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It’s one of the fundamental skills in electronics, required at every level from beginner to rocket scientist. Everyone knows how to solder . . . or at least think they do! Yet kit suppliers will tell you that 99.9% of failures in home-built kits are due to poor soldering. Let’s try to help lower that statistic! Soldering by MAURIE FINDLAY 72  Silicon Chip www.siliconchip.com.au A LMOST ALL OF the constructional articles apppearing inSILICON CHIP involve soldering to make electrical connections. With modern tools and solder, most readers are able to do a good job. However, an understanding of the soldering process, plus some practical experience, can make for reliability and professional appearance. Typically, a project will involve a PC (printed circuit) board plus some ICs, discrete transistors, diodes, resistors, capacitors and so on: all new and shiny with leads finished with materials specially designed for easy soldering. You have a length of solder wire and a small soldering “iron”. The solder is placed to touch, say, a resistor lead and PC board track to be joined. The soldering iron tip is applied to the solder (more often than not, in the form of a thin wire). The solder wire melts and molten solder flows over the two component leads. The soldering iron tip is then moved away and the solder solidifies in a few seconds and leaves a reliable connection. In most cases, it is that easy. But it’s not always so. For example, the solder may not flow over the component leads – it may look like it has but the lead is not actually coated. Or it may have moved before the joint solidifies. Or the heat of the solder might have damaged the component . . . Soldering is not quite as simple as it sounds. For this reason, it’s well worth knowing a bit more about the fundamentals of soldering so that you can handle situations which are not so straightforward. In the electronics industry, solder is used to make the majority of electrical connections. Even an ordinary domestic television receiver may contain thousands of connections between components, PC boards and cables and in most cases the failure of one connection can make the receiver inoperative. Satellites, military equipment and so on make use of solder joints in much greater numbers – and require even greater reliability. It’s not easy whipping out the hot stick when the PC board is a few hundred kilometres out in space! But so much have soldering techFacing page: the budget-priced “Auto Temp” soldering station. About $185 from Dick Smith Electronics. www.siliconchip.com.au Fig.1: solder, a mixture of lead and tin, has a lower melting point than either lead or tin. Lower heat means less likelihood of damage to sensitive components. niques improved over the years that most electronic equipment is now extremely reliable. What is solder? First of all, a fundamental question: what is solder ? Solder is an alloy, or mixture of metals, used for joining other metals. While general-purpose solder is almost always a mixture of tin and lead, other metals can be included for special purposes. For example, a small amount of copper is sometimes added to help preserve soldering iron bit life. And there are specialised solders which don’t contain any lead at all. But they are not the types you will normally come across in electronic work. The tin and lead are closely mixed together but not chemically bonded. The tin-lead alloy has a very useful characteristic – it melts at a lower temperature than that of tin or lead alone. What’s more, the melting point can be controlled by altering the proportions of the two metals. For fine electronic work, an alloy of 62% tin and 38% lead (by mass) is a good choice. It melts at 183°C, much lower than for either metal alone (lead melts at 327°C and tin at 232°C). This lower melting point means that there is less chance of damage being done to components, the PC board and other parts. At the other end of the scale, a different tin/lead mix solder is used for joining sheet metal. Examples are galvanised iron roofing components and tin-plated food containers. Plumbers use solder in relatively large quantities and there usually isn’t A beginner’s generalpurpose soldering kit, with a 25W mainspowered iron, a soldering iron stand complete with tipcleaning sponge, a roll of de-soldering wick and some solder. All up? – about $35.00. (Courtesy Jaycar Electronics). April 2003  73 the solder wire, in tiny hollow tubes, there is flux. Flux helps the solder to flow and to “wet” the metal being soldered, especially the more difficult-to-solder metals. Because the flux melts along with the solder, exactly the right amount is applied to the joint as you solder it. Most general-purpose flux is made from rosin – often (but wrongly) called resin. This Micron 60W temperature-controlled The flux itself is a ressoldering station from Altronics has a LED in, made from rosin. readout to tell you the exact tip temperature. Thoroughly confused You can dial up the temperature you want and it will now? hold it there within 2°. It includes the tip cleaning sponge shown but the solder holder on top is an You need to rememoption. ber that as the flux melts, it releases fumes a problem about overheating the comand these fumes may adversely affect ponents being soldered. The solder some people. There is also some evisticks they use may be about 300mm dence that melting solder releases lead long and up to 80mm2 in cross section. fumes which could also be dangerous. Lead is cheaper than tin and an alloy The moral of the story – don’t breathe of 50% tin and 50% lead, melting at in fumes when soldering. 210°C, is used. Incidentally, you can buy nifty little fan units which suck fumes away from Flux your soldering area. If you’re worried Sheet-metal solder should not be about your health, they are worth used for electronic work – not so looking at. much because of the different alloy Before finishing with flux, there is mix but because of the type of flux another common type of solder (the used (if at all). type you find in hardware stores) By far the majority of solder you will which contains an acid flux and is use in day-to-day electronics work has intended for sheet metal work. more in it than tin, lead and perhaps Never use this for electronic work – some other metals. Down the centre of the acid will quite quickly eat away the copper on the PC board and probably the component leads as well. Tools OK, so what do you need besides the solder? For starters, you need a soldering iron) and perhaps something to remove solder if you inadvertently put a component in the wrong place. Some fine tweezers to hold components in place and a heatsink to clip onto the leads of heat-sensitive components would be worthwhile extras. A heatsink, by the way, is merely a device to draw away heat from a device’s lead(s) so that the heat from the soldering iron doesn’t reach the sensitive parts of the device. Heatsinks are often made like alligator clips (but with flat blades, rather than serrated teeth), which clip onto a device’s leads under spring pressure. Which iron? There are lots of soldering irons to choose from. At the bottom end of the scale, a simple tool will set you back about $20 or less and will do a good job if your only need is to assemble a few PC boards with small components. Typically, it will plug directly into the 240V mains, will be rated at 25-40W, and will be set up for a tip temperature of about 370°C. This temperature is about right for most situations, considering the losses in transferring heat to the components. The disadvantages of some “el cheapo” irons may not be obvious – after all, they solder, don’t they? A very basic selection of hand tools but probably all that the novice constructor needs. On the left is a pair of sidecutters (sometimes called nippers or nipping pliers); next is a pair of needle-nose pliers. The red gizmo is a heatsink (these come in various shapes and sizes) while rounding out that group is a pair of pointy-nose tweezers. At right is a set of flat-nose and Philips screwdrivers. They are bigger than “jewellers” screwdrivers but not much bigger – a hobbyist would normally have some larger flat and Philips (or Pozidriv) screwdrivers. All of these tools are from Jaycar Electronics. 74  Silicon Chip www.siliconchip.com.au Well, yes . . . but cheap irons sometimes are too hot for very small components, yet not able to maintain a high enough temperature to ensure good, sound joints with larger ones. of work where it is difficult to set up an electrical supply – eg, service work in the field. Butane gas, supplied from an internal reservoir, burns to heat the tip. They will usually operate for about an hour on a single refill and at a moderate temperature. However, they are a bit fiddly and you normally wouldn’t consider them against the electric version for general bench work. Electrostatic damage Some cheap mains-powered irons (and even some more expensive ones!) can cause damage to some sensitive components due to electrostatic discharge. This can occur if the iron tip is not properly earthed (and even irons which are properly earthed when new can develop this problem with age). What happens is that a relatively high electrostatic voltage builds up on the iron tip which can exceed the rating of the component being soldered. The result: one “cooked” component – and not by the heat of the iron! To avoid this problem you can run a strap (such as a length of wire fitted with two alligator clips) between the barrel of the iron and the earth plane of the job. Next up the list is a similar type of iron but with some sort of temperature control. A power rating of 60W would be typical and the maximum temperature set would be around 350°C. This will handle bigger jobs than the $20 iron while being kinder to very small components. You could expect to pay more than $100 for a simple temperature-controlled iron, powered directly from the mains. The comments above regarding an earth strap still apply. The next step up in temperature-controlled irons gets us into real money – but you get what you pay for. If are doing quality work over a period of time, you should consider a variable temperature-controlled iron (actually they’re normally called “soldering stations”) in the price range $200-$500. There are quite a few brands to choose from. Most operate through a mains transformer, with the heating element rated at 24V and about 50-60W. The tip temperature is controlled by a circuit which switches the power on and off. In some cases, the switching action happens as the AC voltage passes through zero, ensuring that no transients appear at the tip. You can set the actual temperature via a control knob – some have a scale behind the knob while others www.siliconchip.com.au Suckers! It might say “for soldering and tinning most metals” but this soldering fluid – and most fluxes – are a definite no-no when it comes to electronics. The roll of solder at right might look the same as you see at your electronics shop but this higher-melting-point type is meant for copper pipes, etc. It has a 50/50 tin/lead mix (instead of the normal 62/38 mix) and is also quite a lot thicker than most electronic solder. provide a digital display for the set temperature. The beauty of a temperature-controlled iron is that its tip temperature stays much closer to that set, whether the iron is at rest or supplying its maximum heat. Weller produces a relatively inexpensive but very reliable soldering station that uses a series of tips to select the temperature. The system makes use of the “Curie” effect, where a metal can be designed to lose its magnetic properties at a particular temperature. Unfortunately, this system does not allow for zero voltage switching. There are other versions of electric soldering irons which don’t run from the mains (well, not directly anyway). These use batteries (usually, but not always, rechargeable via a mains plugpack or adaptor) and are handy for use away from a power outlet. Other versions of low-voltage irons run from 12V and are designed to operate from a car battery – either connecting directly with large alligator clips or plugging in via the cigarette lighter. These are obviously intended for automotive uses. Gas irons Just to complete the story, we must consider gas-powered soldering irons. They are not too expensive and are very handy for doing a small amount Whether it’s to remove solder on joints soldered incorrectly (hey, we all make mistakes!) or to remove faulty parts, a “solder sucker” is all the go. It is a cylinder with a piston, the latter set in sharp motion by a spring. Air is sucked into the cylinder via a tip that concentrates a partial vacuum above the molten solder, drawing it into the cylinder. These usually sell for between $10 and $20. There are also professional solder suckers which have an electric vac­ uum pump but these generally cost the earth. While not out of place on a service bench, they’re overkill for most hobbyists. Smaller amounts of solder can efficiently be removed from PC boards, in particular, by means of de-soldering wick. This is a woven copper braid impregnated with flux, which solders very easily. As you start to build more projects, devices such as this mechanical “third hand” become almost essential. The PC board can be held at any angle and, importantly, easily flipped over for soldering. April 2003  75 GAS IRONS Jaycar TS1620 kit Typical of gas irons, this “Vulkan” from Jaycar is very handy if you’re away from a power source. They run from butane gas which can refill the iron in seconds. You place the braid over the soldered joint and then apply the iron – the braid sucks up the solder from the joint (just like a wick, hence its name). Other tools Pliers and tweezers for holding and bending components can be very handy aids to soldering – but don’t go overboard and purchase every one in the catalog until you are sure of what you actually need. In general, the hobbyist can get away with one pair of fine (needle-nose) pliers, one pair of heavier pliers, one small pair of sidecutters and (perhaps!) one larger pair of sidecutters. Usually (though not always), you get a better tool by spending a bit more money. We mentioned heatsinks before. They are essential if you are dealing with components which may be damaged by high temperatures. Heat that would otherwise flow the length of the lead is shunted to the heatsink and the component kept cooler. Most small parts, including semiconductors, can be soldered safely without a heatsink, provided the usual 60/40 solder is used and the joint made quickly. Lets start soldering! Now let’s look at the actual technique of making a good solder joint. First and foremost, the parts to be soldered must be clean – oxidation of component leads and PC board tracks is one of the main causes of poor solder connections. Sometimes we have to use compo76  Silicon Chip nents that have been stored for a long time or for some other reason do not “tin” easily. And believe it or not, many a solder joint has failed simply because the clear insulation on the wire (eg, on coil wire) was not scraped off. The idea is to get the surfaces mechanically and chemically clean by removing oxides, sulphates and other substances that may come from handling or from the atmosphere. Bright copper (and bright tinned copper) solders very easily. Oxidised copper and tin does not. Wiping with a clean rag will often do the job. Stubborn cases may need a touch of fine emery paper or even scraping with a blade – but beware of too much abrasion if you are dealing with plated components. The wires should be bright and shiny before soldering. If in doubt, do a “trial run”, pre-soldering the wires to see if the solder takes properly. If it doesn’t, you don’t have much of a chance of making a good soldered joint. The main point to keep in mind is that both of the parts to be joined must always be raised to a temperature above the melting point of the solder. Ideally, the tip of the soldering iron would be applied to both parts, left for the necessary short time and then the solder wire (with its resin core) applied. The resin melts, spreads across the surface with a cleaning action, followed by the molten solder. The soldering tip is then removed and the solder solidifies to give a sound mechanical and electrical joint. If, as is the case with most modern small components, they wet very easily with the molten solder, it is OK to place the solder wire across the parts to be joined, and apply the iron to the solder wire which then melts and, by conduction, raises the temperature of both parts to the necessary temperature. This is exactly what we do when assembling PC boards: touch the joint with the solder wire, apply the iron, remove the iron, wait a sec and bingo! Just to get things into perspective, easy to solder metals include: gold, tin-lead, tin, silver, palladium and copper. Slightly harder to solder are brass, bronze, Monel and nickel silver, while metals that are difficult to solder include Kovar, nickel-iron, nickel, steel and zinc. Metals that are almost impossible to solder without special techniques and/or equipment include aluminium, alloyed steel, chromium, magnesium, molybdenum, tungsten and beryllium. Some components may have tinplated steel leads. The steel wire provides the mechanical strength needed to support the component while the tin plating makes it easy to solder. Sometimes, and particularly in the case of high-density ICs, the coating of easy-to-solder material is only a few microns thick and gentle cleaning methods are required. If you remove the coating, it may be impossible to make a good joint at a temperature that is safe for the component. Other methods of soldering With the increasing complexity of electronic equipment, assembly methods and soldering techniques have undergone a revolution. These two solder suckers from Jaycar are typical of springpowered, low-cost models. On the left is an economy type with plastic body, while the one on the right is of metal construction. Both have hightemperature, replaceable Teflon tips. Powered solder suckers are also available. www.siliconchip.com.au Component and equipment manufacturers go to enormous trouble to ensure that their products are easy to solder and reliable. Most of the advances in soldering techniques have occurred over the last 40 years or so, in parallel with the increasing sophistication and reliability of semiconductor devices. Before then, most components had wire leads and were strung between tag strips, switches, valve sockets and so on. Interconnections between various parts of the circuit were made with wires and, when there were a number of wires going in the same direction, they were made up into looms. The idea of having many of the interconnections made by conductive tracks on an insulating board (“printed circuit”, or PC board) made it possible to eliminate many of the wires and tag boards. Some of the earliest PC boards were made to accommodate valves! Initially, the sort of components used for tag board construction were the only ones available and they were used in PC boards by bending the leads and pushing them through holes where they were soldered, by hand, to copper tracks on the board. These are still used and are known as “through hole components” – see Fig.2. With this technique, the most expensive part of production was often the hand-soldering operation. Indeed, high-quality equipment produced in small quantities with through-hole components is still hand-soldered. Wave soldering Wave soldering was introduced to allow consumer electronics items (eg, VCRs, radios and TV receivers) to be manufactured cheaply and in quantity. Before assembly, the areas of the board which are not to be soldered are coated with a “solder mask” which, as its name suggests, prevents the copper underneath being soldered. The boards, loaded with components (often by “pick and place” robots) are then placed on a conveyer system. The components are on the upper side of the board with the leads pushed through the holes and pointing down. The excess lead lengths may either be clipped off before soldering or left until afterwards. The conveyor draws the board over a bath which applies flux and then over www.siliconchip.com.au Fig.2: through-hole assembly and surface-mount assembly techniques. Note that surface-mount assembly usually requires special equipment. a heater which brings the underside of the board and the component leads up to a temperature just below the melting point of solder. From there, the board moves over a bath of molten solder which is pumped to form a wave of the liquid. The crest of this wave comes in contact with the underside of the PC board, which stays in the wave just long enough for the tracks and the leads to reach a temperature above the solder melting point. Solder then flows over the tracks and leads and completes the joints. Finally, the conveyor takes the soldered board away from the solder bath. Sometimes the board is simply allowed to air-cool but there are some processes which actually drop the whole PC board into a bath of cold, fresh water. This has the added feature of “shocking” the soldered joints, revealing any weaknesses or poorly soldered joints. If the component leads have not been pre-cut, the cooled board is then taken through a saw which trims all the leads to the required length. For wave soldering to succeed, flux, preheating and solder flow adjustments are all critical. The board itself must also be carefully designed so that solder bridges do not cause shorts. It takes experience to get good results. Reflow soldering Another technique called reflow soldering is used where complex circuitry and high volume are involved. (Did anyone mention computers?) This makes use of special “surface mount” components and requires a substantial investment in plant and operator training. As such, it is not a technique that’s suitable for home conFig.3: solder works by combining metallurgically with the surface of another metal to form very thin, brittle intermetallic layers. It is these layers which actually form the electric and mechanical connections in the soldered joint. April 2003  77 If you’re worried about fumes from soldering, this powered fume filter from Altronics could be the answer. It is designed to suck the air in from around your work and filter it, so you dont breathe in the fumes. struction but it should be mentioned that there are special hot-air-flow hand tools available for attaching and/or replacing surface-mount parts. Increasingly, there are components that are available only in surfacemount versions and the serious home constructor may well wish to use them. Mostly, they are smaller than similar through-hole components and keen eyes (or a good magnifying glass!) and steady hands are needed to place even a small number on a PC board. For professional assemblers, a wide range of resistors, capacitors, transistors and ICs is available. Indeed, most components are now available in surface-mount packaging and some exclusively so. In principle, the idea of reflow soldering is very simple. A paste made up of fine particles of solder and flux is placed on the tracks where the solder joints are to be made (probably tin-plated copper). The component leads are then placed in the paste, heat is applied from above and the solder paste melts (its flow being assisted by the flux) so that it forms a bond between the component lead and the track. In practice, it is somewhat more complicated than this. Three expensive machines are required: a screen printer, a pick-and-place machine and a reflow solder machine. The paste is applied to the PC board by a screening process similar to that used to screen-print signs or T-shirts. The screen may be made of metal rather than silk in order to maintain precise dimensions and handle the solder particles mixed with the flux. The paste is thick enough to keep the components in place while the board is transferred from the pick-and- place machine to the reflow solder machine. A PC board may have hundreds (if not thousands) of components, each of which has to be placed in an exact position. Often, polarity is important as well. The components are supplied on a continuous tape that is wound on a reel – maybe several thousand components on each reel. The machine can usually handle a number of reels at the same time, pick the components from the tapes and place them in precisely predetermined positions on the board. In the solder machine, the carrier moves the board slowly through the several stages of the process. The time Fig.4: the basic principles of wave soldering – see text. Compare this with the wave soldering system shown in the photo on the facing page. Fig.5: reflow soldering doesn’t use a soldering iron at all – temperature-controlled hot air is used to melt the solder “paste” applied to the component and copper tracks to be soldered. The board passes through the hot air, the solder paste melts and presto – a soldered joint. Before soldering Looking through the PC board, with the components on the bottom, here's the lead ready for soldering. 78  Silicon Chip Notice how the tip is applied to both of the bits to be soldered at once and not to the solder? Here’s what you’re aiming for: a bright, shiny fillet-shaped solder joint which has taken to both surfaces. www.siliconchip.com.au taken for it to emerge complete and soldered is in the order of 10 minutes. During the first few minutes, the board is raised to a temperature of about 100°C and held at that temperature to ensure uniformity. At this temperature, the flux surrounding the solder is activated. Further into the machine, the board is brought up to a temperature of about 170°C and held again, to make sure that the heat distribution is even. The board then moves on via the conveyer to the soldering phase where the temperature increases to around 215°C (30°C above the melting point of solder) and held at this for a period that can be from a few seconds up to one minute, depending on the components. As it mover further along, the assembly is allowed to cool naturally and comes out of the machine only a little above room temperature. There are a several different methods currently used in the industry to provide the heating but the general trend is to use a forced hot-air flow. Not only does the time and temperature of the reflow soldering process have to be carefully controlled but the design of the PC boards requires considerable care and experience. For example, the solder pads used for surface-mount components have to exactly match the components. Provided this is done, the surface tension of the molten solder will pull the components into their exact positions during soldering. Here’s wave soldering in action. The PC board is carried along over the solder bath by a conveyor. At one point, the solder is forced up in a “wave” so that the bottom of the board passes through it. The components and copper tracks are soldered and the board then emerges from the bath. Photo: Ohio State University. UHF) but when this is not a consideration, it’s usually easier to stick to components with leads. At this point, it is appropriate to consider the idea of soldering surface- mount components when they are used in home projects. For ICs and transistors, where there is a short lead with some flexibility, a very fine solder tip and a steady hand can result in good work. The problem arises with surfacemount resistors and capacitors, most of which have a ceramic base. You can usually solder one end of the component to the PC board with no problems but when it comes to soldering the other end, the cooling process places the component in a state of mechanical stress. This raises and the possibility of breaking the ceramic body and hence ruining the part. This stress does not occur when both ends of the component cool down together. Be aware of this problem if using an ordinary soldering iron to attach surface-mount components: always check each component after it is in place. SMD resistors and capacitors have the advantage of low series inductance (important when working at VHF and “Dry” joint no. 1 . . . “Dry” joint no. 2 . . . A brittle joint . . . Oh no! The solder hasn’t taken to the PC board track at all – it’s just made a blob on the lead. This is a “dry” joint. Here’s another type of dry joint. Some solder has taken to the PC board but only flux has stuck to the lead. Not a “dry” joint but one destined to fail. It is brittle because something has moved as the solder hardens. SMDs for the hobbyist www.siliconchip.com.au Health considerations Finally, a reminder: solder contains lead and lead compounds are poisonous. There does not appear to be any hard evidence that people doing occasional hobby or service work are exposed to any real health risks although on production lines, an exhaust fan is often used. Commonsense would suggest that you avoid breathing the vapours given off when soldering. Likewise, fumes from molten flux should also be avoided. Finally, always wash your hands after soldering, especially before eating. Provided you follow these simple precautions, you should have nothing SC to worry about. April 2003  79