the electrical and electronics industries, the soldering iron has long been an essential tool for many DIYers and hobbyists. Just recently, Hakko has developed a new soldering iron, suitable for all abilities from beginner to consummate professional, which delivers on a vast range of soldering applications. The FX-601 is a lightweight, well-balanced and versatile soldering iron for most applications, i.e. Stained glass windows, electrical and electronic. Available options for this unit include an iron holder and fume extractor.

The FX-601 eliminates the need for a controller and instead uses an LED to notify the user when the temperature setting has been reached. It has a wide temperature range of 240-540°C and can be used with a variety of different tips. The FX-601 is supplied as standard with a 6.5mm wide chiseltip, which is well suited for stained glass and heavier applications. However, for lighter work, the T19 series of soldering tips have been configured to achieve high heat recovery without increasing power consumption.

4. Are there metals that cannot be soldered?

There are certain metals that cannot be soldered but the solderability depends upon the type of solder alloy being considered. Noble metals, e.g. Tin, 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.

5. What is the difference between soldering, welding and sweating?

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

In soldering, 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 for a strong joint when cooled.

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