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  • Impedance control on PCBs

    Dear Members!

    I would like to ask your opinion about impedance control on PCBs. I am hardware developer and I design high speed PCBs at a big company where we have more high speed interfaces on the boards, like oLDI, HDMI, FPD link, APIX, LPDDR4, MIPI CSI etc. Until now we did not use impedance control on our PCB-s. Our maximum datarate is currently 3Gbit/s.


    I think that impedance control is very important but most of my colleagues say that until now there were no problems then why should we use...
    What is the border where we should begin to use impedance controlled pcb-s? Do you know with what kind of tolerance pcb manufacturers work with and without impedance control?

    I know that these problems could occur:
    - video distortions (pixel errors and synchronization gaps)
    -- data interfaces will have transfer gaps, bit errors
    -- DRAM errors
    -- significant increased EMC (impedance jump -> causes reflections -> causes emissions)
    -- Impedance jump from Chip-out to trace and on Board-to-Board routings

    The problem is that I have never seen such problems in our projects (we sell from one product 300-400 thousand pieces / year) and I do not know how I could prove that without impedance control we are bad.
    And we use higher and higher datarates. For example we use in our new projects LPDDR4 instead of DDR3, where we have instead of 533MHz a clock frequency of 1600 MHz, so rise and fall times are also much shorter. That's why I am thinking that maybe the time has come to use impedance control

    What do you think about this topic?
    Thank you for your answers in advance!
    best regards,
    Andras

  • #2
    Hi,
    Was the PCB's designed by you? OR You're a part of the hardware design team?
    I hope the below-attached links cover some of your questions :
    https://docs.toradex.com/102492-layout-design-guide.pdf
    https://www.youtube.com/watch?v=JNu-1V4KYPo
    https://www.youtube.com/watch?v=ZyoqhJB_E9Q&t=1336s

    Manufacturer's usually keeping a tolerance of +/- 10%.

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    • #3
      andras_t that is very interesting! I will ask others what they think about it. Maybe your boards are small or the impedance of tracks is still close around 50OHMs? What I would be more interesting to know, if they do not care about impedance, does it mean, they do not care about stackup? And what about length matching?

      PS: Personally I would go for impedance controlled PCB. I always try to design as good as possible.

      PSS: When something is wrong, most of the failing I have seen were visible:
      - in the environmental chamber (when temperatures were changing)
      - in burn in test (when whole board was running under a very high stress tests - all interfaces running at full speeds and loads)
      - in long run test (boards running for months)

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      • #4
        In theory I would say that if you know the (estimate) the final thickness of the prepreg and core layers, you could calculate the trace width.

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