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➊ ➋ Solder is placed on the top-right pad & the IC is positioned alongside the pad. The IC is placed on the pads and then solder tacked in place at two diagonally opposite corners. ➌ Solder is now placed on all the pins. The substance around the IC is flux from the solder. How to hand-solder very small surface-mount ICs Provided you have the correct tools and a syringe of no-clean flux paste, soldering very small SMDs (eg, TSSOP devices) into place is easier than you think. Here’s how to do it. By NICHOLAS VINEN Y OU HAVE TWO choices when it comes to soldering in the two surface-mount ICs used in the Stereo Digital-To-Analog Converter described in the previous article: either handsolder them or use a homebuilt reflow oven. In the latter case, you’ll need to follow the instructions in the article titled “How to Solder Surface Mount Devices”, SILICON CHIP, March 2008. However, not many constructors will go to the trouble of setting up a homebuilt reflow oven unless they handle surface-mount devices (SMDs) on a regular basis. Fortunately, hand-soldering finepitch SMDs only requires a few basic tools and a little care. At the very least, you will need a small temperaturecontrolled soldering iron, a magnifying glass (preferably a magnifying 42  Silicon Chip lamp), some desoldering braid (or solder wick) and a syringe of no-clean flux paste (Altronics Cat. H-1650). You’ll also need two pairs of tweezers, one straight and the other pair with angled tips. Don’t try to attempt the job without these basic tools, otherwise you could wreck both the ICs and the boards. It’s also vital to have lots of patience. You must treat each IC gently and avoid applying heat for long durations (more than 3-4 seconds at a time). The pins on TSSOP and SSOP devices are quite thin – in fact, they are noticeably easier to bend than larger surfacemount packages like SOIC/SOP (let alone DIP). Soldering iron A temperature-controlled soldering iron is the best iron to use here. Set the temperature to somewhere in the 350-400°C range. The lower end of that range is appropriate when applying solder while temperatures around 400°C should be used when heating the solder wick, as described later. You don’t need to use a very thin tip on the soldering iron. In fact, using a thin tip can actually make the process more difficult when it comes to applying enough heat to the solder wick and getting the solder to reflow properly. The standard tip supplied with most good irons should be sufficient and a medium to fine conical tip works well. Be sure also to use fine, good quality solder (eg, 0.71mm diameter). Step-by-step procedure The step-by-step procedure for soldering in each SMD IC is as follows: (1) Place the board flat on the workbench, copper side up. (2) Apply a tiny amount of solder to the top right pad (top left if you are left-handed). To do this, briefly touch the pad with the soldering iron and add a dab of solder – just enough so that you can see smoke from the flux – then quickly remove the iron. You should now be able to see a small solder bulge on that pad (check with siliconchip.com.au ➍ ➎ ❻ A thin layer of flux is applied to the pins & the excess solder removed using solder wick (eg, four pins at a time). This shows the IC after the remaining pins have been cleared of the excess solder using solder wick. Here the IC has been cleaned with isopropyl alcohol, to remove the flux residue (optional). a magnifying glass if you are unsure). (3) Clean the tip of the iron with a damp sponge to remove any excess solder. (4) Place the IC next to (but not on) the pads. If you are right-handed, place it slightly to the left of the pads and vice versa. Ensure that the dot or divot in the corner of the IC (nearest pin 1) is oriented correctly. (5) Grab the IC by the ends using a pair of tweezers. (6) Use the soldering iron to melt the solder on the top-right pad, then gently slide the IC along the board and into place. Remove the soldering iron immediately it’s in place. This process should only take a couple of seconds, to avoid overheating the pad. Don’t worry about getting it in exactly the right place the first time. Just try to avoid getting any solder on the other pins. As long as you do that, repositioning the IC is easy. (7) If the IC is not exactly lined up with the pads, simply re-melt the solder and nudge the IC until it is. Wait a few seconds between each attempt. You need to get three things right: the vertical position, horizontal position and rotation. When it’s correctly lined up, the pins will all be centred on the pads. (8) Once you are happy with the alignment, rotate the board 180° and solder the pin at the diagonally opposite corner. The IC may still move a little during this step, so check the alignment again and adjust it as necessary. (9) Now solder the remaining pins. Start in one of the two remaining unsoldered corners and apply solder to each pin. Do not worry about bridging them – in fact the simplest technique is to apply a small blob of solder between each pair. Make sure that all pads have solder flowed onto them but don’t go overboard as you need to remove the excess later. (10) Once all pins are soldered, apply a thin layer of flux paste along both rows towards the outside. A thin layer should be enough (you can always add more later if necessary). (11) You now have to remove the excess solder. Begin by placing a length of solder wick immediately alongside (but not on top of) some of the pads. Now place the soldering iron on top of the solder wick, pressing it down onto the board, while gently sliding the wick towards the solder on the pads. As the wick heats, it will start to melt the flux and the excess solder, creating visible smoke. At that point you can slide it right up against the pins. Most of the excess solder should then be sucked into the braid. Finally, slide the wick along the board away from the pads and lift it and the soldering iron off the board. Do not apply any pressure directly onto the IC pins during this procedure. At all times, you should be pressing down onto the PC board only while sliding the wick along it. The whole process should take no more than about 5-6 seconds. Don’t worry if some solder is left behind – rather than applying the heat for too long, it’s best to remove what’s left with a second pass. When you are finished, the pins should be left with a near-perfect amount of solder and no bridges (see photos). (12) Repeat this process all the way along both edges of the IC, moving the wick along a few pins each time. Don’t do it twice on the same set of pins as most of the flux is used up in the process and the solder won’t flow properly without it. (13) Once you have gone around the entire IC, inspect the pins using a magnifying glass to check for any remaining solder bridges. There will likely still be some bridges after the first pass. Be sure to check high up on the pins where they enter the package, as sometimes solder can find its way up there. If there are solder bridges, apply a little more flux to the affected pins and then repeat the process with the solder wick. Do this until all the pins are clear. If you are using no-clean flux (ie, the recommended type) then you don’t need to remove the flux residue. However, if you really want to, pure SC alcohol will dissolve it. Issues Getting Dog-Eared? Keep your copies safe with our handy binders Available Aust, only. Price: $A14.95 plus $10.00 p&p per order (includes GST). Just fill in and mail the handy order form in this issue or ring (02) 9939 3295 and quote your credit card number. siliconchip.com.au October 2009  43