Solder and Soldering Guide

Solder and soldering - the factors for success

Soldering success in all these environments depends on three factors - the solder and flux used, the soldering techniques, and the soldering tools or equipment utilized for the operation.

In this article we look at how these factors can contribute to successful soldering results in manual production and independent hobby work, with a focus on soft soldering, which is the best-known method of joining metallic materials to make reliable electrical and mechanical connections for electronics and other light applications.

Even if we focus solely on the role of soldering in mounting electronic and electrical components onto printed circuit boards (PCBs), the activity takes on many diverse forms in different applications and environments. Hobbyists and field service technicians can use soldering workstations or portable, possibly battery-operated soldering irons. Alternatively, they may use a soldering gun that heats very quickly.

Soldering in electronics production facilities is usually handled by automated ovens that process complete, populated PCBs as they pass through on a conveyor belt. Many PCBs today are populated mostly with surface-mount (SMT) devices and require ovens using reflow soldering. However there are usually a number of through-hole components also mounted on the PCB; these are typically installed using hand soldering stations in the production area. Conversely, PCBs containing only through-hole devices can be processed using wave-soldering ovens.

Inevitably some boards going through a production process will need to be reworked, due to faulty components, wrong components used, or modifications for other reasons. This task will be handled by a technician at a rework station, which will include desoldering as well as soldering capability, and possibly also fume extraction. Rework stations are also used by technicians in service centers to handle equipment returned for repair or upgrade. Using rework stations has become more challenging with the advent of complex SMT devices and lead-free solder.

Below, we describe the soldering issues you should consider, whether you are a hobbyist or a technician working in a service or manufacturing environment. We discuss the constituents of solder and their impact on soldering; this includes lead-free solder, which must now be used on most applications. We then look at the techniques essential for successful soldering, and finish by discussing the soldering tools available – from simple soldering irons to small, manually-operated soldering ovens.

Solders and fluxes

Irrespective of its alloy composition, a good solder has a number of desirable properties. It should liquefy at relatively low temperatures and moisten the metal to be joined. It should flow easily on the metal, attach to the metal surface or form an alloy, and on hardening become as strong as possible without being brittle. Solder is available in various package sizes and diameters. The smaller diameters enable precision work to be carried out as required on printed circuits.

For many years the most commonly available solder comprised a tin/lead alloy, with tin concentrations ranging from 5% to 70% by weight. These alloys are still in use today, although as we will see, their use is restricted because of health and safety risks and legislation. The greater the tin concentration, the greater the solder’s tensile and shear strengths. All alloys are solid at 183°C, but their melting points vary with their tin/lead ratio. 60/40 tin/lead alloys, for example, become liquid at 188°C, and are accordingly said to have a plastic range of 5°C. Their consistency is pasty within this plastic range.

By contrast, 63/37 alloys have no plastic range; they melt at 183°C. Alloys that have a single melting point like this, rather than a melting range, are known as eutectic. They are ideal for applications such as wave soldering that require a low melting point, while avoiding a plastic range that would give components an opportunity to become misaligned before the solder freezes. For other applications such as hand soldering, these considerations are not critical – so 60/40 alloys are favored, as they are slightly lower cost. Under conditions of slow cooling, 60/40 may give duller joints than 63/37 but this is a purely cosmetic effect. Fig.1 shows how melting point temperatures vary with tin/lead ratios.

Fig.1

A disadvantage of tin/lead alloys used on electronic assemblies is that they dissolve copper. This means that they can erode thin copper wires as used for electronic component leads, and thin copper films, as used on printed circuit boards. One solution is to use an Ersin Multicore Savbit alloy, which contains a precise amount of copper, sufficient to prevent copper absorption taking place during soldering. Erosion of copper becomes 50 to 100 times slower when Savbit alloys are used; the life of the soldering iron bit is prolonged as well.

A far more serious problem with tin/lead soldering alloys is their use of lead; this has high toxicity, coupled with a tendency to leach into the environment from PCB assemblies. As a result, lead-based solders are among the hazardous materials identified for restriction or banning by the European Union’s Restriction of Hazardous Substances Directive (RoHS) and Waste Electrical and Electronic Equipment Directive (WEEE), which took effect in July 2006. In North America, California passed the Electronics Waste Recycling Act of 2003 (EWRA) which covers four heavy metals restricted by RoHS. The California Lighting Efficiency and Toxics Reduction Act applies RoHS to general purpose lights or other electronic devices that provide functional illumination for indoor residential, indoor commercial, and outdoor use.

The electronics industry has responded by producing a range of new 'lead-free' alloys, consisting primarily of tin/copper (Sn/Cu), tin/silver (Sn/Ag) or tin/copper/silver (Sn/Cu/Ag). Compared with solder containing lead, lead-free solder compounds are duller in appearance and more temperature-sensitive during the hardening process. For hand soldering, a limiting factor with lead-free solders can be its availability in wire form, as some alloys such as tin/bismuth are not easily drawn into wire.

Currently, the most popular alloys used to make wire solders are Sn/Cu/Ag and Sn/Cu. Wire solders for hand assembly, as shown in Fig.2, are readily available in these two alloys, as they are also popular as lead-free bars for production users. 68% of SMT assemblers and 50% of wave assemblers have chosen Sn/Cu/Ag, while 20% of wave assemblers have chosen Sn/Cu.

Solder wire used for hand soldering typically contains a central rosin flux core which is released on heating. Flux content is a critical factor in determining wetting behavior for lead-free solder, as these wet a little more slowly than 63/37 solders tested under similar conditions. Lead-free solder wires should contain at least 2% flux by weight; the 1% levels typical of lead solders will not work well with lead-free. If 2% flux results in sluggish wetting, 3% levels may be used; however this will create higher residues, not always cosmetically appealing in no-clean applications. Addition of flux with a squeeze bottle is normally unacceptable due to over-application issues.

The flux should be specifically designed for lead-free applications and therefore able to withstand higher soldering tip temperatures without charring, spattering and decomposition. Some fluxes may smoke more when using hotter tip temperatures.

Acceptability requirements for soldering are covered in the IPC Association's IPC-A-610D Section 5. This covers connections of all types, including SMT, through-hole and terminals. It also refers to three classes of equipment and environment, and their demands for soldering quality; Class 1 refers to general electronic products, Class 2 to dedicated service electronic products and Class 3 to high performance electronic products, such as life-support systems or other equipment where performance on demand is critical. Overall, fluxes are subject to the IPC Joint Industry Standard J-STD-004 or equivalent.

Soldering techniques

A key characteristic of soft soldering for electronics assemblies is that solder’s melting point is lower than that of the metal parts being joined; the metal parts therefore remain solid while the molten solder flows between them. The majority of electronic soldering joints are situated between a wire and a soldering lug or between a wire and a printed circuit board. The solder cools to leave a strong, tight, electrically-conductive and heat conducting joint.

To ensure a perfect soldered connection, all grease, corrosion, oxidation or other contaminants must be removed from the joints with isopropyl alcohol before soldering as shown in Fig.3. A sound mechanical connection is also required. Accordingly, twist the stranded wire and wrap it around a terminal. Otherwise, thread the wire first through a hole in the soldering lug or board. Bend wire in soldering lugs by approximately 90° and in boards by approximately 45°. The soldering joint should be a minimum of 1.6 mm from the components so that the latter are not damaged by the effects of the heat. The weak point of a copper wire is the point where the wire emerges from the soldered joint. Therefore, after soldering is completed, the wire must not be bent upwards.

To make the soldering process easier and achieve a smooth, electrically perfect soldering joint, the wires should be tinned before soldering. The soldering tip should be cleaned immediately before use on a moistened sponge. Use solder sparingly, as excessive amounts can flow into sockets, jam switches or cause short circuits between wires or pads.

Never heat the solder wire directly. Instead, heat the wire or terminal to be soldered to the correct working temperature, then introduce the solder. This melts and flows into the joint. After finishing soldering, carefully remove the soldering iron to leave a clean, smooth soldered joint. This is particularly important when using soldering guns – keep the power triggered on until the soldering tip is well away from the joint, to avoid leaving solder residues on the tip. To extend a solder tip’s life for as long as possible, tin it before returning to storage.

Fig.3

Electronic components must not be heated excessively during soldering, as this can damage or destroy them. Use a pair of long-nose pliers as a heat sink, or for very sensitive components use equipment such as a Weller soldering station with temperature control. Also, complete the soldering as quickly as possible, as excessive heating can damage copper wire, particularly stranded wire. There may also be remnants of solder, which can cause short-circuiting or result in a defective soldered joint.

Soldering using lead-free soldering wire presents some special challenges, as previously mentioned, because of the higher melting temperature of lead-free soldering alloys such as Sn/Cu/Ag and Sn/Cu, which are 30 to 40°C higher than leaded solder. As a result, lead free hand soldering requires stable dynamic temperatures. To achieve this, soldering irons must have more power and an efficient method of transferring thermal energy to the soldering iron tip. Attempting to overcome the heating problems by running lower-power soldering irons at higher temperatures to increase the heat energy in the tip are unsatisfactory.

Firstly, this thermal energy is quickly dissipated as soon as the tip is applied to the joint. The soldering iron's characteristics make replacement of this energy a slow process, so the tip cools down. Secondly, an excessively high starting temperature can cause serious problems. Possibilities include burning of the flux, increased oxidation of the solder joint and damage to the components. The service life of the soldering tip is also reduced.

The best solution is to use equipment that has been designed to cope with lead-free soldering. Weller soldering equipment, for example, generates heat particularly quickly and conducts it to the soldering tip.

Soldering equipment

We have seen how the right solder alloys and fluxes, and the right soldering techniques contribute to the quality of soldered joints. However solder joint quality depends also on using the most suitable and best-available soldering equipment; accordingly, we finish our look at soldering with a review of the soldering equipment currently on offer.

We can start by categorizing the available products into soldering irons, soldering guns, desoldering tools and rework stations, and soldering ovens. Although large-scale automated soldering ovens are beyond the scope of this article, we mention the availability of smaller benchtop reflow ovens.

Soldering irons

Soldering irons are available in power ratings from a few watts up to hundreds of watts. All irons should have interchangeable tips, as there is a wide range available. The soldering iron can be matched to the soldering task’s requirements in terms of both power and tip type. Before use, check that the tip is not loose, corroded or oxidized as this will impair heat transfer. These problems can be mitigated with Weller’s plated and oxidation-prevented ‘longlife’ tips. Only a well tinned soldering tip guarantees a perfect soldered joint and optimal heat transfer.

For small-scale soldering jobs, an iron of 40 W or less, with a tip of 6.3 mm or 5.0 mm is recommended. For larger-scale electrical soldering tasks and sheet metal working, 120 to 200 W irons with tips up to 20 mm wide are recommended. For service applications, cordless irons that are either battery- or gas-operated have proven to be very effective.

Soldering irons are available as standalone tools like the example in Fig. 4 that can be plugged into a mains supply, or they may be part of a soldering workstation that feeds the iron at a low voltage and provides a means of setting and displaying the operating temperature. If the station is described as not requiring calibration, this means that the actual tip temperature can always be expected to correspond closely with the displayed temperature to within a tight tolerance. This is important if the soldering station is to be used within an organization contracted to work to IPC standards.

This is because IPC specifies soldering temperature tolerances. Within J-Std-001, Appendix B specifically addresses soldering irons, voltage spikes and temperature considerations. Section B-2 (Benchtop and Hand Soldering Systems) states: Temperature controlled soldering equipment (at rest) should be controlled within (+/- 5 degrees C [+/- 9 degrees F]) of the idle tip temperature. Constant output (steady output) tools in compliance with A-2a, d, e, and f (Other parts of the Standard) may also be used.

Fig.4

Soldering guns

The advantage of a soldering gun is that the soldering tip heats up extremely quickly and reaches soldering temperature in less than 10 seconds. It can easily be operated with one hand. Another advantage is that it cools rapidly when not in use, making it safe to leave on a bench. Soldering guns tend to be used more for heavy electrical connections than for PCBs.

Rework stations

Removal and replacement of Ball Grid Array (BGA) and SMT components in a rework or service environment must be done manually; this is a demanding operation with scope for many types of error if not performed correctly. For example if an IC is heated excessively, the IC itself, the PCB or neighbouring components can be damaged. Alternatively, if an IC is not sufficiently heated, and the solder fully melted before removal, pulling the IC up may tear some of the pads from the PCB, causing possibly irreparable damage. For this reason, rework stations need accurate heating profiles to allow successful BGA/SMT removal and replacement. These profiles approximate to those used in reflow ovens.

The rework stations usually have one or more external thermocouples to allow temperature profiling to be based on actual board temperatures. Heat is applied by hot air convection, and fed through a nozzle onto the component. Nozzles can be interchanged for different component sizes. The PCBs to be reworked can be gripped on a board holder, and the device to be removed can be lifted by vacuum. Fig. 5 shows an example of a modular rework station with many facilities for operator comfort and improved productivity.

Reflow ovens

Fig.6

Conclusions

In this article we have seen how significantly the quality of soldering joints is affected by the soldering materials used, together with the techniques of the operation and the choice of appropriate equipment. We have also seen how in many industrial applications soldering quality is mandatory and must be calibrated – especially if the electronics assemblies being produced are for life-critical applications.

Solder and Soldering. Date published: 15th September 2015 by Newark