Author Topic: PCB Capacitor Burnout  (Read 728 times)

0 Members and 1 Guest are viewing this topic.

Offline ipek.grgcTopic starter

  • Contributor
  • Posts: 15
  • Country: tr
PCB Capacitor Burnout
« on: June 20, 2020, 09:48:19 am »
Hello, hope everyone is doing well.

I have designed an STM32F1 board variation. I have multiple power sources. One of them is USB 5V and the other is JTAG 3V3. 5V is converted to 3V3 through a MIC5205-3.3YM5 regulator with a 2.2uF bypass cap on its output. The thing is, when i plugged the JTAG connector and powered the circuit from JTAG 3V3 input, after a little while, the 2.2uF bypass cap of the regulator started to smoke. Then i checked, there was short circuit. I tested again after 10 mins, and the short circuit was gone. Then i plug the USB this time, and after a while the regulator input pins started to smoke and there was little sparks. I measured the reg output, it was 3.3V then there was a short again. Now i reflowed the reg and caps again (thinking maybe there were a tiny solder paste leftover would have been stuck in between the legs) and plugged the USB back. I am measuring the reg output and it is currently 1.8V, no smoke or sparks. And when i unplug the USB and plug the JTAG 3.3V again, the same cap starts to smoke again, creating a short. I am lost, i can't find the reason behind this. I am attaching the reg schematic and PCB layout. I am happy to hear any ideas, thanks!

 

Online Siwastaja

  • Super Contributor
  • ***
  • Posts: 9337
  • Country: fi
Re: PCB Capacitor Burnout
« Reply #1 on: June 20, 2020, 09:53:02 am »
What is the capacitor body size?

Ceramic capacitor cracking failure is a possibility, especially if the cap is larger than 0805 (say, 1206 for example). Replace the cap. Make sure there is no board flex. Do not touch the capacitor body with the soldering iron, and solder the pads quickly (a few seconds per pad) without applying excessive heat.
 
The following users thanked this post: ipek.grgc

Offline mikerj

  • Super Contributor
  • ***
  • Posts: 3382
  • Country: gb
Re: PCB Capacitor Burnout
« Reply #2 on: June 20, 2020, 11:04:28 am »
What type of capacitor did you fit?  The datasheet for this regulator states that low ESR capacitors (e.g. ceramic) can cause oscillation.
 
The following users thanked this post: ipek.grgc

Offline ocset

  • Super Contributor
  • ***
  • !
  • Posts: 1516
  • Country: 00
Re: PCB Capacitor Burnout
« Reply #3 on: June 20, 2020, 11:18:25 am »
c20 and c19 are near board edge and may have suffered depanelisation damage. As discussed, replace and try again.
 
The following users thanked this post: ipek.grgc

Offline ipek.grgcTopic starter

  • Contributor
  • Posts: 15
  • Country: tr
Re: PCB Capacitor Burnout
« Reply #4 on: June 28, 2020, 10:04:36 am »
I replaced C19, C20 and the Regulator. Now everything works fine, maybe the problem occurred during the SMT assembly. Thanks a lot for the help.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf