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AWR1243: Alternative mmWave boards plating

Timur Suanov
Intellectual 290 points
Part Number: AWR1243

Hello, TI experts!

In your Application report SPRACG5 you wrote that

"Electroless Nickel-Immersion Gold (ENIG) is not a good choice for mmWave boards due to its high losses".

Did you estimate these losses in any way (dB value)? Did you tried any other PCB plating in the production of mmWave EVMs? And can you suggest an alternative surface finish applicable for mmWave boards (immersion Sn, electrolytic Au coating)? 

Thanks,

Timur

  • 0
    TI__Guru*** 147884 points

    Hi,

    We are checking with the pcb design team and will get back to you

    thank you

    Cesar

  • 0 in reply to Cesar
    TI__Mastermind 18470 points

    Hi Timur, 

    We have not tried any other copper plating other than the immersion silver finish on our EVM. Mostly this was us avoiding the additional modeling, validation and functional risk associated with switching to a plating method that did not have similar (or superior) copper conductivity. We wanted to keep the EVM RF designs as close to an ideal copper + air environment as possible. The EVM are just a functional starting point for developers so our goal was making sure everything worked first. 

    In full production systems an analysis of the trade-offs of different finishes, masking, and conformal coating may be required to ensure long-term quality of the RF structures, while still achieving the required phase and power requirements. TI has not done such an analysis. 

    Some searches online do pull up a few relevant articles from RF PCB substrate vendors such as Rogers on the ENIG vs. bare-copper comparison topic. One article here: https://www.microwavejournal.com/blogs/1-rog-blog/post/28496-finish-makes-a-difference-in-final-pcb-performance

    "For example, for the microstrip circuits, very little difference in conductor loss was apparent between the unplated (bare-copper) and ENIG circuits though about 2 GHz, although the higher losses of the ENIG circuit became more noticeable above that frequency. Compared to the bare copper circuit, the microstrip with ENIG measured about 0.25 dB higher in loss at 10 GHz, increasing to more than 0.5 dB higher at 40 GHz. The loss difference between GCPW circuits using bare copper conductors and using ENIG finish are even more dramatic, with about 0.5 dB more loss at 10 GHz and greater than 1 dB more loss at 40 GHz. The measurements clearly show the frequency dependence at higher frequencies but also how a particular circuit type can also contribute to the impact that a plated finish can have on a circuit."

    The article is only referencing conducted losses, not radiated losses, but the effect will be similar and the losses will be worse at the higher 76-81GHz bands AWRx operates. 

    Thank you,

    -Randy

  • 0 in reply to Randy Rosales
    Intellectual 290 points

    Hi, Randy,

    Thank you very much for your detailed response and reference!

    Regards,

    Timur