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Thread: killed a Teensy 4.0 because of PCB layout "bug" -Suggestion-

  1. #1

    killed a Teensy 4.0 because of PCB layout "bug" -Suggestion-

    Hello Guys,

    i brought my first Teensy4.0 to make my lasercutter faster (using grblHAL).

    I made an adapter to connect the Teensy to a common Arduino Uno compatible CNC shield.
    This made it nessesary to cut the 5V usb power pads apart.

    I did this with a X-Acto knife like i do on all my pcbs. I even measure that i did indeed cut "enough" to break the connection.

    I supplied 5V externally and used the USB connection from my PC.
    Everything went fine until i started to program the board.

    The programming application crashed and displayed that i needed to press the programming button.
    I did but nothing happened. No led, no blinking, no USB connection.

    I read posts thinking i bricked the teensy because the firmware didnt upload right. Tried the 15s program button press but nothing.

    I started measuring voltages. 5V where ok, core did show 0,95V which seemed right. However, 3,3V measured at 5V what?
    I checked every connection thinking my external stuff did inject 5V into a wrong pin but no. Without anything connected, the 3,3V still measured at 5V.
    After close inspection, i came to the conclusion that cutting of the jumper (5V / 5V USB) connected the 3,3V layer thats directly underneath the 5V jumper to one pad of the jumper.
    A little sliver of copper that the blade pushed into the layer below made contact to both the pad and 3,3V layer.
    This injected 5V USB into the 3,3V rail killing the teensy in the process. (does not talk anymore, so i assume its dead)

    I am a PCB layouter myself, i work with pcbs daily. Accidentally cutting too deep can happen to everybody.
    These layers do not have much seperation (the board has many layers most likely 8).

    I would suggest that the layout gets modified to change the layer below the "cut-area" to ground. (assuming there is space to do that)
    No need to change the whole layer flood fill from 3,3V to GND. Just a little area below the jumper.
    Adding GND would just produce a short. (in case of a break trough that happed on my teensy) A short would trip USBs over current protection or would bog the voltage down but not kill the uC.
    If someone did have something with enough current supply capability connected, it would likely just burn the sliver of copper away. (he should use a fuse anyway..)
    I could miss something but I find ways that a ground fill would help, not hurt.


    I am quite sad that this happened as i will not have the funds to buy a new one anytime soon. It was also quite a bad user experience for my first Teensy.
    Sure, its my fault, i should have measured the resistance not only to 5V but also to 3,3V.
    I have also learned something: It seems to be good design practice to keep the area under a user cutable jumper clear of any copper or expect a short to the lower layer. Especialy if the jumper is placed on a dense multilayer board. I will certainly employ this in my work and hobby projects.


    BR
    Steph

  2. #2
    Administrator Robin's Avatar
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    We would like to replace your Teensy. I've sent you an email to the address you used to register with the forum.

    -Robin

  3. #3
    Senior Member PaulStoffregen's Avatar
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    This suggested change has already been made on Teensy 4.1. The next batch which is entering production in a couple weeks with have this change to its inner layers.

    I'm also planning to do the same on Teensy 4.0. But we have to wait for the next batch, which will probably be made in a few months.

  4. #4
    We would like to replace your Teensy. I've sent you an email to the address you used to register with the forum. -Robin
    Quote Originally Posted by PaulStoffregen View Post
    This suggested change has already been made on Teensy 4.1. The next batch which is entering production in a couple weeks with have this change to its inner layers. I'm also planning to do the same on Teensy 4.0. But we have to wait for the next batch, which will probably be made in a few months.
    PJRC is amazing.

  5. #5
    Wow!

    i did not expect this response. Thats customer support right here.

    Did anybody else experience this issue so you implemented this change already or did you just add this change based on my post?
    Seems way too fast to be true .

    Quote Originally Posted by Robin View Post
    We would like to replace your Teensy. .. send a mail ..
    I registered with a one time e-mail to avoid spam. (i experienced leaks before so i use one time emails on every forum to avoid spam)
    I will send you a PM with a valid mail adress.
    Thanks for offering this! I appreciate this!

    BR
    Steph

  6. #6
    Administrator Robin's Avatar
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    Quote Originally Posted by grblHALluser View Post
    Wow!

    i did not expect this response. Thats customer support right here.

    Did anybody else experience this issue so you implemented this change already or did you just add this change based on my post?
    Seems way too fast to be true .

    I registered with a one time e-mail to avoid spam. (i experienced leaks before so i use one time emails on every forum to avoid spam)
    I will send you a PM with a valid mail adress.
    Thanks for offering this! I appreciate this!

    BR
    Steph
    The change has been in the works for a while - on a list of minor improvements to the board. PCB changes take some time to implement with manufacturing and production lead times.

    email me directly about the replacement - robin at pjrc. I tend to not check the PMs very often.

  7. #7
    Quote Originally Posted by Robin View Post
    The change has been in the works for a while ...PCB changes take some time to implement...
    Thats what i was thinking.

    I send you the mail.
    Thanks again.

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