Introduction

Traditionally, electronic circuits produced by the home enthusiast were made using through hole technology (THT) components assembled on to either a printed circuit board (PCB) or other media such as stripboards or wire-wrap boards. These components and methods still have a valuable role for the less experienced constructor or for the rapid production of prototypes or low numbers of circuits. For an example of what can be achieved using THT components and stripboard, take a look at the PWM Spindle DC Motor Driver project. Many SMD PCB based projects will have a number of THT components such as off-board connectors or jumper terminals in addition to the SMD components, so it is important to have a solid technique using a soldering iron and wire solder before moving on to SMD PCB assembly.

Modern circuits are most commonly produced using surface mount technology (SMT) using surface mount devices (SMD). These components are assembled directly to the solder pads on a PCB and do not have leads that pass though holes in the boards. This means that the components can be manufactured to be much smaller and cheaper and so produce higher component densities when compared with THT components. The small size of the component connections also dictate the use of solder paste rather than solder wire, as it is much easier to apply the correct amount of solder paste to make a good solder joint and is also easier to physically position the small components and not have them move during the soldering process. With the solder paste pre-applied and the SMD component positioned, it is simply a case of heating the assembly to the correct temperature to melt the solder paste, producing the required joints.

Commercially a stencil will be used to enable the right amount of solder paste to be applied to the pads on the PCB, then the components will be positioned using an automated process involving a Pick and Place machine. Finally, the assembly will be placed in an oven in order to carefully bring it to the correct temperatures for the solder paste to melt, fusing the components to the PCB and forming good electrical and mechanical joints.

In order to assemble a SMD PCB by hand these processes need to be emulated and whilst this is not especially difficult, it might take some practice before you are happy with the results. The techniques described here do work and can be used to produce quite professional looking results if carried out carefully. There are numerous other ways that others have found to hand assemble SMD components and it is worth doing some web research and adapting your methods until you have something that works for you.

Lead and Non-Lead Solder

You will find that you can purchase solder wire or solder paste in may different formulations, but the most important distinction is whether the solder you are purchasing contains lead or not. The environmental considerations of whether to use solder that contains lead will not be discussed here as much has been written elsewhere on the subject. This article will limit itself to the practicalities of using the different solder types in the home constructor or small volume production areas.

Firstly, if you intend to produce assembled PCBs for anything other than your own personal use the decision has been made for you - RoHS, REACH and other regulations worldwide dictate that you must use non-leaded solder. If you are assembling a PCB for your own use only then you still have a choice.

Why would you choose to use solder that contains a toxic substance (lead)? Simply put, because it's easier to use. Leaded solder is typically a mix of tin (Sn) and lead (Pb) in a 60/40 ratio that will melt at relatively low temperature of 183°C - 190°C. The lead content also makes the solder 'flow' more easily and in practical terms you will find it much easier to produce a good quality solder joint with leaded solder. For this reason it is recommended that a beginner to soldering gain experience using leaded solder where possible. Electronic components can be damaged by excessive heat caused by the soldering process and the lower melting point temperature of leaded solder gives a little extra leeway for the beginner.

Lead-free solder paste and solder wire are typically composed of tin (Sn), silver (Ag) and copper (Cu) in different ratios depending on the desired characteristics of the resulting solder. The most common formulations are tin / copper in the ratio 99.3 / 0.7 which has a melting point of 227°C, tin / silver / copper in the ratio 99.0 / 0.3 / 0.7 (known as SAC0307) which has a melting point of 217°C - 228°C and tin / silver / copper in the ratio 96.5 / 3.0 / 0.5 (known as SAC305) also with a melting point of 217°C - 220°C. The cost of lead-free solder wire or paste can be almost twice that of leaded solder and the cost also increases as the silver content goes up. The higher silver content solders are easier to use however, as they have slightly lower melting points and also 'flow' more easily. With experience you can decide for yourself which of these different solders suits your purpose, but initially it is recommended that the SAC305 formulation is the way to go, both in paste and in wire solder.

Regardless of your choice of lead or non-lead solder, you should ensure that you work in a well ventilated environment in order to avoid breathing in the fumes produced. You may also wish to purchase a solder fume extractor to help with this. Note that both lead and non-lead solder produce toxic fumes due to the flux content of the wire or paste used.

Equipment Required

Soldering Iron and Hot Air Gun

In order to successfully assemble SMD PCB boards you will need some basic equipment, the most important of which are a soldering iron and a hot air gun for electronics work. Ideally the soldering iron will be temperature controlled as this makes it much easier to work with the different solder types described previously. The hot air gun should be both temperature controlled and have the ability to adjust the rate of air flow. This is vital for work with the smaller SMD components as too high an air flow will either move the components from their mounting pads or at worst have you searching the floor for these tiny devices. The soldering iron and the hot air gun can be purchased as separate units or as a combined piece of equipment, according to preference. As with any tools or equipment, the well known brands sell at a premium price but will have the advantage of better build quality and back-up should you have any issues. Good results can be obtained with lower cost equipment however if this better suits your budget and if you are prepared to experiment a little more to find the correct settings.

You should try to purchase a selection of soldering iron bits with your soldering iron to enable working with a range of components sizes. You will definitely need bits in the range of 0.5mm - 4mm in size, preferably with conical, bevel and chisel tips. Make sure you also have a suitable soldering iron stand with a 'wet sponge' to keep the bits clean and free from residues from the solder or flux used.

A temperature controlled soldering iron in the power range 60W - 70W is ideal in this application, as it will heat up quickly and be able to provide sufficient heat to solder larger joints. You should be able to adjust the temperature through the range 200°C to 350°C; most soldering irons on the market will cover a greater range than this.

Some of the electronic components that you will handle may be sensitive to damage from electro-static discharge (ESD) and so you should also check that the soldering iron you choose has a correctly grounded tip.

A range of nozzle sizes and shapes is also essential for use with the hot air gun that you choose. It is important to be able to direct the hot air to where it is needed and also away from where it is not and a selection of nozzles will help you do this.

The hot air temperature should be controllable in the range 100°C to 350°C or better. The lower end of the range is not particularly useful for soldering, but makes the gun useful for warming heat-shrink tubing or similar jobs. As previously mentioned, the ability to control the air flow is essential. A nice additional feature is the ability for the gun to detect when it is in it's cradle and automatically cool down and switch off the air flow.

Solder Wire and Solder Paste

The issues regarding lead versus non-lead solder have been discussed previously, so having made your choice you should purchase a reel of solder wire. A good diameter of wire for all round electronics use is 0.7mm and this will be fine for the uses outlined here. A non-lead solder in SAC305 formulation was used in this guide.

You will also need to purchase some solder paste in the same formulation. Solder paste can be purchased in tubs or cartridges of various sizes, but for small scale production or home use a syringe, as illustrated will prove easier to use and contains sufficient paste for many boards. Solder paste does have a shelf-life as it will slowly dry out over time and eventually become unusable. The usable life can be greatly extended by keeping your solder paste in a refrigerator between uses and in this way your paste should still be fine after many months of storage. You will need to remove your paste from the refrigerator a few hours before you wish to use it in order for it to return to room temperature.

It is also recommended that you obtain some 'No Clean' flux paste. A small syringe as shown will be sufficient for the assembly of many PCBs.

Multimeter

It is essential that you can test the quality of the solder joints in your PCB assembly, check for short circuits and also test components to ensure they are working as expected. In order to do this a multimeter should be purchased. It does not have to be expensive and auto-ranging multimeters of the type shown are more than capable of meeting your requirements in this regard. The meter should be capable of measuring voltage, current and resistance as a minimum, but it isn't difficult to find a meter such as this which can additionally test diodes and measure capacitance, temperature and frequency.

Additional Tools

Some other tools will also be useful for any electronic assembly work. Small pliers and side-cutters are useful for any THT components that may need to be assembled and many good kits containing a range of pliers and side-cutters are available from numerous sources.

A range of tweezers of various types is essential for hand assembly of SMD PCBs. Curved and straight tips in a range sizes allow for the handling of delicate SMD components of greatly differing sizes and shapes without damage. They should have an anti-ESD coating and non-magnetic tips so that components can be released as easily as they can be picked up.

SMD PCB Assembly

For the purposes of this guide, a SMD PCB for the PWM Spindle DC Motor Driver project is shown being assembled. The same techniques could be applied to any SMD PCB however.

The assembly process starts with a bare PCB. The pads that will be used to assemble the SMD components can be clearly seen and these should be clean and entirely free from grease or other marks. Avoid touching the pads with your fingers and if necessary wipe the board over with isopropyl alcohol to ensure that it is clean before you begin assembly.

The basic order of assembly is by component size, starting with the smallest components. This enables you to get the nozzle of the hot air gun fairly close to the smaller components without being physically obstructed by the larger ones. Placing the smaller components with the tweezers is also easier without the larger ones being in the way.

Any THT components will be assembled last as these tend to be the largest and will also stop the board sitting flat on your work surface once they have been soldered in place.

Lead-free solder wire and paste in the SAC305 formulation were used for this example assembly and so any temperature settings for the hot air gun or the soldering iron are written with this in mind. Should you choose to use a different solder formulation, then your temperatures will need to be adjusted accordingly. As a rough guide, if you are using leaded solder, then subtract around 40°C from the temperatures shown here.

Step 1 - Apply the Solder Flux

A small amount of solder flux should be applied to the pads of the components that are to be soldered in place. Do not apply solder paste to pads that are not yet going to be soldered or it will partially heat up, become tacky and have to be cleaned off. The solder paste can be applied to the pad only using a small brush - you really only need a thin layer, any more will just need to be cleaned off later.

Step 2 - Apply the Solder Paste

Some solder paste should now be applied to each of the SMD pads that are to be soldered. It is suggested that some paste is put onto a non-porous surface (a scrap of old PCB material will do fine) from the syringe, then a small amount of paste transferred to the SMD pad using a blunt needle or similar pointed object. These pads are too small to apply the paste directly from the syringe. You will learn the correct amount of paste to apply through experience, but the inset in the photograph can be used as a starting point.

Step 3 - Position the Components

The components should now be carefully positioned on the pads using appropriate tweezers. Gently press the components down into the solder paste until they lie flat on the surface of the PCB.

Step 4 - Heat the Solder Paste

The solder paste should now be heated using the hot air gun. In this project, the temperature of the hot air was set to 300°C and the air flow set sufficiently low so as to not move the components when heated. The gun is angled so as be facing almost vertically downwards at a distance of between 10mm and 20mm above the PCB. The solder paste should start to melt after around 10 seconds. When it does melt, allow it time to 'flow' over the pad and the component. You will also see that it becomes shiny at this point and loses the previous dull appearance. This will take a further 4 -5 seconds. Once this has happened remove the hot air gun and allow the PCB to cool before moving it.

Even though the solder paste should melt at around 217°C, the hot air needs to be considerably above this temperature in order to melt the solder paste quickly. The longer the component is heated for, the more likely it is to suffer damage. Due to the variation between different pieces of equipment and materials used, you will have to experiment to find the correct temperature and air flow settings for your situation. A scrap piece of PCB material is ideal for this. It is not recommended that you go above 320°C as you are very likely to damage components or even burn your PCB.

Ensure that you work on a surface that is able to withstand the heat of this process!

Step 5 - Assemble the Remaining Small Components

All the remaining small components should now be assembled in small groups, repeating steps 2 to 4 as necessary. Allow the PCB to cool between groups, otherwise the flux will run instantly and become ineffective and the solder paste will lose the correct consistency.

Step 6 - Diode D2 and Inductors L1 and L2

Although these components are significantly larger than the 0603 package components previously assembled, exactly the same techniques can be applied and so should present no real extra challenges. Apply flux to the whole of each pad using the brush as before. Follow this by adding solder paste, this time a small amount directly from the syringe as the pad is bigger. Try to visualise adding sufficient paste so as to give a thin (around 0.2mm) layer across the whole of the pad. When you position the component and press it in to place, then this paste should just cover the pad without too much excess.

Use the hot air gun to heat the solder paste as before but as the components are larger, angle the gun to heat and flow the solder paste on one pad at a time. You will find it does take a little longer to flow the solder paste as there is more of it. Once one pad has flowed correctly, move to the second pad on the component, which will already have pre-heated and should not take long to flow. Only assemble one component at a time.

Step 7 - MOSFET Transistor Q1

MOSFET transistor Q1 is typical of many components found on SMD PCBs with its TO-262 package (or similar) being used for many different transistors, voltage regulators and the like. The pad design not only provides electrical contacts, but also acts as a heatsink for the package. For this reason, the pad is quite large and therefore presents different problems when using solder paste and a hot air gun to assemble the component. The problem lies in the fact that while the heatsink usefully draws heat away from the component in normal operation it also does so while trying to heat the solder paste during assembly, leading to difficulties heating the solder paste sufficiently for it to melt and flow. This problem can be overcome with a slight variation in technique.

Initially cover the pads with a thin layer of flux as before. Then add the solder paste using the syringe directly as a dispenser to any pins that are used (the centre pin of the device is not used in this example) as well as a pattern of dots that when compressed will disperse to cover the whole of the large pad. The image shows this more clearly.

The component should then be carefully positioned and pressed in to place until it lies flush with the top surface of the PCB. This should disperse the solder paste fairly evenly and some will emerge from the sides of the device.

The solder paste on the pin pads should then be heated as normal using the hot air gun until it melts and flows. This will hold the component in place while the solder paste on the large pad is worked on.

Firstly, the solder paste around the edges of the component on the top surface of the board should be heated using the hot air gun. If this melts and flows within a reasonable length of time (20 seconds or so) then great but if not transfer the hot air gun to the bottom side of the board, heating the large pad from underneath. Now, the reason for the additional vias in the large pad can be understood as they help transfer heat through the board to the solder paste lying under the component on the top side of the board. It shouldn't take long before the solder paste will melt and flow and this can be seen in the via holes.

If necessary once this has been done, transfer the hot air gun back to the top side of the PCB, melting and flowing the solder paste around the edges of the component to finish off the assembly. The assembled component is shown in the image.

Step 8 - Electrolytic Capacitor C1

The preparation of the pads for the large electrolytic capacitor C1 is the same as for previous components, a thin layer of flux being followed by the dispensing of a small amount of solder paste directly from the syringe. The component is then carefully positioned and pressed in to the solder paste as before.

The difficulty presented by heating the solder paste for this component lies in providing sufficient heat to melt and flow the solder without damaging the component, especially unintentionally melting the black plastic base the component is mounted on. This can be done by using a very narrow focussed nozzle on the heat gun. If this is not available or does not work for you in practice then an alternative is simply to use a narrow bit on a soldering iron set to around 300°C and apply the soldering iron directly to each pad in turn.

Step 9 - Through Hole Components

The final assembly step is to solder any THT components such as off-board connectors in place, using a soldering iron set to 270°C.

Finishing Work

Your fully assembled PCB should be cleaned with isopropyl alcohol to remove any excess flux that might remain around any of the solder joints.

It is also recommended that you thoroughly test your PCB with a multimeter, ensuring that all the solder joints are electrically sound and that no inadvertent short circuits have been created. With care and some basic knowledge of electronic components you can also test to ensure that many of the passive and some active components have not been damaged during the assembly process. For example it is easy to test that the value of a resistor looks 'about right' (its value will not be the same in circuit) or that an LED lights up with a diode test.

The final step is where you get to enjoy the outcome of your hard work - power up your circuit and enjoy using the professional looking SMD PCB that you have assembled!