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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): $70 12 issues (1 year): $130 24 issues (2 years): $245 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: Silicon Chip PO Box 194 Matraville NSW 2036 Phone: (02) 9939 3295. ISSN: 1030-2662 Printing and Distribution: Editorial Viewpoint PCB assembly pitfalls Now and then we get requests to supply circuit boards with some or all parts pre-soldered to them. While it seems like an attractive idea on the surface, we generally have not done it for a few reasons. The first is the financial risk involved. We would have to pay for the PCBs to be made, plus the (potentially expensive) parts, then for the assembly, all before we know how many we are going to sell. However, my biggest concern was the things that could go wrong in the process. What if the parts or the boards we receive are faulty, or even the soldering? Regardless of whether we picked up these problems before or after they reached customers, we’d be unlikely to recover any of the money we had spent. We would either have to abandon it altogether, or start over and hope to avoid the same problems the second time. There’s also the fact that acting as a quasi-manufacturer distracts us from the task of publishing the magazine, which is considerably more important. When the Pico 2 Computer project was published in our April issue, we knew that the designers had built multiple prototypes that worked. They also provided us with all the files we’d need to get boards made, and even instructions on how to go about ordering them. So, I thought it was finally time to give it a try. After all, they had practically handed it to us on a plate! What none of us were expecting was that JLCPCB (the company we paid to assemble the Pico 2 Computer boards) had been delivered a batch of apparently defective CH334F USB hub chips. They started using those to assemble boards just before we ordered a large number from them. These are the hardest chips to solder or desolder on the whole board. And, of course, there is apparently no other source of these chips than JLCPCB and their supplier, LCSC. So we couldn’t order parts from a different batch in the hope that they would function correctly. This could have been a disaster. It was lucky that someone else had ordered some boards not long before we did, found they didn’t work, and contacted Peter Mather (the Pico 2 Computer PCB designer) for help. He swapped the CH334F chips between one of the new boards and a known good one, and found that the fault followed the chip, confirming that was the problem. It was also very fortuitous that there was an easy way to work around this problem. The defective part of the chip was involved in sensing an over-­ current condition on the USB ports and cutting power to them. This was sensed via a resistive divider. Removing the two resistors in that divider disabled the function, and the chips then worked perfectly. The board has a PTC thermistor as a second line of defence to limit the current drawn from the USB ports if there is a fault. So disabling this active current monitoring feature isn’t really a problem. We had to remove the resistors from all the assembled boards we received and added notes to the kits explaining that. This approach allowed us to salvage those boards and avoid a bad experience for our customers. Ordering these Pico 2 Computer boards was a way for us to ‘dip our toes in the water’ with regards to possibly providing assembled PCBs in future when necessary. I don’t want to do this as a matter of course, because I think it side-steps an important part of hobby electronics; it’s mainly when we can’t avoid using chips that most people would struggle with soldering. Inevitably, more and more new chips only come in packages like QFN and BGA. That’s simply the way that electronics is heading. In summary, I like the idea of having boards professionally assembled, but we must proceed with caution. There is still a lot that can go wrong, as this experience demonstrated. by Nicholas Vinen 9 Kendall Street, Granville NSW 2142 2 Silicon Chip Australia's electronics magazine siliconchip.com.au