16
\$\begingroup\$

I am a beginner in PCB design. While using sensors for my project I thought it would be a great idea to connect my sensors using RJ-45 connector and in my device, I'm thinking of using a RJ-45 female connector for PCB. The female connector looks like this:

enter image description here

To make the female connector firmly attached to the PCB board, the 8 pins are soldered to the PCB board. But, besides it, I don't know the use of these cylindrical holders. How to use it?

\$\endgroup\$
3
  • 25
    \$\begingroup\$ Cut them off, install it, trip over the cable, and see how "firmly attached" the 8 pins are. \$\endgroup\$
    – Alexander
    Apr 17, 2018 at 16:30
  • 2
    \$\begingroup\$ Note that "PCB board" means "printed circuit board board". \$\endgroup\$
    – Ruslan
    Apr 18, 2018 at 6:51
  • 4
    \$\begingroup\$ That's 8P8C, not RJ-45 \$\endgroup\$
    – Agent_L
    Apr 18, 2018 at 9:39

4 Answers 4

Reset to default
35
\$\begingroup\$

These are strain relief fittings. They spread the horizontal force that's exerted when you insert or remove the connector from the socket. This helps prevent the electrically connected pins being damaged over time. To use them, follow the datasheet layout for the part and simply put the corresponding hole in the PCB. Those fittings will clip into those drilled holes and hold the part in place.

As pointed out by @DaveTweed in the comments, there are also the metal pins connected to the shield, which counteract the normal force trying to lift the connector upwards. These pins should be soldered to the board, and probably connected to ground.

\$\endgroup\$
2
  • 12
    \$\begingroup\$ Those two metal fingers attached to the shield help anchor the connector, too. Because they get soldered to your ground plane, they provide resistance to normal forces (those that try to lift the front of the connector away from the board), while the plastic pins provide resistance to shear forces (parallel to the board). \$\endgroup\$
    – Dave Tweed
    Apr 17, 2018 at 14:19
  • 4
    \$\begingroup\$ To clarify, the shield pins should have their own holes with pads (and in most cases should be connected to the ground plane as Dave mentioned); the holes for the plastic pegs do not need pads or any other special treatment -- just don't oversize them, as some snugness is necessary to provide the strain relief function these are intended for. \$\endgroup\$
    – Doktor J
    Apr 17, 2018 at 15:45
16
\$\begingroup\$

They hold the connector in place by snapping in and provide some strain relief.

When you make the footprint, it's better to use unplated holes for the locating snaps. They should not have pads.

They don't need a net connection, so they don't need to show up on the schematic symbol.

The shell pins should have pads and net connections. For example:

enter image description here

Follow the datasheet recommendations as to hole sizes and exact positions, and pin numbering (or figure it out if it's not shown, as sometimes happens).

The above footprint would be a bit better if pin 1 was indicated, perhaps by a square pad.

enter image description here

\$\endgroup\$
2
  • \$\begingroup\$ Why is it better to use unplated holes? Is it just the finished diameter tolerance or something like more friction of FR-4 vs a plated hole? \$\endgroup\$
    – Wesley Lee
    Apr 20, 2018 at 2:36
  • 1
    \$\begingroup\$ @WesleyLee both reasons you cite, at least with HASL. For ENIG mostly tolerance. \$\endgroup\$
    – Spehro Pefhany
    Apr 20, 2018 at 15:43
6
\$\begingroup\$

They also serve to hold the connector in place while it's being soldered.

\$\endgroup\$
0
\$\begingroup\$

Not to be ignored if your PCB is of any value to you. I recently ignored ensuring these mounts were secure and ended up trashing several boards when the cables connected to them were pulled on too tightly. Although I was able to get the connector body to sit back onto the board, the wires inside had flattened such that they would no longer make a reliable connection to the CAT cable.

\$\endgroup\$
3
  • \$\begingroup\$ I agree. Note that the question is "how to use them" - can you elaborate on this? Do you melt them, silicone them, or what? \$\endgroup\$
    – SDsolar
    Apr 20, 2018 at 2:25
  • \$\begingroup\$ Also, this is borderline anecdotal with very little meat. Remember that Stack Exchange is not a traditional forum (thankfully!) where people just tack on their thoughts. \$\endgroup\$
    – pipe
    Apr 20, 2018 at 6:48
  • \$\begingroup\$ I see boths points very clearly now. Thank you for the guidance! \$\endgroup\$
    – Thuddwin Smith
    Apr 22, 2018 at 2:09

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.