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Thread: PCB design - grounding tabs on SMD RJ45 jacks?

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    Question PCB design - grounding tabs on SMD RJ45 jacks?

    I'm ready for revB of my board. It's a small board supporting 32 outputs. One side is full of 8 RJ45 jacks; the other side has all SMD devices. I generally have used the thru-hole RJ45 jacks, but both the thru-hole pins as well as the un-plated mounting holes take up too much space on the back of the board. I'm now using SMD RJ45 jacks for the first time.

    In reviewing the "artwork" before sending to fab, I noticed that the (electrically-isolated) mounting tabs on the RJ45 jacks create big islands of unconnected copper. It occurred to me that if I add them to the GND net, I can then have them part of the ground plane/copper pour (with thermal relief as normal for GND connections).

    Is this a bad idea (or good idea/general practice)? By adding these tabs to the GND net/copper pour it creates a pretty solid ground plane and will (possibly/hopefully) help isolate the signals on the other side.

    BTW, I'm using TE Connectivity (Tyco) 338088 jacks, drawing found here: http://www.te.com/commerce/DocumentD...txt=1-338088-3

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    Default Re: PCB design - grounding tabs on SMD RJ45 jacks?

    Since the connection is isolated, attaching it to your copper pour should be fine, and for designs where you are dumping heat into a ground plane, the more copper the better. However, my experience with SMT connectors, especially RJ45 where there is quite a bit of force associated with undoing them, is the pins can pull up off the board pretty easily. I've actually found in several designs I've done that the PTH connectors actually take up less room, and make a much more robust connection. SMT is desired for fully automated production lines of course. Just my opinion.

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    Default Re: PCB design - grounding tabs on SMD RJ45 jacks?

    Quote Originally Posted by Utimgr View Post
    ... I've actually found in several designs I've done that the PTH connectors actually take up less room, and make a much more robust connection. SMT is desired for fully automated production lines of course. Just my opinion.
    @Utimgr I was looking for an alternative to the RJ45 but couldn't come up with anything that looked good. I am running 4 twisted pairs (signal/ground) through cat5 cable, thus the RJ45. I'm not familiar with "PTH connectors". Google didn't help much - can you point me towards something? Thanks.

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    Default Re: PCB design - grounding tabs on SMD RJ45 jacks?

    Quote Originally Posted by ags0000 View Post
    @Utimgr I was looking for an alternative to the RJ45 but couldn't come up with anything that looked good. I am running 4 twisted pairs (signal/ground) through cat5 cable, thus the RJ45. I'm not familiar with "PTH connectors". Google didn't help much - can you point me towards something? Thanks.
    PTH - Plated through hole. Something like this
    Last edited by Coaster; 04-12-2018 at 01:39 PM.

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    Default Re: PCB design - grounding tabs on SMD RJ45 jacks?

    @Coaster Oh, that should have been obvious. I was thinking "Pluggable Terminal Housing" (like the Phoenix connectors used in many controller designs, with screw attachment to cables and pluggable to the board matching mate) or even Pluggable Through Hole.

    The problem with the PTH RJ45 is more than just the terminals, it's those *giant* mounting pegs. I've used these TE jacks many times. They just take so much room from the backside of the board that it was not possible for me to use them this time (without significantly increased board size.

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    Default Re: PCB design - grounding tabs on SMD RJ45 jacks?

    Has anyone ever considered the other "RJ" connectors for special lines? RJ11, RJ12, etc.

    With all the different types of signals running in, out, and through our boards, there is bound to be the mistake of plugging an E1.31 (Ethernet) cable into a SPI port. Other possibilities, and combinations are possible.

    As far a mounting them on a board, the smaller footprint of say an RJ11 would take some stress off of the soldered connection points, now wouldn't it?

    Just my $.02 thought.
    Live, Laugh, Love.

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    Default Re: PCB design - grounding tabs on SMD RJ45 jacks?

    Yea sorry on the PTH, sometimes you think everyone knows the acronyms. Instead of individual jacks, have you looked at using gang'ed assemblies? 1x4 or 2x4? If you have to use SMT, and sometimes you do, I would consider epoxying them down so they don't rip off the board. Repairing those are a pain. Good luck!

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    Default Re: PCB design - grounding tabs on SMD RJ45 jacks?

    Do you have a pdf of the gerbers? Maybe the group can make other suggestions depending on your layout.

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    Default Re: PCB design - grounding tabs on SMD RJ45 jacks?

    Hi, I think the advantage of RJ45 is that you can buy an RJ45 to RJ45 off the shelf if you dont need your own crimping tool. That said you can use cat5 in IDC connectors which gives you a different footprint, I'm not sure if it is smaller. Interestingly both RJ45 and IDC connectors work with 24G 8 core alarm cable if you want a flexible connector (I needed to exted some of my P10 connectors at short notice).
    I'd stick with RJ45 if you can.
    Last edited by Barnabybear; 04-14-2018 at 11:29 AM.

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    Default Re: PCB design - grounding tabs on SMD RJ45 jacks?

    From what I understand the only time you donít want to fill in or have ground planes with rj45 is when your using magnetics (rj45 Ethernet). Which make sense because of all the coil inductance can introduce some undesired ground effects


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