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Home of the G7CKF range of electronic kits

How to solder surface mount devices

Introduction

There has been a quiet revolution in electronic construction over the last few decades. The components we are all used to with pins and wires that locate in holes drilled in PCBs have slowly become obsolete, replaced by much smaller surface-mount components that attach directly to the top of the PCB. This process has reached the point at which traditional through-hole construction is rapidly becoming the preserve only of hobbyists, and exciting new components are not being made available in through-hole variants. Thus to deliver those exciting components Language Spy have to make some surface-mount kits.

If you are used only to through-hole work, this may seem rather daunting. Surface-mount components are so tiny, how could you possibly solder them! The truth is though that while surface-mount components are undeniably smaller than their through-hole counterparts there is nothing to stop someone competent in through-hole work making the jump to surface-mount in a painless manner. Careful selection of manageable sized components, the right tools, and the mastering of a few simple new techniques are all you should require, so we have assembled this guide to the tools and techniques for soldering surface mount devices by hand.

Before you start

You will probably already have most of the tools and accessories for soldering. However some of what you will need for surface-mount work differs from that for through-hole, so it is worth quantifying the contents of a simple surface-mount workbench. So, you will need:

A good light source. Surface-mount components are tiny, and easily lost. Plenty of light ensures you will be able to see them clearly. A good downward pointing desk lamp should suffice.

A clear high-contrast surface. Because surface-mount components can be difficult to see, it helps if they are manipulated over a bright white surface. A fresh sheet of white printer paper on a desk makes a suitable working area.

Good hands-free magnification. Unless you are fortunate enough to have amazing eyesight, you will need a good quality magnifier to work with surface-mount components. The “Helping hands” type on a stand is suitable, or perhaps the headband type.

A small flat-blade screwdriver. You will need to hold surface-mount components down while you solder them. If you have a set of jeweller's screwdrivers then usually one of the smaller ones will be suitable, below about 1.5mm in width.

A good-quality pair of precision tweezers. You will need these for picking up, manipulating, and turning over surface-mount devices. Metal tweezers are likely to be more useful than plastic ones, as they can be used on hot items during soldering.

A craft knife. Surface-mount components are packaged in a tape for use in automated board assembly machines. The tape has a thin plastic cover that needs to be carefully peeled back to reveal the components. This is best done over a well-lit surface under a magnifier using the tip of a craft knife. If the component falls out of the packaging onto the floor or a badly lit surface it is unlikely that it would be found.

A fine-tipped soldering iron. With care Language Spy kits can be assembled with a standard fine tipped iron suitable for use with conventional 0.1” pitch through-hole components. However the size of surface-mount components means that you will require a greater level of skill to build it using the larger screwdriver style iron tips. Thus the finer the tip, the better for assembly. It is also recommended that you use a temperature controlled iron at a suitable temperature for your solder, if you have one.

Lead-free solder, with a flux core. Language Spy kits are RoHS and WEEE compliant products, so to maintain that compliance they should be soldered with RoHS lead-free solder. If you only have leaded solder or are more comfortable with that variety then it will do as good a job, however it is worth being aware of the RoHS compliance. Any decent quality flux cored solder designed for electronic use should be suitable for assembling the kits. Solder intended for plumbing may not be suitable.

Flux. Any decent quality non-acidic soldering flux should be suitable for use with Language Spy kits. Use liquid flux in a syringe designed for surface-mount assembly if you can afford it, otherwise flux paste applied with a cotton bud.

Desoldering braid. Decent quality desoldering braid is a must for surface-mount work, as some of the techniques involve an excess of solder which must be removed. You may also find a set of side cutters useful, for cutting off used braid.

Solvent cleaner. This is not essential, but it is desirable to remove excess flux from your board when you have finished construction. Which solvent cleaners you have available will depend on where you are in the world, however a good electronic parts supplier should carry suitable products. We use an electronic-grade aerosol cleaner and an old toothbrush to scrub away the flux.

Finally, you should be aware of the possibility of damage to semiconductor devices from static electricity, and take care to use a suitable earthing arrangement if you think it may be necessary.

How to solder SMDs

It is assumed that anyone embarking on a Language Spy surface-mount kit will already be familiar with soldering through-hole components as they are not kits for the novice solderer. Thus this “How to solder...” page will not deal with any through-hole components the kit may contain, instead concentrating only on the surface-mount elements.

Surface-mount construction differs from through-hole in that the components are not automatically held in place by a hole in the board. Thus surface-mount components face a hazard through-hole solderers may not be used to: they are subject to the forces exerted by the surface tension of liquid solder as it solidifies. A surface mount-resistor for instance will rise up on end, so-called “tomb stoning” as the solder solidifies if it is soldered at one end without being held in place. Techniques for hand surface-mount soldering therefore differ from those for through-hole soldering in that the emphasis is on securing the component as well as soldering it.

It is worth recommending here a YouTube search for videos of SMD soldering techniques.

Chip resistors and capacitors

Before soldering a chip resistor or capacitor, ensure that the pads it is to go on are clean, flat, and ready tinned. The boards included in Language Spy kits are already tinned, however this step is included here as general advice for surface-mount soldering. If tinning a pad leaves a domed blob of solder on it, remove the blob with some desoldering braid and your soldering iron.

Apply a thin layer of flux to both pads.

Carefully peel back the plastic tape on the top of the tape containing the chip to release it. It is suggested you do this immediately above the area of board onto which it is to be soldered, it is easy to lose a loose a small component. If the chip has landed the wrong way up, turn it over with your tweezers.

Carefully nudge the chip into place with the tip of your small screwdriver so that its conductive ends are centred on the pads.

Place the tip of the screwdriver on top of the chip and apply gentle pressure to hold it down while it is soldered.

While holding down the chip, pick up a small amount of solder on the end of your soldering iron, and solder one end of the chip to its pad. When the solder has solidified, test that the chip is soldered to the board by giving the chip a nudge with the screwdriver.

Solder the other end of the chip. Use as little solder as you can and use as little contact with the iron as you can, you need to avoid melting the joint at the first end of the chip.

If your chip has a large blob of solder at each end, remove them with some desoldering braid. Capillary action will have drawn enough solder underneath the ends of the chip to make a good contact without the need for a blob of solder.

Larger surface-mount components

Some Language Spy kits include electrolytic capacitors which are much larger than the chip components, with much larger pads. Similar techniques are used to solder them, but they are much easier to handle and position.

As with the chip components, before soldering an electrolytic capacitor ensure that the pads it is to go on are clean, flat, and ready tinned.

Apply a thin layer of flux to both pads.

Peel back the plastic tape on the top of the tape containing the capacitor to release it. The electrolytic capacitors are large enough to pick up and place by hand.

Carefully place the capacitor on the board so that its connection tabs are centred on the pads. These components are polarised, so be sure to place them on the board the right way round. The angled corners should align with those on the outline printed on the board.

Hold the capacitor in place with the tip of your finger.

While holding down the capacitor, pick up a small amount of solder on the end of your soldering iron, and solder one tab to its pad. When the solder has solidified, test that the capacitor is soldered to the board by giving it a gentle push with your finger

Solder the other tab. If the capacitor has a large blob of solder at each end, remove them with some desoldering braid.

Integrated circuits and similar components

You will find integrated circuits and RF transformers in Language Spy kits, all of which have a similar outline with two rows of closely spaced pins. Soldering these pins individually would be a difficult task even for an extremely advanced solderer, so we will use a different technique of soldering multiple pins at once with an excess of solder which we will then remove with desoldering braid.

As with the chip components, before soldering an IC ensure that the pads it is to go on are clean, flat, and ready tinned.

Carefully peel back the plastic tape on the top of the tape containing the IC to release it. It is suggested you do this immediately above the area of board onto which it is to be soldered, it is easy to lose a loose a small component. If the IC has landed the wrong way up, turn it over with your tweezers.

Gently nudge the IC into place with the tip of your small screwdriver so that its pins are centred on their pads. Take particular care to ensure that the IC is on the board the right way round. The ICs used in Language Spy kits have one bevelled edge on top, with the IC the right way up and the bevelled edge on the left pin 1 will be at the top left. The bevelled edge is marked on the board as a white line down the left hand side of the IC space, in addition pin 1 is marked with a white dot and a white semicircle to represent the notch found at the top of older dual-in-line ICs. Pin 1 of the RF transformer is indicated by a white dot on the top of the transformer, and a corresponding white dot on the board.

Place the tip of the screwdriver on top of the IC and apply gentle pressure to hold it down while it is soldered. Take care to keep it steady so that its pins do not move off their pads.

While holding down the IC, pick up a small amount of solder on the end of your soldering iron, and solder one corner of the IC to its pads. It does not matter if you cover a couple of pins with a blob of solder, the aim is merely to fix it down at this point. When the solder has solidified, test that the IC is soldered to the board by giving it a gentle nudge with the screwdriver. Inspect the IC to ensure that its pins are still correctly aligned with their pads. If it has moved you will need to very carefully melt the solder you have just applied and nudge the IC into place with your screwdriver.

Apply a thin layer of flux to each row of pins.

Solder the other side of the IC from that whose corner you have just soldered. Your aim is to hold a blob of molten solder on the end of your iron and run it down the row of pins in one fluid movement. Starting at one end melting the end of your solder wire over the IC pins you should be able to draw the resulting molten solder across the row. The flux will ensure that a good joint is made, and any surplus solder or solder bridges between pins will be later removed with desoldering braid.

Solder the first side of the IC in the same way as above.

Your IC should now be securely fixed to the board, but will certainly have an excess of solder and will probably have some solder bridges between pins. Carefully remove this excess with desoldering braid. Capillary action will have drawn enough solder underneath the IC pins to make a good contact without the need for a blob of solder.

After soldering

After soldering surface-mount devices using these techniques it is quite likely your board will be rather a mess with a lot of surplus flux on its surface. When you have completed all the surface mount soldering it is therefore worth removing all this flux with solvent cleaner. It won't stop the kit from working, but it looks unsightly.