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Thread: Teensy 4.0 PCB layout suggestion

  1. #1
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    Teensy 4.0 PCB layout suggestion

    Hi there... I have been working with Teensy 4.0 and it has been a dream, I have learned so much. But I digress..

    I have a project where I am using all the underside surface mount pads... it has been quite a challenge to attach to them via solder with a header that is reliable... but more specifically where I don't cause all kinds of maddening solder bridges.

    After building several PCB's I find that my consistent issue revolves around Pin 31, C17 and C29. When I attempt to solder onto Pin 31, I inevitably bridge between C17 and C19 which is a problem since they are so close together.

    Even with Pin 33 and L3, it becomes a problem.

    Do you think that in future versions, that you could potentially move these components a scoche out away from the solder pads?

    Thank you, I so much appreciate your consideration.

    Jess
    Fort Collins, CO

  2. #2
    Senior Member PaulS's Avatar
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    Yeah, understand your frustration.
    What I used to do, not specifically on Teensy, is to use a piece of kapton tape to cover the relevant components, like so:

    Click image for larger version. 

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    That usually worked out quite well.
    What also might help is to cut that specific pin of the header you are using, a tad shorter than the other pins.
    And use solder as little as possible. Normally I pre-tinned the pads only and soldered the header on the pads without any additional solder [flux will help].

    Paul

  3. #3
    Senior Member skpang's Avatar
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    To solder those pads I use a fine iron tip and 0.3mm solder wire. Works ok.

    The 0.3mm solder wire helps not to get too much solder onto the join.

  4. #4
    Senior Member PaulStoffregen's Avatar
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    Quote Originally Posted by mhavoc View Post
    Do you think that in future versions, that you could potentially move these components a scoche out away from the solder pads?
    Unfortunately the bottom side parts placement is tightly constrained by many factors on the other 5 PCB layers.

    C29, C24, C18 have critically important placement to minimize the loop area where the DC-DC switching current flows.

    C17 is meant to decouple the power for the flash memory chip, which is on the other side of the PCB from the pin 31 pad. Ideally it should be located where that pad is. It's already shifted off the center of the underside of the flash memory. But even if we wanted to move it farther away from the pin 31 pad, it just can't really go any farther away because that area has necessary vias needed to connect the flash memory signals. Those vias can't move much, because they're located in the area between the BGA chip and flash chip.

    Maybe C17 could shift up every so slightly, which would give a bit more gap between it and C29. But that would lengthen C17's connection to the flash GND pin.

  5. #5
    Senior Member PaulStoffregen's Avatar
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    Looks like I can move C17. It's not much, but the distance from C29 has always been so close than even a small change can make an improvement, which hopefully will make solder bridging less likely.

    I have a change planned for the pads on U4, just a very slight adjustment in pad side and connecting the unused pads to GND, which could let us (maybe) use an alternate voltage regulator if Diodes Inc can't deliver more AP7366. Parts we ordered many months ago are running late. We have only enough AP7366 remaining to last until about February (assuming NXP delivers enough IMXRT chips, of course). I believe we have PCBs to last until about April-May.

    So if I make this C7 position change (still not 100% sure, but seems do-able) and if we get the voltage regulators, the change to next PCB rev would come around April-May time frame after we use up all the PCBs already made. But if those voltage regulators don't come, and if we're able to get an alternate part (still a pretty big "if"... so far 3 alternate chips are under consideration) then the new PCB rev would likely come in February when we're forced to change by not getting the AP7366 chips as planned. But when/if those AP7366 regulators do arrive later in 2023, we'd have a couple months of using up today's PCBs. That is, unless some other part shortage also forces yet another PCB rev. But as far as I know, we have a good buffer inventory on all the other Teensy 4.0 part until mid-2023. Sadly the same can't be said for Teensy 4.1, with the SD sockets repeatedly delayed, so I have yet another PCB rev to consider...

    Maybe that's more than anyone wanted to know about our supply chain challenges. But on the C17-C29 solder bridge, I am seriously considering moving C17 slightly to increase the distance from C29.

  6. #6
    NO! Thanks for all that information! It is always good to get additional insights into what it takes to get from idea to production. I "play" with a number of different micro-controllers and electronics but [I suspect like a lot of "hobbyists"] have never ordered a PCB nor produced production products. I am always interested in what goes on behind the scenes and you being a small business have to personally deal with EVERYTHING involved. It is good to hear what you go through and what drives your decisions. I am professionally in the software development field and even ran my own software/network support company years ago so I have a [small] clue as to what is involved but what you do is an order of magnitude greater/more complex than my experiences. You going into detail on your situations and what drive your decisions helps everyone understand how your products get to where they are and where they are going.

    I would say keep up the sharing as much as you are willing!

  7. #7
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    Hi All.... thanks for all the responses and support on this.. Im psyched to try the Kapton tape idea on my existing Teensy production. What I ended up accidentally doing was bridging those two ends of C29 and C17 which unfortunately are GND and +3.3V... so I guess good in that it just shut down my USB power and didn't damage the Teensy. No matter how much flux and wick I used I couldn't get it out of there and it was too tight to put a solder sucker in there. (I had pin headers on the sides already too which made the exercise more challenging than it needed to be). In the end, I just had to take off C17 and then get the solder bridge off the pad of C17 and C29. I was able to get it eventually and keep C29 in place.. So I have a slightly degraded Teensy but this is my test device so it gets put through the ringer anyway.

    Paul, I appreciate all the insights for sure... always good for everyone hear all the challenges because things aren't a simple as design and go these days... I can only imagine the complexity of your 5-layer design!

    Cheers!
    Jess

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