Burning Teensys through USB cable

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I have had a tough day with several Teensy boards. I don't know what's going in on in my workflow today but I think I just fried a Teensy 3.2 and another 3.6 just by soldering the pins. I used very thin solder and it's not the first time I've done this, it looks fine. I plugged them to the computer via the usb cable and the led started to blink, then I soldered the pins and did the same thing. The led blinked for a moment and then stopped. No way to get them back on Teensyduino. What could I have done wrong? What's the best way to check if the boards are totally gone?
 
From what I worked the other day and some recommendations Paul S gave on some post, you can press the reset button a while so it gets totally resetted so you can then upload your code again... it blinks the red led a little and it then appears again on the port options on the general Arduino IDE...

But I also want to know how to test if there are issues with soldering or else on a teensy a + request on this!
 
I will sometimes use a multi-meter and check things like does there appear to be short between VIN/VUSB and GND?
If you plug it in to USB measure is there 5v going to VIN? Is there 3.3v on the 3.3v pins...

Also sometimes when I am planning to use external power I cut the VIN/VUSB jumper on bottom and forgot that I did so and nothing happens when I plug it in to USB.

And as mentioned I at times will plug in the board while holding down the program button. Also as Frank mentioned I try a different USB cable. I actually have a few Teensy boards that don't always make proper contact with some cheaper color coded USB cables I purchased recently. Works with most of them, but some don't like them...
 
I just measured the voltage between GND and all pins with the Teensy connected to the computer throught the USB cable. It gives me 4.9V between GND and VIN, all others are below 3.3V. I also tried to connect the cable with the button pressed but it doesn't make any difference, no LED is blinking.
 
Teensy is sensitive to shortcuts on conductive surfaces, ESD pad, metallic workbench, .... a bare board needs to sit on a non-conductive surface, especially with pins soldered.

Teensy are sensitive to ESD, that is if you touch the Teensy while plugging the USB cable, a discharge can happen and destroy an arbitraty part of the chip. So use ESD protection.

Use another USB cable. I remember I had some of these cn specials, that once worked and many times not. Even a Samsung phone cable had this behaviour. Recycle.

Did you unplug the Teensy before soldering?

Did you use an ESD-proof soldering iron? Absolutely not a solder gun with that hot wire bow, that is directly connected to voltage, these kill a micro controller on the spot.

Soldering: Use a solder wire that is thick enough for the job. The thicker (within reasonable limits of course), the faster you get enough solder to the spot, shortening heat impact time and minimizing risk of overheating. With fine soldering wire you need several seconds to feed enough solder, wasted time and risky as well.

Try to recover the Teensy:

Unplug, hold the Program button and plug in USB, keep holding until blink comes up again.
 
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Thank you all for your replies. I don't think it's the cable, I have tried several already and different ports. ESD might be the culprit. I used a TS100 which was not grounded, and I unplug the Teensy before soldering. However, does it make any sense that the boards worked for a little while when connected to the computer, and then failed? I have another board, the last one, which I will solder now with a ESD-proof soldering iron. Should I solder on a ESD pad or not? Thanks!
 
If the power supply to the TS100 is grounded, then the TS100 is as well. The tip itself is floating, so there should be no conductive path, except for the capacity to the heater (about 100nF). The TS100 itself can not be the issue, as long as the power supply is grounded correctly, or, if it is floating, connected to the ESD protection. Unfortunately, many switching power suppllies generate stray A/C charges on their outputs, that could break though to the soldering tip.
That's what the grounding connector at the TS100 is for!

Here is a good thread about the TS100 and power supplies and ESD:
https://www.eevblog.com/forum/reviews/ts100-doesn_t-function-properly-when-earthed/

Some tips for ESD safer workspace:
https://www.ifixit.com/Wiki/ESD

About the failure after a short runtime:

Could it be a thermal issue? That is, have you selected a higher CPU clock than default? In the overclocked setting, a Teensy can crash once it overheats.

There has been an issue in older Teensyduino where some strange effects happened, but that is resolved long ago. Have you installed Arduino 1.8.13 and the latest stable Teensyduino 1.53 and compiled and flashed the blink from that?
 
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