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I am driving the enable on a boost regulator and the boost is not turning on. When I checked the voltage on the enable, it was 2.5V. The boost worked on an identical set of boards I tried before this (I didn't measure it then). Is this likely a cold solder joint? I used flux.

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    \$\begingroup\$ Have you checked for shorts? It is possible that the regulator has a short somewhere and as a result is drawing unusually large currents from through the enable pin - this would explain why the io pin voltage would be dropping. \$\endgroup\$ May 4 '16 at 2:38
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    \$\begingroup\$ How are your measuring the voltage - with a meter or with a scope? Ie, are you sure it is a steady 2.5v, and not something more interesting in the time domain? \$\endgroup\$ May 4 '16 at 3:41
  • \$\begingroup\$ I cut the wire from the GPIO to the boost regulator. It still shows 2.5V, so a problem with the micro. Current draw is constant at 14ma at the battery, so I don't think it is a short at the regulator. Initially SPI didn't work on this micro board so I reflowed it with the hot air gun and it began working. Somehow the reflow wasn't good enough to fix this GPIO. \$\endgroup\$
    – BSEE
    May 4 '16 at 22:16
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If the issue only happens on a single board from a set of identical ones, then the issue could be a bad ic or bad soldering. Reflow the pins of the IC, check for shorts at the regulator and test again.

Slight differences in ICs may result in some not responding exactly the same. Check the data sheets for the minimum/typical/maximum input voltage level needed to turn it on, and check all boards to see if they meet this.

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It turned out to be a bad reflow. When I removed the part (a 28QFN), I saw pins had sunk in the solder but not wetted (stuck to the pins). I reflowed a new micro and made sure it was really hot and now my boards work. You can kind of see it in this photo, but the pin in the upper left corner on the left side.

enter image description here

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