Upsidedown chip problem - Page 1
Forum section: electronics / projects

author: naxxfish
author_rank: Contributor
timestamp: August 31, 2015, 04:24:33 pm
content: So, this is a tad embaressing...Currently doing a project where I'm using some MAX3250's to get some isolated RS232 down a "long" bit of wire to some "remote" boards to send them some commands. They're a SOIC package, 28 pins.Unfortunately, when I made the footprint for the part in KiCAD, I inexplicably numbered the pins in a mirror image (i.e. pin 1 is top right instead of top left).Bugger!What I'm trying to work out now is what is the best way to fix this .... I've attempted to flip the chip upsidedown, bend the pins down and solder them, but I'm not sure how well that's working out.Pretty sure there's some shorts hidden under the pins...Is there a better way of doing this? Would using mod wire and have the chip floating be better? Bit worried I might end up breaking the wire-bonds in the chip with all that bending, too. I've got about 12 of these chips all with the borked footprint to sort outThanks....

author: JVR
author_rank: Regular Contributor
timestamp: August 31, 2015, 04:39:00 pm
content: SOIC should handle that just fine. Just do the bending properly the first time. And take case when soldering. I'd also suggest straightening the pins before bending them the other way round.

author: Siwastaja
author_rank: Super Contributor
timestamp: September 01, 2015, 06:22:51 am
content: You are doing it right. I'd suggest using a small delicate bench wise: insert the chip fully and straighten all the pins on one side on one go; then remove the chip and insert it partially, to bend the pins at once. Solder bridges shouldn't be any more of a problem than they normally are. Just solder like you always should. Once you get into this, it takes 15-30 seconds to straighten and bend the legs and the chip is ready to be used like normally. PCB re-spin avoided.Bodging with small wires is HELL and you'll never want to do it, especially with a part with this many wires.No need to be afraid of damaging the chip. The legs protrude quite deep into the plastic before they are bonded.

author: naxxfish
author_rank: Contributor
timestamp: September 01, 2015, 11:35:39 am
content: This is good to know. Board respin not really an option here, so it was really the only solution. Those ICs are quite pricey, since they're nicely isolated RS232 (although a bit cheaper than an equivalent performing circuit with optocouplers, all told), so appreciate the reassurance that I'm not Doing it Wrong.I've done a pin-swap with modwire before, on a QFN - but never a complete inversion. Never made such a stupid mistake before!I found an interesting idea on the Arduino forums where this was done on a single sided boardhttp://forum.arduino.cc/index.php?topic=96381.0- where they routed out a hole and dropped the chip in upsidedown! Also reduces the height, as a nice side effect. Not an option for me, though, as I've got traces under the chip, but something worth remembering in case board height clearance is a problem!The one thing I'm going to have to be careful of is getting those pins re-formed nicely - as in the picture above I still had a bit of the second bend in the pins so they're not sitting quite flat on the pads. Maybe I can make a small form out of wood or something that I can put over the chip and press down on to flatten them out. Then again, maybe a decent straight-edge would do the job just as well!Cheers!

author: naxxfish
author_rank: Contributor
timestamp: September 08, 2015, 03:54:02 pm
content: As it happens I tried working with this but I couldn't form the pins well enough for it to work well. They ended up keeping their shape a bit and didn't sit flat on the pads well enough to solder solidly, so I ended up going for a board respin as there were a few other niggles that I wanted to clear up anyway.

author: C
author_rank: Super Contributor
timestamp: September 08, 2015, 05:53:21 pm
content: Instead of working at soldering one pin at a time, think of many pins at a time.If you take some bare wire and wrap it around a bolt as a form you can get a nice spiral.Solder spiral to board pins.Set chip on top of spiral and solder.Cut out chunk of spirial away from chip edge.You now have a C shape section of wire connecting board to chip pin.C

author: ajb
author_rank: Super Contributor
timestamp: September 08, 2015, 06:10:20 pm
content: I believe Lindstrom have pliers designed to form SOIC leads, or at least I seem to recall seeing them in a catalog. Not sure if they're a stock item. Depending on where you get your boards made, a respin may well be the cheaper option, though.

author: Howardlong
author_rank: Super Contributor
timestamp: September 08, 2015, 06:19:18 pm
content: For some unfathomable reason, sometimes chip manufacturers give the pinouts from the bottom of the device rather than the top. TI does it sometimes on QFNs. I've never figured out quite what they are thinking.Here's an example.www.ti.com/lit/ds/symlink/tlv320aic3254.pdf

author: ciccio
author_rank: Frequent Contributor
timestamp: September 08, 2015, 08:03:36 pm
content: Maybe because QFNs pins are not visible from top...I understand that this may be an inconvenience, but I learnt to check twice, after designing my first board at work (around 1977) with tape and stickers, and discovering that I mirrored all the ICs, so I had to solder them on copper side...Best regards

author: Howardlong
author_rank: Super Contributor
timestamp: September 08, 2015, 08:17:57 pm
content: But why the inconsistency?

author: sunnyhighway
author_rank: Frequent Contributor
timestamp: September 08, 2015, 08:27:15 pm
content: Glad to see I'm not the only one to make these kind of silly mistakes.Luckily, I had no traces underneath the center of the chip on the pcb.This is my personal recipe for these kind of mistakes:Ingredients:- 2 TSSOP28 to DIP conversion boards- 2 14 pin pin-headers- 2 zebra strips cut to size.- smallest bolt and nut i could find.- Optional: 2 isolation rings.- Optional: 1 bottle of premium beer.Procedure:- Put the beer in the fridge.- Drill a hole in the pcb in the dead center where the offending chip should have been.- Drill a hole in the dead center of 1 conversion board.- Solder the pin headers on the conversion board with the hole in it.- Place the zebra strip on the pads on the pcb.- Carefully bolt that conversion board to the pcb (wedging the zebra strips) (Add to taste or desire: 2 plastic or nylon isolation rings)- Solder the chip to the second conversion board.- Solder the second conversion board to the pinheads of the first conversion board.- Get the beer from the fridge, open it and celebrate your victory.

author: Bud
author_rank: Super Contributor
timestamp: September 08, 2015, 08:29:39 pm
content: You can't beat Minicircuits who marks with the dot the Last pin on Some of their rf transformers. Go figure.

author: SL4P
author_rank: Super Contributor
timestamp: September 09, 2015, 01:31:48 am
content: Yeah... a lot of us have done something similar!I used Lindstrom flat blade jewellers pliers to flatten/straightenallthe pins out to the side of the package - then to re-shape them in the other direction - to roughly the same bend dimensions.Worked perfectly on 100 boards x 2 chips per board!it took a while, but if you all the chips through each step (less tool &I work area change so) it runs pretty smoothly.