Magic Smoke on First SMT PCB - Page 1
Forum section: electronics / beginners

author: CharlesK
author_rank: Contributor
timestamp: January 04, 2021, 05:00:40 pm
content: Hello everyone,Today I received my first SMT PCBs! So exciting! Unfortunately, it let out the feared magic smoke. I received 5 pcbs, first one I added a micro usb port and when it smoked I assumed it was my shoddy soldering that shorted something. Next try I connected the wires to separate pins on the board to prevent shorting, still no luck. got the smoke again. Third time decided to film it when it happened. Now I can see where it went wrong!https://youtu.be/UOIiwplJGpMThe component that burns is a ESD5Z7.0T1G - LCSC Part # C133093https://lcsc.com/product-detail/TVS_ON-Semicon_ESD5Z7-0T1G_ON-Semicon-ON-ESD5Z7-0T1G_C133093.html/enAccording to the (ESP32) BOM it should be a LRC LESD5D5.0CT1G - LCSC Part # C383211https://lcsc.com/product-detail/Diodes-ESD_LRC-LESD5D5-0CT1G_C383211.htmlThe part was changed due to it not being available during manufacturing.The part is located between usb VCC and GND.I'm no expert but looking at the datasheet, it seems do be a slightly different kind of component?Is there a way to get one of these boards to work? I only have 2 left!Thanks, regards!

author: Manul
author_rank: Super Contributor
timestamp: January 04, 2021, 05:20:22 pm
content: I will not hide that I giggled a little. Seriously, do not burn the last two. Even one burned board is enough to start serious investigation. Do no believe in luck.The replacement TVS is unidirectional and obviously they put it the wrong way. So you may just remove it or change direction.Did you design this board yourself?

author: Manul
author_rank: Super Contributor
timestamp: January 04, 2021, 05:27:23 pm
content: Or you are powering it with reverse polarity or too high voltage.

author: CharlesK
author_rank: Contributor
timestamp: January 04, 2021, 05:32:20 pm
content: Quote from: Manul on January 04, 2021, 05:20:22 pmThanks for the quick answer! I did design this board myself. Will desoldering and turning it 180 degrees work? There are three of these components on this board. How do I know if the others are or aren't placed in the wrong orientation? On future boards I'll make sure to panic when the first one starts to smokeAlso, the power should be fine. It's from a usb port on my pc that I always use for esp32 boards.

author: CharlesK
author_rank: Contributor
timestamp: January 04, 2021, 06:04:40 pm
content: Quote from: Manul on January 04, 2021, 05:27:23 pmI removed the three diodes in question (of a fried board) and tried it again. It works perfectly now. Connects to pc and is recognised. I am even able to upload a sketch to it! time to find out if the rest of my design works as planned...Also I should figure out what those components were supposed to do...Thanks Manul, you're a genius!

author: Siwastaja
author_rank: Super Contributor
timestamp: January 04, 2021, 07:01:04 pm
content: I'm really stunned (don't know whether in a good or in a bad way, probably both) that in 2021, beginners order fully assembled boards based on a random schematic entry with absolutely no prototyping!You know, just some 2 years ago that used to cost around $500 and 6 weeks typical.Even now, it still costs many orders of magnitude more time and money than it needs to be.In other words, prototype! For prototyping to be efficient, you need to solder, use lab power supply (preferably with current limit) and use basic measurement instruments (a $10 digital multimeter carries you far). For simple designs (a few dozen components, without complex BGA/LGA packages), you often can work without a PCB at all to test if your ideas / circuit design behave in real world, so that once you order a PCB you have most of the things ironed out.

author: CharlesK
author_rank: Contributor
timestamp: January 04, 2021, 07:11:43 pm
content: Quote from: Siwastaja on January 04, 2021, 07:01:04 pmPlease don't get me wrong. I've been developing this board since april 2020. I've made many iterations using different components. I've had many pcb's made before however I would always solder existing modules on to these pcbs using header pins. This is the first time getting SMT service. I transferred the schematics of the modules onto a single board and combined these. I must admit that it was a total shot in the dark to wether it would work or not. But as you say, it is cheap enough now to give it a shot.Thanks for the replyFor the ESP32 I used the official Espressif BOM, schematic etc. for the other modules I used easyeda user created module scematics and double checked them with my multimeter on the real modules to see if they were drawn correctly.

author: rstofer
author_rank: Super Contributor
timestamp: January 04, 2021, 07:19:56 pm
content: Quote from: CharlesK on January 04, 2021, 05:32:20 pmI would NEVER risk my PC powering up an unknown board. At a minimum, I would use a powered USB Hub. Better still, a lab supply where I can measure the board current and, more important, limit it to some reasonable number. May 50 mA or 100 mA depending on what's on the board. This will usually prevent the smoke from escaping. If the power supply sees an overcurrent, it will fold back the applied voltage.

author: Manul
author_rank: Super Contributor
timestamp: January 04, 2021, 08:32:48 pm
content: Quote from: CharlesK on January 04, 2021, 05:32:20 pmWell, it is possible that they put them in random orientation, so all of them need to be checked for correct orientation. Some of them might be correct, so don't just flip them all. Unidirectional TVS will have a diode forward voltage drop (~0.6V) one direction and rated breakdown voltage (Vbr) the other direction (similar like a standard zenner diode). Vbr (7.5V in this case) will be for sure out the range for multimeter, so it will measure open. Also, being unidirectional, they should have cathode marked. So the orientation can be checked both with multimeter and visually. Obviously you do not want power supply voltage to be clamped by 0.6V, so you can deduce the correct direction easily.On the other hand, these diodes being 7.5 Vbr are a questionable benefit for your circuit. You might just remove them all. They will not protect much in my opinion. If you want to protect from 5V power rail overvoltage, then choose a 5V TVS and a series fuse. In other places (like I/O) it migh be better to use the classic schottky diodes going to power supply rails.

author: wraper
author_rank: Unknown
timestamp: January 04, 2021, 08:56:40 pm
content: Original part was bidirectional TVS diode which can be placed in any orientation. Replacement is unidirectional TVS diode for which orientation is critical.

author: thinkfat
author_rank: Unknown
timestamp: January 04, 2021, 09:04:27 pm
content: The good news is that you learned an important lesson with no real damage done. Just make sure to correct the orientation of the remaining parts. The boards should still be OK.

author: langwadt
author_rank: Super Contributor
timestamp: January 04, 2021, 09:26:00 pm
content: Quote from: Siwastaja on January 04, 2021, 07:01:04 pmor spend time carefully reading the datasheets, review and practice "defensive design" iow add a a few zero ohm resistors wherein doubt and get a usable first item instead of wasting time trying to do a mockup with smd parts that might even have disappointing performance unless mounted to a proper pcb