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I built a board that combines a number of sensors, an ATmega2560 and a TPS62125. The board worked, I was able to measure 3.3V coming out of the buck regulator, and was able to program the ATtmega via ISP. Then, trying to fix another issue, I applied flux to all the pins of the ATmega and reheated them all with an iron, trying to clean them up a bit (it's a TQFP package, and I had some bridging that I originally cleaned up with a wick, but it left behind some crap).

After that, it stopped working. I can only measure about 12mV on the power rail now, but the circuit is still drawing 16mA @ 12.3V from the battery (which is about where it was before). The cause and effect seems pretty clear - I either created a short with the iron that I can't see now, or I fried the chip and there's an internal short. That said, is there anything else I can look at to verify this? I'd have thought that a short to ground would at the very least draw more power. The buck regulator is rated for 300mA so I'd expect a short to ground to draw the full watt or so (300 * 3.3V), but it's only drawing about 200mW.

I checked that the enable pin is high on the regulator (it's pulled up to the battery). Anything else I can look at?

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  • \$\begingroup\$ What kind of flux did you use? If not resin flux, it could be conductive and needs to be cleaned off carefully. \$\endgroup\$ – Olin Lathrop Aug 29 '14 at 13:44
  • \$\begingroup\$ "Rosin soldering flux" from radioshack. A quick google doesn't show whether it's conductive, I'll dig some more. \$\endgroup\$ – kolosy Aug 29 '14 at 13:50
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    \$\begingroup\$ Or likely a pin has lifted. Go over it with a microscope. \$\endgroup\$ – Passerby Aug 29 '14 at 13:52
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    \$\begingroup\$ Rosin flux is OK since it's a insulator. That's not the problem then. \$\endgroup\$ – Olin Lathrop Aug 29 '14 at 14:11
  • \$\begingroup\$ @Passerby - would a lifted pin cause a short though? \$\endgroup\$ – kolosy Aug 29 '14 at 14:50
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Flux is not going to cause that much of a short circuit. (1 Meg ohm between pins maybe.) I'd first look closely at all the joints and see if you can find a solder blob or some other thing that is causing the short. If that fails you can map out the very small voltage drop along the power trace (with a DMM) and figure out the approximately where the short is.

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  • \$\begingroup\$ Flux can absolutely cause these kind of problems, just not the particular flux the OP is using. Some types of "water clean" or "no clean" fluxes are quite conductive and need to be carefully cleaned off a board. We had problems with this when we first started doing QFN packages. It was enough to hold the reset pin low, and can be just a few 100 Ohms. Rinsing the board in clean water helped, but the fix was to remove the part, clean the board, then solder it back using solder with non-conductive flux. This is a very real problem. \$\endgroup\$ – Olin Lathrop Aug 29 '14 at 14:14
  • \$\begingroup\$ @OlinLathrop, Oh I've had plenty of issues with different fluxes.. mostly at the meg- ohm and above range. It's hard to see how a no-clean flux could leave a residue resistance that was ~1 meg ohm and still be useful to people. Now the water clean fluxes may be different... those need to be cleaned.. with water :^) But even if it was ~100 ohms, that's still not going to short out his power supply. \$\endgroup\$ – George Herold Aug 29 '14 at 14:37
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You could try washing the board in distilled water, then in pure iso-propyl alcohol (isopropanol). That should remove any flux that might be causing a problem.

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