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SILICON SILIC CHIP www.siliconchip.com.au Publisher/Editor Nicholas Vinen Technical Editor John Clarke – B.E.(Elec.) Technical Staff Bao Smith – B.Sc. Tim Blythman – B.E., B.Sc. Advertising Enquiries (02) 9939 3295 adverts<at>siliconchip.com.au Regular Contributors Allan Linton-Smith Dave Thompson David Maddison – B.App.Sc. (Hons 1), PhD, Grad.Dip.Entr.Innov. Geoff Graham Associate Professor Graham Parslow Dr Hugo Holden – B.H.B, MB.ChB., FRANZCO Ian Batty – M.Ed. Phil Prosser – B.Sc., B.E.(Elec.) Cartoonist Louis Decrevel loueee.com Founding Editor (retired) Leo Simpson – B.Bus., FAICD Silicon Chip is published 12 times a year by Silicon Chip Publications Pty Ltd. ACN 626 922 870. ABN 20 880 526 923. All material is copyright ©. No part of this publication may be reproduced without the written consent of the publisher. Subscription rates (Australia only) 6 issues (6 months): $72.50 12 issues (1 year): $135 24 issues (2 years): $255 Online subscription (Worldwide) 6 issues (6 months): $52.50 12 issues (1 year): $100 24 issues (2 years): $190 For overseas rates, see our website or email silicon<at>siliconchip.com.au * recommended & maximum price only Postal address: PO Box 194, Matraville, NSW 2036. Phone: (02) 9939 3295. ISSN: 1030-2662 Printing and Distribution: 14 Hardner Rd, Mount Waverley VIC 3149 54 Park St, Sydney NSW 2000 2 Silicon Chip Editorial Viewpoint Myths about SMD soldering I often see people recommending that a soldering iron with a fine tip should be used for soldering surface-­ mount devices (SMDs). While a fine tip sometimes comes in handy, most of the time it is not what an experienced technician will use to solder SMDs. The problem is that small tips have poor heat transfer; you need good heat transfer to solder SMDs properly. I think there are a few reasons that this advice persists. To start, it seems to make sense if you have little experience soldering. To solder small parts, you need a small tip, don’t you? Another is that I suspect many people are not using enough flux, or the right flux, when soldering. Using the right flux is like magic. With it, solder seems to ‘know’ where it should go! I generally use a medium conical tip (the kind that often comes with the soldering iron) for most SMD work, as well as most through-hole components. Perhaps that is out of laziness. But it works pretty well, even for finepitch ICs. The only time it doesn’t work is for parts like QFNs where you have to get in really close to the device, and the larger ball end of the tip has trouble making contact. I’ve seen experienced soldering technicians recommend using large chisel tips because they overcome that problem; you can angle them to get into tight spaces, but they still have a large tip with a high thermal mass so that they don’t lose temperature as soon as they touch cold solder. They also have a large surface area to transfer heat when you need it. Regardless, you may be doing yourself a disservice if using a fine tip for general SMD soldering. I’ve tried it and it’s so frustrating trying to get heat into the joints. Sometimes such a tip isn’t even capable of transferring enough heat into joints on larger parts to form proper fillets! The thing is that when you add a proper flux paste or gel (not liquid – that’s for different applications), enough to coat the pads and pins, all you need to do is touch molten solder to the pin/pad interface and it’ll be pulled into place. Do it quickly, with the right technique, and you can perfectly solder a whole side of an IC, with perhaps 14 pins, in a few seconds. The excess solder will simply stay on the iron tip; only the amount needed flows onto the part or pad. I must warn against a technique I’ve seen some people use where they apply solder paste to a device pin and then touch it with the iron to melt it and reflow the joint. It sounds like a good option, but the hot iron hitting the cold solder paste can cause the tiny, invisible solder balls to fly off at high speed, landing who knows where. They could cause problems later. Solder paste is best used with a hot air wand or reflow oven, where it can be melted slowly and in a controlled manner. If you don’t want to deal with hand-soldering SMDs, those are good options, but I primarily use a hot air device for removing parts, not soldering them. I’m always worried I will blow parts away during soldering! Still, obviously it is possible with the right technique. Writing this reminds me of a YouTube video where Louis Rossmann (a major figure in the Right to Repair movement) demonstrates replacing an SMD display connector on a MacBook that has dozens of small pins. He uses a large chisel-tip soldering iron with a lot of flux gel, and you can clearly see what he is doing under the microscope: https://youtu.be/z1EOTP51fz0?t=1116 by Nicholas Vinen Cover background image: https://unsplash.com/photos/shooting-star-in-night-sky-5LOhydOtTKU Australia's electronics magazine siliconchip.com.au