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SMD Soldering Tips & Tricks While the only difference between SMD and through-hole components is how they are soldered to the PCB, there is a lot of jargon surrounding SMDs and new techniques required to work with them, especially the smaller types. This article accompanies our SMD Trainer project (starting on page 38) and provides a lot of detail to help you become an SMD soldering master. Image source: www.pxfuel.com/en/free-photo-qhfan By Tim Blythman U ndoubtedly, some people would prefer to learn how to solder SMDs by getting a hold of the Trainer board (see page 38) and some parts and just getting stuck into assembling it. However, soldering SMDs is a lot easier if you know the tricks. You might find the information in this article helpful even if you don’t plan on building the SMD Trainer. There’s plenty of general advice and hints here, so it’s well worth a read. However, keep in mind that this article is intended to accompany the Trainer; it does not describe less common components and SMD packages that do not appear on the Trainer PCB. If you have some SMD experience but still might have something to learn, you could read through this article and skip over any sections about subjects that you already understand. SMD component sizes and packages Many of the components used in our Trainer design (including the resistors, 30 Silicon Chip capacitors and diodes) have two leads (terminals) and are in so-called ‘chip’ style packaging. These are small, flat and roughly rectangular. These tend to be the most numerous type of components in any design based primarily on surface-mount parts. Some passive components come in different types of SMD packages. For example, it’s common to see small electrolytic can capacitors sitting on a small plastic base with SMD-style leads protruding. While smaller than most electros, they are still larger than most surface-mount passives, so they are not hard to work with. The parts in chip packaging are often described by a four to six-digit code, and there are both imperial and metric versions of this code. For example, a common 3216 metric sized part would be interchangeably known as 1206 under the imperial system. Confusingly, there are some parts with the same codes in both systems (including 1206), but they are very different sizes! Australia’s electronics magazine One way of differentiating these is to use the “M” prefix for metric sizes; this is what we prefer, and we will usually quote both to resolve ambiguity. For example, you will often see (M3216/1206) in our parts lists. This is the largest resistor and capacitor size that we have used in the SMD Trainer. Larger parts are available, though; the next step up is usually M3226/1210 and then M4532/1812. The first two digits determine the component length, while the other digits determine the width. Most parts are longer than they are wide, so the first two digits will be greater, but this is not always the case. Usually, the leads are along the short sides, but in cases where the leads span the longer sides, the numbers might be reversed (eg, M1632/0612). The metric digits are in tenths of a millimetre, so an M3216 part measures 3.2mm long by 1.6mm wide. Also note that the two terminals will be situated at opposite ends, lengthwise. Under the imperial system, each siliconchip.com.au pair of digits accounts for 1/100th of an inch, so a 1206 part is 0.12in by 0.06in, close to the metric equivalent. Table 1 summarises some of the more common two-lead sizes. Note the last row showing a five-digit imperial code (with a dimension under 1/100th of an inch or 0.25mm!). You can also see how, confusingly, some codes (such as 0603 and 0402) are present in both rows. On our Trainer board, the parts around IC1 are all M3216/1206 size. This is one of the largest sizes for which there is a comprehensive range of parts, so it is a good choice for using SMD parts where there is no need to go smaller. The LEDs around IC2 vary from M3216/1206 through M2012/0805, M1608/0603 and M1005/0402 down to M0603/0201. Each has a corresponding resistor of the same size. Another two-lead package that you might see is often used for diodes and is known as SOD-123 (small outline diode). These are similar in appearance to the transistor packages we’ll describe below, but only have two leads. Components with three or more leads IC1 and Q1 on our board are also in commonly-available SMD packages. For parts with more than two leads, there are often variants with differing pin counts but otherwise identical pin pitch and spacing between rows. Parts called SOIC or SOP (small outline IC or small outline package) typically have pins with 1.27mm or 0.05in pin pitch. This is exactly half the pitch of most DIL (dual in-line) through-hole parts. IC1 is in a SOIC-8 package with a 3.9mm body (plastic part) width. Like Table 1 – common passive SMD component sizes Metric M3216 M2012 Length 3.2mm 2.0mm Width 1.6mm 1.2mm Imperial 1206 0805 Length 0.12in Width 0.06in M1608 M1005 M0603 1.6mm 1.0mm 0.6mm 0.4mm 0.8mm 0.5mm 0.3mm 0.2mm 0603 0402 0201 01005 0.08in 0.06in 0.04in 0.02in 0.01in 0.05in 0.03in 0.02in 0.01in 0.005in DIL parts, width tends to increase as the pin count increases, to allow room for the internal leads to fan out along with larger silicon dies. The package we have chosen for transistor Q1 is called SOT-23 (“small outline transistor”). There are also variants with extra pins opposite each of these, called SOT-23-6, plus SOT23-5, which is much the same as SOT23-6 but lacking a middle pin on one side (see Fig.1 below). The basic SOT-23 parts (Mosfets, small-signal transistors, dual diodes etc) are quite easy to work with, as they will only fit their pads one way, and the pins are fairly well spaced and accessible. But they are getting to the point where their size means they are more likely to be misplaced, lost or simply fly into the distance without a trace if not handled carefully. A clean workspace of uniform colour is the best strategy against losing these tiny parts. The package size of IC2 on our Trainer board is the next step down, called SSOP for “small shrink outline package”. You’ll also see these with other modifiers, such as TSSOP (thin small shrink outline package). Either way, they’ll have a 0.65mm pin pitch, about half that of SOIC. Besides being thinner, TSSOP packages are also narrower than SSOP, so watch out – some M0402 footprints will suit either, but not all. Integrated circuit packages Another common IC package that is suited to hand-soldering is the QFP (quad flat pack) and its many variants, such as TQFP (thin quad flat pack). These come with a variety of pin pitches, with 0.8mm down to 0.4mm being typical. They are often used where more pins are needed in a small space, such as for microcontrollers. While the packages are not much smaller, with the pins arranged around four of the sides, they can be more tricky to align correctly. We’ve placed a QFP-44 (10x10) footprint on the rear of the PCB for reference; it has 44 pins (11 along each side), while 10x10 refers to the plastic case dimensions in millimetres. It has a pin pitch of 0.8mm. You can test your skills if you have a suitable part, although it won’t do anything. It could also be useful as a reference for checking dimensions and pin pitches. While it’s usually the tiny size of SMD parts that makes hand-soldering difficult, there are other reasons too. For parts smaller than SSOP, a designer might choose a QFN (quad flat no-lead), BGA (ball grid array), VTLA (very thin leadless array) or WLCSP (wafer level chip scale packaging). Fig.1: some of the more common surface-mount component footprints are shown at left (eg, SOT23, SOIC-8, SSOP-16, M3216/1206) along with pin numbering. siliconchip.com.au Australia’s electronics magazine December 2021  31 These parts are not intended to be soldered by hand, depending on a reflow process or similar to be soldered correctly. That’s not to say that they can’t be hand-soldered at all, but it is very difficult. Some parts can also have large ‘thermal’ pads on the underside of their packages that need to be soldered. Unless the PCB is designed with a via through the PCB to allow the solder to be fed from the other side, it isn’t practical to solder these by hand either (although a handheld hot air reflow tool can be used with great success). The packages and parts described so far are all standard to a degree. There are also numerous SMD parts that come in unique packages. Our SMD Trainer has two parts like this; the mini-USB socket and the coin cell holder. SMD component markings Markings on SMD parts can be cryptic, even when present, but resistors (above a certain size) are thankfully quite straightforward. Instead of a colour code, they are simply printed (or laser etched) with the numeric equivalent of the colour code. A through-hole 10kW resistor would have coloured stripes of brown, black, orange or brown, black, black, red, indicating 10 followed by three zeroes or 100 followed by two zeros. An SMD 10kW resistor would simply be marked ‘103’ or ‘1002’. Note that there is no tolerance code. Unfortunately, the common ceramic chip SMD capacitors are not usually marked at all. In this case, all you can do is make sure that the parts are well labelled in their packaging and only work with one value at a time. ICs can be tricky, too, as they usually have cryptic codes etched into the smaller space that’s available on their tops. SOIC parts may be large enough to have a sensible code, but SOT-23 parts are too small for this. Some manufacturers may even use the same code that another manufacturer has used for a different, incompatible part. The part’s data sheet usually indicates what code(s) they have used. ICs also have a mark indicating their orientation. Usually, the marking is intended to highlight pin 1. This may be a dimple in the plastic moulding or a bevel along one edge. Or it might be an etched symbol on the part top. Referring to the data sheet is the 32 Silicon Chip best way to find out what this mark will be. We usually mark the location of pin 1 on the PCB silkscreen with a small dot or “1”. Some SOIC parts will have a notch and bevel marked on the silkscreen too, corresponding to these features that might exist on the IC. Note, though, that different manufacturers of equivalent parts can use different methods for indicating pin 1. Since the smallest SMD components are not intended to be placed by hand, they generally have no distinct markings. Instead, a computerised pick and place machine is programmed to know how they are orientated in the tape reel on which they are supplied; the data sheet will often show this. As LEDs are polarised, they too usually have a polarity mark. It can vary, but it is usually a green dot or T-shape marking the cathode, or a small triangle that matches the direction of the triangle in the diode symbol and thus also points to the cathode. Tools & consumables This article is intended for relative beginners, so we will assume you mainly have tools intended for soldering through-hole parts. That means a soldering iron (temperature-controlled ideally) and some solder wire. You could use those tools to assemble the first section of our SMD Trainer Board with a bit of care, although a few extra items will be helpful. Tweezers You’ll need something to hold the parts in place while soldering. The small size means that you can’t use your fingers; even if they were small enough, they would get burnt very quickly! Fine-tipped tweezers are ideal. Kits like Jaycar’s TH1752 or Altronics’ T2374 are perfectly adequate, although precision points can be helpful for smaller parts. Just about anything that can be described as tweezers will be better than nothing. Flux Practically all electronics solder contains flux or resin, usually sufficient for through-hole construction. But you probably won’t realise the benefits that a separate flux can bring until you start using it. While you might be used to solder wire ‘just working’, it’s actually the resin core (the resin from certain trees makes an excellent flux) that is largely responsible for this. There are other, more modern and even synthetic fluxes, but resins (called “rosins” after purification) continue to be used as they are quite effective. If you’ve ever tried reusing solder, you’ll know that it doesn’t work as well as new solder. That isn’t due to its age, but because flux has been consumed. This is primarily due to the metal oxides that build up over time as metals react with oxygen in the air. One feature of flux is that it is a reducing agent; Tweezers are useful for holding components when soldering. You can also purchase tweezers with heating cores, which can be used for desoldering as shown in this photo. Source: https://commons.wikimedia.org/wiki/ File:Soldering_a_0805.jpg Australia’s electronics magazine siliconchip.com.au For applying flux there's a variety of different tools you can use, such as this flux pen above. We generally recommend using a flux gel syringe over a pen or container of paste because it's easy to apply and doesn't boil off immediately circuits but must be removed from mains circuits before applying power. The impurities captured by the flux can create a conductive path that would be dangerous at such voltages. You should also clean the flux off the PCB to be able to inspect it properly. Flux and slag can obscure solder bridges and poor solder joints. It’s best to clean as you go, rather than leave it all until the end, as flux is easier to remove when warm. Clean up using the appropriate chemicals. It’s best to use Nylon brushes and/or lint-free cloths since you don’t want to leave fibres behind on the board. Don’t just spray or pour the cleaning solution onto the board; you need to remove it after it has had a chance to dissolve the flux. Sometimes letting it sluice off will carry away much of the flux, but you’ll still need to dab it dry. You may find that the cleaning process is imperfect or, even worse, reveals a soldering failure. There’s no choice but to go back and fix the problem, then clean and inspect it again. the simple explanation of this property is that it can reverse oxidation. The flux reacts with the oxides to leave a pure metal that will bond better. Many fluxes also form a layer to keep out oxygen and prevent further oxidation, which also applies to the solder itself, PCB pads and component leads. Another feature of flux is that it should be heat-activated and only work near the soldering temperature. This prevents it from being used up prematurely. Flux can also enhance heat transfer. Since all surfaces need to be heated above the solder melting (eutectic) point to enable good solder bonding, flux can help get heat into where it is required. The flux can be applied directly to the parts and PCB in surface-mount work, facilitating heat transfer from the iron to all components. The flux also reacts with the various oxides and contaminants to neutralise their negative effect on the soldering process. The reaction products are referred to as slag. This is due to the reactions with the various oxides. The result is often a dark, sticky substance that collects on the tip of the soldering iron. Flux can also be a potently corrosive chemical and can damage a board if any is left behind. Your flux should have a data sheet that explains this aspect in detail; those marketed as ‘no-clean’ are less likely to leave a corrosive residue. Liquid fluxes, flux pens and flux pastes are available; our preference is for a paste or gel as it is easier to apply and control and sticks around longer. Even for the amount of soldering we do, a fairly small syringe lasts for years (or at least until it expires), so there is no need to buy a huge amount of flux paste. For ease of handling, we recommend getting a small syringe, such as Altronics’ H1650A Flux Gel Syringe. The syringe allows for the precise application of small amounts. soldering, especially if you use a lot of flux (which is not a bad idea since it results in more reliable joints). You’ll probably find that you’ll need to clean your iron’s tip as you go. A cleaning sponge is the most common choice here; lightly moisten it, just enough to prevent the iron from burning the sponge. We’ve seen brass sponges that work pretty well, but they don’t seem to have the ability to capture all the residue. In a pinch, a lightly-moistened paper towel works well. Cleaning A solder sucker is better for removing a larger volume of solder, while a braid is better for smaller jobs such as SMD components. Source: https://commons. wikimedia.org/wiki/File:Solder_sucker.jpg It’s important to clean up after siliconchip.com.au Solvents Most fluxes will also recommend a cleaner (even the so-called no-clean fluxes). Isopropyl alcohol (isopropanol) is a reasonable all-around choice. Some fluxes and their slags are sticky and might require scrubbing to be cleaned up properly. Therefore, an even better option is a specialised flux cleaner like Chemtools’ Kleanium Deflux-It G2 Flux Remover (siliconchip.com.au/link/ abad). Take care with these solvents. Many, including isopropyl alcohol, are flammable, while some are poisonous or can damage the skin. The solvent datasheet or MSDS is the best place to find advice and information about these things. The presence of flux should not inhibit testing of most low-voltage Solder wicking braid You might also hear this called desoldering braid or solder wick; it is a length of finely woven copper wire that has usually been impregnated with some sort of flux. It is used to wick away (or absorb) excess solder. A typical use is removing the excess solder which has formed a bridge between two pins, or cleaning solder from a pad after removing a defective part and before fitting a new part. It is pretty cheap; you can purchase a small roll over 1m long for a few dollars from Jaycar (Cat NS3020) This is a close-up of some solder wick braid. It's normally sold on a reel and is used for cleaning solder. Source: https://commons.wikimedia.org/wiki/ File:Solder_wick_close_up.jpg Australia’s electronics magazine December 2021  33 You might need to use the zoom feature (even digital zoom will be very helpful) to see a reasonable amount of detail. If your device has a macro mode, then that will be better suited for close-up viewing too. But we generally find that it’s handy to have a fixed magnifier that can be rigged up in place above a PCB, as well as a small handheld unit that can be picked up and aimed as needed. Lighting While it doesn't need to be an all-in-one package, a magnifying glass, PCB holder and good lighting will help to make soldering small components easier. This is the Jaycar TH1987 mentioned below. or Altronics (Cat T1206A). A typical use might consume a few millimetres of braid, so it too will last for quite a while. PCB Holder Many boards that use SMDs are quite small, and it can be helpful to secure a PCB in place while working on it. It’s also handy to be able to move it around to access a particular component at a certain angle. Tool’s like Jaycar’s TH1982 Third Hand PCB Holder or Altronics’ T2356 Spring Loaded PCB Holder are ideal. The PCB is held in place but can be adjusted, or the entire tool rotated, to allow access from different angles. While these tools are not expensive, even something like Blu-Tack or a similar reusable putty can be a handy makeshift substitute. While the heat from the iron will likely soften and tarnish the Blu-Tack, we’ve never had any trouble using it to hold a PCB in place. Magnifiers Being able to clearly see the tiny parts and features involved with SMD projects is paramount. There are two important ways that you can improve the way you see: magnification and illumination. If you have keen eyes and you’re working with some of the larger parts in SOIC and M3216/1206 packages, you may well do fine without magnification. But it is still vital to peer closer 34 Silicon Chip to inspect your work and check that everything is as it should be. Fortunately, there is a vast range of things that you can use for magnification, and you might well already have some of these, like a simple handheld magnifying glass. Some PCB holders include a magnifier of some sort, including Jaycar’s TH1987 PCB Holder with LED Magnifier. That one includes a soldering iron stand too. The other extreme is a microscope. While certainly not as cheap, not much magnification is needed. Many microscopes also provide excellent illumination. These days, there are many USB and digital microscopes available. A smartphone camera is a suitable piece of gear that most people will already have in their pocket. A digital camera with an LCD viewfinder is a similar option. Good lighting is paramount for successful SMD work. A diffused light source is best, as point sources can cause shadows that obscure parts of the PCB, especially between component leads where bridges might form. If you only have point sources, then aim them from opposite sides to cancel shadows. You can diffuse the light by reflecting it off something white like a wall, ceiling or sheet of paper. As long as you’re happy you can see what you need to see, then you probably have enough light. Fume extractions Remember that flux also generates smoke which is unhealthy to inhale. A fume extraction hood is the recommended way of dealing with this but can be expensive. A small fan (such as a computer fan) can work too, set up to blow away from you. If you can’t manage some sort of active fume control, working outside (or near a large open window) is another option. Top gear If you don’t already have them, the items we’ve mentioned so far are all available at reasonably low prices. We’ll also briefly touch on a few items that can further enhance your SMD experience. Some form of fume extraction is important if you're working in an enclosed area. While this Hakko FA430 (August 2011; siliconchip.com.au/Article/1121 siliconchip.com.au/Article/1121) may be out of the budget of some hobbyists, you can instead just use a small fan to blow the fumes away. Australia’s electronics magazine siliconchip.com.au As we noted earlier, a basic soldering iron is probably adequate to work with larger SMD parts. When you start to get into the smaller parts, then some optional features become essential. Two aspects will help. A fine tip will allow more accurate soldering as you generally want to make contact with just one pin at a time (but see the section below about drag soldering; larger tips can be better with those techniques). The edge of a chisel tip can be narrow enough to work down to relatively small sizes. A soldering station with adjustable temperature is an advantage when working on larger parts. Many of these come with stands and sponges, which also help. Finally, a hot air rework gun can be very handy for desoldering SMDs or reflow-soldering some of the trickier parts. These are available at surprisingly low prices and are well worth having if you plan to do much work with surface-mount components. Using your tools To sum up the advice given above, make sure you have some flux paste, a soldering iron tip-cleaning sponge and some appropriate solvent for your chosen flux. Use the flux generously and keep your iron’s tip clean. Soldering techniques If you’ve read any of our SMD construction articles before, then the following will be familiar. We’ll even go into quite some detail about how you use the tools we’ve just mentioned. You can also follow along with the photos we’ve included. Apply flux to the pads of the components in question. It is a good idea to work in small groups of similar components. For example, you might plan to work with all the 10kW resistors if there are many of them. If there are a small number of different values, then they can be worked in parallel. One exception to this are capacitors, which, as we noted earlier, do not usually have any distinguishing markings. In that case, we recommend sticking to a single value at a time. Roughly place the components on their pads. Flux gel or paste will generally be sticky enough to hold them in place. You might find that your tweezers pick up small amounts of flux and will then stick to components. That’s another reason to keep everything quite clean. Adjust the component with the tweezers so that it is centred on its pads. The amount of PCB pad visible will dictate how easy it is to apply the soldering iron, so symmetrical placement is not just neat, but crucial to ease of soldering. For tiny leads, it can help to apply some flux to the top of the lead too. Clean the iron’s tip and apply a minuscule amount of fresh solder to it. Gently hold the component down flat against the PCB with the tweezers and touch the iron to both the pad and lead together. Hold it there for a second to allow the parts to heat up and bond with the solder. You should see the solder flow from the iron and onto the part and pad. Remove the iron and continue to hold the part in place while the solder solidifies. One second will be sufficient for small parts with fine leads, perhaps longer for larger components. If the part has moved or is not flat against the PCB, grip it with tweezers and apply heat to melt the solder. Adjust its position until you are happy. If the part looks like it is still wellaligned and flat against the PCB, apply some fresh solder to the iron and work through the remaining pads. Medium conical tips are used for general soldering including through-hole and larger SMD components. They have the advantage of being usable at virtually any angle. Finer conical tips are able to make contact with smaller leads, so they are more suitable for soldering large-to-medium SMDs, while still working with smaller through-hole parts. The wide contact area of chisel tips makes them handy for applying solder wick to remove solder, as well as heating SMD tabs or reflowing the pins on one side of a device. Like the chisel, the knife tip can make contact with a large area of the board at one time. Its angle makes it more comfortable for running down the sides of ICs. Bevel tips can contact an even larger area but the larger tips like this one are generally too large to get near smaller components. Smaller bevel tips are not only more manoeuverable but you can also angle them to make contact on just one edge, or the whole face when needed. An SMD flow tip is similar to a bevel tip but it has a depression in which to hold molten solder. This makes them ideal for drag soldering many pins at once. siliconchip.com.au Australia’s electronics magazine December 2021  35 Metric 0402 0603 1005 1608 2012 2520 3216 3225 4516 4532 5025 6332 1 x 1mm Imperial 01005 0201 0402 0603 0805 1008 1206 1210 1806 1812 2010 2512 0.1 x 0.1in 1 x 1cm This diagram shows common SMD component sizes at actual size. The metric 0402 component is so small that it is barely visible! An example of wave soldering showing the PCB leaving the heater portion of the machine and being moved to the solder wave. Source: https://youtu.be/ VWH58QrprVc For very narrow or fine pads, place the iron onto the pads first. The solder mask on the PCB will help to prevent the solder from flowing where it shouldn’t. We try to enlarge the pads in many of our SMD projects to make this easier, although you won’t find this in all designs. Depending on the iron, pad and flux, the solder may be drawn onto the pad and lead by surface tension alone. The advantage of this is that the iron does not obscure the view of the lead so that you can observe the joint forming. The behaviour of solder and its surface tension at the small scales used for SMDs is critical, so this will help you get a feel for what works. You might have seen parts being soldered with solder paste in a reflow oven; when the solder liquefies, the part snaps into the correct location. This is due to the surface tension, pad location and the importance of the solder mask. Surface tension also pulls solder exactly where it is needed. Only a tiny amount of solder is required if the parts are flat against the PCB. If you see clean, curved fillets of solder, that is a good indication that the joint is well-formed. You can use surface tension to apply a generous amount of solder to ensure a strong joint. A bulging but clean and glossy joint is sure to be more functional and solid than a tiny fillet that cannot be seen, just as long as it doesn’t bridge out against any other part! These movements are what has to be practised. The timing will also depend on things like your iron temperature and choice of (tin-lead or lead-free) solder. If you experience a solder bridge, and as long as the part is correctly aligned, continue to solder the remaining leads. Then sort out the bridge. Use the technique described earlier to remove solder from bridged leads. Apply more solder if needed (especially if you can’t easily access the bridge). Apply flux, braid (see below) and then the iron. Allow the braid to absorb some solder, then carefully slide both away. Inspect the part closely with a magnifier. If the joint appears dry or unclean, then apply fresh flux and gently touch the clean iron tip against each lead in turn. You’ll find that even this step of refreshing each lead will help distribute solder to where it should be. Drag soldering When SMD components have pins that are very close together, it becomes impractical to solder them individually. The only component on the SMD Trainer PCB that we would consider having such tight pin spacings is IC2, in an SSOP package with 0.65mm pin pitch. Some chips have an even finer pitch, down to about 0.4mm (eg, TQFP-144). In these cases, it’s easier to drag solder the ICs. Once the chip has been tacked in place and flux has been applied to the pins, a small amount of solder is loaded into the iron’s tip and then gently dragged along a row of pins. Surface tension pulls a small amount of solder from the tip and onto the pins. Done correctly, it forms When drag soldering you'll typically use a flow or bevel tip. The easiest way to learn hown to drag solder would be by watching a video, such as the many found on YouTube. Source: https://youtu.be/ nyele3CIs-U Some common soldering iron tips; most are suitable for SMD work. 36 Silicon Chip Australia’s electronics magazine siliconchip.com.au Table 2 – common types of solder Type of Solder Composition/Name Melting-point Comment Lead-based SnPb 60/40% 188°C Higher tin (Sn) concentrations lead to greater strength SnPb 63/37% 183°C Eutectic – melts/solidifies at a single temperature Sn100C 227°C Silver-free; contains copper, nickel and germanium SAC305 217-220°C Contains tin, silver and copper; used in wave soldering SnCu 217-232°C Contains tin and copper; tin-based lead-free solders are quite often used for reflow and wave soldering SAC387 217-219°C Contains tin, silver and copper Rosin NA Helps to facilitate soldering Non-rosin NA Often contains metal halides such as zinc chloride, hydrochloric acid, citric acid etc; can be corrosive Silver, copper, brass, bronze etc >450°C Often used for jewelry and are designed to have a melting point just below that of the corresponding metal Lead-free Flux Hard solder perfect joints the first time. It’s generally better to apply too much solder than not enough as bridges are easier to see than joints with insufficient solder, and they are easily cleaned up using braid (see below) and more flux. You can get special flow soldering iron tips with ‘wells’ (depressions) to hold the solder for this technique, but you can get away with a standard tip. You just have to add more solder to it more often (eg, every 5-10 pins soldered instead of every 30-40 pins). Even larger-pitch ICs like the SOIC types can be soldered using this sort of technique; it can be quicker (and neater) than soldering them individually. Using braid Solder braid is best for removing small amounts of solder, while a solder sucker is better for removing large volumes. So if you have a lot of solder to remove, start with the sucker to remove the bulk and finish with the braid to tidy up. But at the tiny scales involved with SMD parts, solder suckers become unwieldy and likely to simply inhale your parts as well as the solder you’re trying to remove. The amounts of solder you need to remove will be pretty small too. Before using the braid, it helps to add flux. The word “flux” comes from the Latin word “fluere”, meaning to flow; we want to encourage the solder to flow into the braid. Press a clean part of the braid onto the solder with your iron and allow everything to heat up enough to melt the solder; it should start to soak into the braid. Being made of copper, the braid conducts heat well, so place your grip with care or use tweezers. After the braid takes up the solder, carefully move both the iron and braid away together by sliding away across the PCB. You don’t want to remove the iron first and have the braid soldered to your PCB! It can sometimes help to add more solder where you want to remove it, especially if it’s a solder bridge tucked deep between two pins. The extra volume can give the braid more surface to contact. If there is a dark residue on your PCB after using braid, this is probably the byproduct of the flux working. For areas like this, a cotton-tipped swab dipped in flux cleaning solvent can be used to clean small regions before SC continuing. The basic principles of wave soldering. The PCB 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. ► siliconchip.com.au Australia’s electronics magazine Reflow soldering doesn’t use a soldering iron at all – temperaturecontrolled hot air or IR is used to melt the solder “paste” applied to the component and copper tracks to be soldered. The board passes through the oven, the solder paste melts and hey presto – a soldered joint. December 2021  37