I did an install of a 3.9 in a custom chassis. I tested it first with only USB. Then I tested it with a battery, everything seemed fine. But once I pushed the board and attached it with kapton tape and inserted, it sounded like it had a burn speaker. I took it out and saw some actual smoke. But I haven’t been able to find were. I didn’t saw any obvious short, inspected the solder and seemed right. Any help would be appreciated on how to troubleshoot a short like this.
That would mean a voltage inversion. The battery was well installed. Where could I have a short to blow it? I don’t want to connect it to the battery to avoid any further damage. But it is hard to test the board without powering it. Any suggestion?
No, voltage inversion should be safe.
Q7 damage is usually caused by a short.
Or trying to charge when the battery is hooked up backwards.
Definitely a short, then. Could an excess of solder cause a short? I find it very strange that it was smoking.
If the solder touches something else, sure.
I’m sorry, I meant solder flux. I should try to find the short with the tester. I will ask about normal resistance when I take the measures.
Flux is non-conductive, so it shouoldn’t cause any immediate problems.
Flux is also corrosive, so it’s a good idea to clean it off, because it can cause problems if left for a long time.
I did cleaned it with some absorbing paper. But may be some was filtered through the holes. I’m absolutely baffled. It’s not the first board I break. Is the first I can’t understand where I made the short. 
Burning a board when you start, is tolerable. When you supposedly have experience, not so much. Like the previous one I broke, was because I crossed the D+ and D- in the USB. Apparently some color codes use green for USB D+ and some for D-. It sounded broke. But I have to test it thoroughly. Now I’ve learned that the usual arrangement in the connectors is V D- D+ G. 