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RF430CL330H variation with inductance

Dylan
Intellectual 1255 points
Other Parts Discussed in Thread: RF430CL330H

Hi Team,

Going into production with an NFC device using the TI RF430CL330H,  some variation with inductance on the board has been showing up. TI's reference board layout was used to design the board and there were no modifications to the loop antenna layout. Fine tuning the antenna's tuning capacitor has be figured out, however after the initial production runs there is some slop on the inductance.

Are there any recommendations for controlling the loop inductance during PCB fabrication?

 

How is the loop inductance controlled during the PCB fabrication for the RF430CL330H evaluation board?

 

Is it possible to get the original fab notes or PCB specs that have the instructions for the fab vendors for maintaining the loop inductance within some tolerance? It would be nice to see what those notes are so the same information can be used.

 

Thanks,
Dylan

  • 0
    TI__Mastermind 44625 points

    Hi Dylan,

    I have requested the PCB details for the RF430CL330HTB and will provide this when I receive it from the board house.  Generally, you need to be sure that the board is manufactured with the same PCB thickness and copper weight.  This should provide around +/-3% tolerance from what I have seen.  You could test this by measuring some random samples from a batch.  How much variation are you seeing on your boards?

  • 0 in reply to Eddie LaCost
    TI__Intellectual 1255 points
    Eddie,

    Thank you for looking into this. I look forward to your findings.
    I will update you with the variation numbers once the measurements have been made.

    Thanks,
    Dylan
  • 0 in reply to Dylan
    TI__Mastermind 44625 points

    Hi Dylan,

    Please see the PCB specifications used for the RF430CL330HTB board below. 

    Number of Layers – 2

    PCB Thickness – 0.062”

    Base copper thickness – 1 oz

    Finish Copper thickness – 1 oz (Total 2 oz of Copper)

    Surface finish – Leaded solder

    Green Solder mask

    White silk screen

  • 0 in reply to Eddie LaCost
    TI__Intellectual 1255 points

    Thank you, Eddie!

  • 0 in reply to Dylan
    TI__Intellectual 1255 points

    Eddie,

    Are there any recommendations for impedance control on the loop trace?

    Thanks,
    Dylan

  • 0 in reply to Dylan
    TI__Mastermind 44625 points

    Dylan,

    Sorry for the delayed response.  I have been out of the office on holiday.  As far as impedance control, you should not need to do anything special.  Just stick with consistent PCB properties.  How large of a variation are you seeing in the inductance of the coils?  This may help to point to specific problems.  For example, I have seen some PCB coils where the spacing between traces is too small and there can be some small copper "whiskers" which will short two of the coil traces together effectively reducing a turn and lowering the inductance. 

  • 0 in reply to Eddie LaCost
    TI__Intellectual 1255 points
    Eddie,

    The inductance of the coils typically ranges between 4.7 to 5.1uH. However, there is a bad unit that is measuring around 4uH. Based on the TI evaluation gerbers, how tight of a range should be expect?

    Thanks,
    Dylan
  • 0 in reply to Dylan
    TI__Mastermind 44625 points

    Dylan,

    What tool are you using to measure inductance?  Are you measuring at 13.56MHz?  I know many LCR meters measure at 1kHz, so this may be the reason for the high measurement.  Ideally, use a network analyzer at 13.56Mhz to obtain accurate measurements.   

    4.7uH is too high to reach resonance at 13.56MHz.  The RF430CL330H has 35pF input capacitance which would put the resonant frequency around 12.4MHz.

    High inductance aside, I think the variance of 4.7uH to 5.1uH is within reason.  Typically, you should see +/- 5% or better.  In regards to the 4uH reading, can you look at the coil under a microscope and see if there are any small shorts between the antenna traces.  This would effectively reduce a turn on the coil and lower the inductance.