Manual Soldering - Beyond the Basics

Jonathan Atkins, IEng MIIE - Technical Manager

Introduction Every engineer and technician knows how to solder. The process has found almost universal application as an electrical connection, largely due to its low process temperature (250 to 450ºC) and very low joint resistance. The iron and flux cored solder wire are familiar sights in any workshop, where the rules for their use are long established. However, with changing environmental legislation and the introduction of new materials, we cannot assume that the rules remain the same. Manufacturing Trends The continual technical developments found in electronics manufacturing mean that more and more PCBs are assembled and soldered using automated plant. Materials have changed too, with a variety of solders and fluxes available to suit every technical and environmental application. All have unique characteristics that need to be understood in order to gain the fullest benefit from their use. Despite this trend towards automation manual soldering and the humble iron still have a significant role to play in batch assembly and rework. This article is intended to address the issues involved in hand soldering using these new materials, whilst maintaining joint quality. It is as well to remember that the failure of a single soldered connection has the potential to disrupt a complete electronic system. Developments in Solders One of the most important characteristics of a solder alloy is its melting point. The actual value is relevant, but more important is the range of transition from liquid to solid. Where this is narrow (less than 1ºC) the chances of producing a "dry" joint are minimised. Solders are predominantly alloys of tin, which has a melting point of 232 ºC. Other metals are added to produce the desired physical characteristics at an economic cost, and to achieve a specific melting point. Every alloy has a proportional mix where the rapid transition described above exists. This is known as the eutectic point (from the Greek "well meltable") and an alloy of 62% tin and 38% lead exhibits this property at 183ºC. Figure 1 shows a phase diagram for this alloy.

This ability to produce different melting points is a real advantage to the engineer attempting to rework very dense PCBs. For example an alloy with an increased lead component will melt at around 300ºC. Thus it can be used to make a joint that would not be effected by the flowing and reflowing of adjacent joints made with 62Sn / 38Pb. A thorough treatment of solder alloy composition and other related topics can be found in reference 2. Lead is widely used in solder alloys, but environmental concerns mean this is unlikely to continue. The toxic nature of lead on all forms of life is well understood, and with landfill sites containing more old electronics, the polluting potential is clear. Both the automotive and telecomm industries have medium term plans to remove lead from their products. The suggested alternatives are based on 99% Tin and have a melting point of between 200 and 240ºC, depending upon the alloying metal used. To replace the excellent soldering performance provided by Tin Lead alloys is proving difficult and a large body of research is underway involving the use of silver, copper and other more exotic metals. Developments in Fluxes Fluxes are used in soldering to corrosively clean the joint area and to aid the transfer of heat from tip to joint. They are rated in terms of their activity, with an aggressive acidic flux said to be highly active. Some combinations of materials are more difficult to solder than others and the activity must be chosen accordingly. Any solder or flux supplier can be consulted on this subject. The most commonly used flux compositions are based on the naturally occurring substance, Rosin. RMA (Rosin Mildly Activated) flux will the familiar to most readers. Unfortunately rosin contains colophony, which has significant health and safety implications. When heated to soldering temperatures colophony will liberate fumes that are classed as irritants. A fume extraction system, of high specification, must therefore be provided for users. It is inevitable that a residue will be left on the joint after soldering. If the flux is very active, i.e. the organic water soluble or inorganic acid varieties, then residue the must be removed to prevent corrosion from compromising the joint over time. Most industrial users also choose to clean RMA residues for process reasons, the most efficient medium being CFC solvents. The phasing out of such solvents because of the environmental damage that results from their use, was a difficult challenge to the electronics industry. The response was the development of no-clean flux, the advantage of which are clear from the name ! They differ considerably from RMA fluxes in their composition. The chemical activity of the solids used are not less than those found in an RMA, they just contain less by volume (typically 1% as opposed to 10%). This situation is a good example of compromise between features, activity is lowered to remove the need for cleaning, but this reduces the flux efficiency. The RMA and No-clean also have very different responses to rising tip temperature. Lab tests 3 have shown that the wetting performance of a no-clean deteriorates with a temperature rising from 260 to 370ºC. The RMA actually increases its performance in this range, and will tolerate a far higher maximum temperature. One particular variety of no-clean is classed as rosin-free, and seeks to avoid the health and safety problems previously mentioned. However, it does not completely remove the need for fume extraction and thus has been confined to niche applications such as education. The Soldering Station A typical station (the Antex 660TC) is shown in figure 2. It consists of an iron connected to a unit containing a step down transformer and control circuit.

As has been previously illustrated, controlling the actual temperature of the soldering bit is of great importance when using new materials. The bit can be subjected to high thermal loads, such as soldering to a PCB pad connecting to an earth plane. In this case heat would be sinked into the joint causing a drop in temperature. The most common method of compensating for this is to include a sensor in the heating element. Usually this is a thermocouple but increasingly positive temperature coefficient (PCT) resistors are being fitted. The control circuit in the station uses the classical feedback loop to compare the actual temperature with the desired temperature. From this error signal the required power is developed and delivered to the iron. The US military standard MIL-STN-2000A is generally regarded as a benchmark for professional users. It requires that the control system maintain the temperature to within ± 5.5ºC of the set value. Not every soldering station meets this specification but those employing closed loop control can be expected to give at least ± 10%, which is suitable for most applications. Open loop control can also be used. The user sets the desired temperature on a dial calibrated in ºC, this causes a proportional variation in the power delivered to the iron and thus tip temperature. Large and unpredictable variations in temperature must be expected with this method, particularly with the large loads previously described. Choice and Care of Soldering Bit The primary job of the soldering bit is to transfer the power developed by the heating element into the joint area. It is therefore important to consider the shape of the bit: maximum heat transfer will occur when there is the largest possible surface contact between bit and the joint. When a conical, fine pointed, bit is used the contact area can be reduced to a fraction of a mm2, thus increasing the process time and increasing the risk of dry joints. A good general principle is that conical bits should only be used where necessary for access to fine pitches, and the cross section of the tip flat should be as large as possible. The condition of the actual soldering bit can have a significant effect upon the quality of the completed joint. Bits are machined from copper alloy to ensure good thermal conductivity, unfortunately the copper is rapidly dissolved by the corrosive flux within the solder. Electroplating with iron enables the bit to withstand this attack for a longer period. Ultimately the plate will be breached and a replacement bit will be required, this is known as corrosion failure. The effective life of the bit is therefore very much dependant upon the activity of the flux with which it is used. The standard RMA flux will produce the longest life, whilst the highly active water-soluble flux the shortest. It is always good practice to clean the bit by frequently wiping it on a clean, damp sponge. This will prevent contamination of the bit's surface, but only if a specialist sulphur-free sponge is used. The other major mode of bit failure is dewetting, caused by a lack of fluxing activity. Whenever the hot tip is free of molten solder a layer of hard oxide tends to form. This will prevent subsequently applied solder from flowing across the tip and wetting to it. Where this occurs the natural reaction is to turn the temperature up, which of course only makes the problem worse. No-clean and rosin free fluxes have experienced particular problems in this area. The key to reducing dewetting failures is in good maintenance actions: * Keep the tip temperature as low as possible. It has been shown experimentally 4 that tip life will decrease by 30% when the temperature is increased from 345 to 400 ºC, and by 65% when raised to 455ºC. It is also worth noting that the production of flux fume is also significantly suppressed at lower temperatures. * Turn the iron off, or at least down, when not in use * Keep the bit tinned and in the stand when not in use, wipe on sponge and re-tin, before use. Should a bit become heavily oxidised it is possible to restore it by the use of a commercial cleaning compound. These products should be used sparingly; they consist of a concentrated flux that will increase the risk of corrosion failure. The use of any kind of abrasive on the tip is to be avoided. Summary * When soldering to PCBs, load sensitive control of tip temperature is vital. * Regular maintenance will significantly improve effective life of tips. * The process parameters for different fluxes are not necessarily interchangeable. References 1. EPE Magazine's Basic Soldering Guide www.epemag.wimborne.co.uk/solderfaq.htm 2. R. J. Klein-Wassink, Soldering in Electronics (Electrochemical Publications) 3. E. M. Oh, Soldering & Surface Mount Technology No. 21 4. L. Abbagnaro, Electronics Manufacturing Products, January 1997