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Thread: Can I "re-bake" Teensy 4 to surface-mount it on my PCB?

  1. #1
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    Can I "re-bake" Teensy 4 to surface-mount it on my PCB?

    What will happen if I make a cutout in my PCB for the bottom components in the middle of T4 and use pads instead of the castellated cutouts?
    Then I'll put it in one of these: https://www.amazon.com/Happybuy-Infr...3596831&sr=8-2
    Will all the good things fall off of the poor Teensy or will they hold?
    What if I use the low temperature paste?

    If it works, it will be much cheaper and easier than the castellated cutouts which nobody makes or charge a fortune.

    Did anybody try that?
    I really need the SD pads, the USB host and all those bottom pads.

  2. #2
    Senior Member+ defragster's Avatar
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    Not sure - but I assume with T4 having dual side components - one side is already low temp pasted for initial build?

    You've seen this thread - Is-there-a-third-Teensy-4-0-breakout-board
    @KurtE should be getting a set from OSH soon to see if the cuts all worked. He made a board expecting these to work.

    loglow WIP soldered cable and pins is one way and the Tindie Ultimate breakout uses spring pins as another way.

    In some few months a new T_3.6 sized version of a T4 will have much of this done ...

  3. #3
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    Yeah, I know all this.
    Can't wait a few months and i need it to be small, and with sd card on the same side as usb.
    I think I'll just risk $20 and try it. Going to order the first Pcb next week and get the oven. I need it anyway.
    And i think the paste is sticky enough to hold those tiny parts, even if melted. We shall see.
    But thanks for the reply, as always.

  4. #4
    Senior Member PaulStoffregen's Avatar
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    Quote Originally Posted by Andy Belov View Post
    Will all the good things fall off of the poor Teensy or will they hold?
    What if I use the low temperature paste?
    It all depends on your temperature profile and how you handle the boards.

    When Teensy 4.0 is manufactured, the back side parts are placed first and reflowed. Then the top side parts are placed and reflowed. During this 2nd reflow, the bottom side parts do stay in place despite gravity and re-melting the solder.

    So if you use a lower temperature paste and handle the boards carefully, I'd say odds are pretty good you may be able to get it to work.

    Would you be willing to share your technique? How about writing a brief article with photos and specific info like the type of paste, equipment used, temperature profile, and other key info? If you're willing to share, we could arrange to send you a handful of defective Teensy 4.0 boards (which have all the parts on both sides) so you could test & refine your technique without risking loss of good Teensy 4.0s.

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    Not clear that there is a problem that needs additional solutions, but I have an unproven theory that applying a silicon conformal coating would help.

  6. #6
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    Quote Originally Posted by PaulStoffregen View Post
    It all depends on your temperature profile and how you handle the boards.
    Thanks, @PaulStoffregen

    This is the first time I do the baked SMD. I'm getting this today: https://www.amazon.com/Happybuy-Sold...xpY2s9dHJ1ZQ==

    What paste and temperature profile should I use?
    How do I apply the paste? Any suggestions will save me time and money.
    I have no idea what I'm doing. I just want to fit a million things onto a 9x6cm board.

    I promise to let everybody know how it worked.

  7. #7
    I've only used it a little bit and not recently but I have a similar machine and you'll definitely want to use some junk boards to calibrate the heating profile with your solder. In my experience none of the built-in profiles seemed very accurate to the listed temperatures but a custom profile of the correct "shape" with the temps modified worked ok for me. I also know there has been work on 3rd party firmware for the machine, but I have not installed it so can't speak to whether it's improved enough to make it worthwhile to install.

    I used https://www.oshstencils.com to get stencils and was happy with them, but I'm sure there are many alternatives or you may be able to get one directly from your board manufacturer.

  8. #8
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    Thanks @ecurtz.

    The oven arrives on Monday. I'll do all the modifications.

    For the stencil, these guys offer it with the boards for $7: https://jlcpcb.com/
    Each board will cost me $0.49
    A week to get made and delivered from China to New Jersey, for only $17 4 day DHL. Absolutely amazing.

  9. #9
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    It worked! At least the parts I tested.
    Got the first PCB with SMD parts in my life. All designed by myself.
    Got a bare Teensy put some paste on the PCB, shoved them in the oven.

    First time - too much paste, got silver balls all over the place, a few shorts. Put it back into the oven, opened it at the highest temperature and pulled the Teensy off of the board.
    Cleaned the pads and put it back on the board with no additional paste. Took it out, put the USB host connector, no capacitors around the current limiter - they arrive tomorrow and I couldn't wait.
    Connected a MIDI keyboard and getting this in the serial monitor when I press the keys: Note On, ch=1, note=72, velocity=64

    I can't believe it. The Teensy didn't even cough after I re-baked it. When I pulled it off of the hot board with melted paste, it flew around the tray, but nothing happened to it's components, I checked.
    The oven goes up to 180C at the highest. This is crazy. I can add and remove components now as I pleased.

    Make a few stupid novice mistakes on the PCB, but nothing that stops me from going on with it for the first round.

    Click image for larger version. 

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  10. #10
    I was following this with a vested interest, as I thought you were going to take the Teensy to 250C.
    I'm not 100%, but I think I cannot legally use 180C solder if I intend to re-sell my assembly.
    If anyone is baking teensies at 250C on a regular basis, please shout out and let me know.

  11. #11
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    Quote Originally Posted by Darcy View Post
    I cannot legally use 180C solder if I intend to re-sell my assembly
    Also, please shout out if you know why you can't rely on a low-temperature paste for production.
    The actual melting temperature of the paste is 137C. It's just the oven's lowest stock profile goes up to 180C, measured with an external thermometer, even though the oven says it's 160C.

    The only thing I saw so far is the paste is not good for high temperature devices. I tried to pull off some of the components I soldered so far, including Teensy, and they stayed. I'll try again harder to see what it takes to break the solder.
    If you don't bend it too hard, I don't see why you can't rely on it.
    Looks like metal, smells like metal...

    Please, anybody who noticed that low-temp paste is not good, please let us know.
    No superstitions or suspicions please, bare facts.

    I just found this: https://smtnet.com/library/files/upl...muth-alloy.pdf
    At the bottom:
    Conclusions
    Based on the tests conducted during the evaluation the following was determined:
    1. Sn58Bi, Sn57.6Bi0.4Ag and Sn57Bi1Ag solder pastes show good printing and reflow performance over the variety of
    components tested.
    2. The tin-bismuth solder pastes were found to have good head-in-pillow performance and were acceptable for pin-in-paste
    soldering.
    3. Voiding studies on power transistor components showed low voiding with the developed tin-bismuth pastes with minimal
    effect on voiding from silver additions to the tin-bismuth pastes.
    4. Paste durability studies showed good results over the 5 day print and reflow testing for the developed tin-bismuth paste.
    5. Pull and shear testing data for tin-bismuth soldered components were equivalent or better than Sn3Ag0.5Cu and Sn37Pb
    soldered components.
    6. There was minimal differences in pull and shear testing of Sn58Bi, Sn57.6Bi0.4Ag and Sn57Bi1Ag soldered components.
    7. Cross-sectional analysis of Sn58Bi, Sn57.6Bi0.4Ag and Sn57Bi1Ag soldered components showed good bonding to the
    board and component interfaces.

    Looks good to me.

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