SMA Connector

[Nikos A.]

Hi everyone,

Another question from the newbie..

I designing a PCB board with an isolated converter and an Oven Controller Crystal Oscillator. The output of the oscillator will be 10Khz terminated on SMA connector.
Due to space limitations I want to solder the sma in vertical position. Some like this in the picture



I've found this SMA https://eu.mouser.com/datasheet/2/643/pi-CCS-JOHN-142-0701-201-1290069.pdf but their pins' length (3.94mm) is more than twice my PCB thickness (1.6mm)..

Another option is this one https://eu.mouser.com/datasheet/2/643/pi-CCS-JOHN-142-0711-201-1290310.pdf which looks more user "friendly" but I am not sure how easy is to solder this connector.. For the middle pin I need a plated through hole and I will solder from the bottom side?

One more aspect that I am worried about is the point of termination between trace and SMA connector that will force my signal into a 90 degrees change of direction (from trace through SMA)

Please advice

Thanks

[DaJMasta]

The first one is a through hole connector, of course the pins are going to extend below the pad... It's also less than 2.5mm, how tight is the clearance on the underside of your board?

The second is a surface mount connector and is meant to be installed with solder paste applied and in a reflow oven, the method you've described with a via through the board can work for heat transfer, so you'd apply solder paste/flux to the pad on the top, solder the edge pins in place, then heat the paste in the middle from the back.  Could be a bit tricky to inspect, depending on clearance and how easy it is to see.  If you make the via large enough for solder to flow through, you may be able to see from the underside, but I'd still use the solder paste on the top side to be sure, even if hand soldering.


Right angle signal trace is not going to matter at 10kHz (I assume you meant 10MHz), or 10MHz, or likely even 100MHz.  In fact, 18GHz SMA connectors are REALLY overkill for applications under 1GHz, I'd look into using some other kind of test point, pad, or connector.

[Nikos A.]

Thank you for you answer DaJMasta,

  In fact, 18GHz SMA connectors are REALLY overkill for applications under 1GHz, I'd look into using some other kind of test point, pad, or connector.

But I need a steady terminated connection at the output (I cannot use test point or pad).. Is there any other connector that you suggest?

[ejeffrey]

I recommend against using surface mount SMA connectors for external interfaces.  The through-hole pins provide a lot of mechanical strength which is good.  Hopefully nobody is really banging around your nice OCXO, but it still is nice to have a securely mounted connector.  Surface mount connectors are fine for internal connectors that will be used for board-to-board or board-to-bulkhead jumpers.

[Nikos A.]

The through-hole pins provide a lot of mechanical strength which is good.  Hopefully nobody is really banging around your nice OCXO, but it still is nice to have a securely mounted connector.  Surface mount connectors are fine for internal connectors that will be used for board-to-board or board-to-bulkhead jumpers.

You are right, this is a good point that I didn't take into account.. Due to soldering difficulties as well as the mechanical durability I will proceed with the through hole option despite the fact that I don't like the extended pins below the board.. I am gonna live with that 

[ejeffrey]

Electrically it is no big deal at 10 MHz.  If this is for a one-off you can grind down the pin after soldering if you are worried about it shorting out.

SMA is overkill but fine, and that connector is pretty cheap.  BNC would also work but I usually prefer SMA for its small size.

There are even smaller RF connectors like SMP, MCX, or u.fl but they are more fragile and really more suited for connecting once during assembly and then leaving alone.  For lab use, SMA is great.

[pigrew]

At <8 GHz, don't worry about the little stub of the signal pin going through the board. Note that the stubs of the GND pins matter even less. I also agree with avoiding SMD if you will be going through many connection cycles as mechanical strength will be an issue. The 90 degree bend will also not matter until many GHz.

If it is still using too much board space, look into MCX, MMCX, or SSMCX connectors. There is a trend where smaller connectors are rated for fewer connections, so keep that in mind.

EDIT: Is it a square wave or sine wave that you are outputting? If square, what is the edge time that you are trying to achieve?

[coppice]

I recommend against using surface mount SMA connectors for external interfaces.  The through-hole pins provide a lot of mechanical strength which is good.  Hopefully nobody is really banging around your nice OCXO, but it still is nice to have a securely mounted connector.  Surface mount connectors are fine for internal connectors that will be used for board-to-board or board-to-bulkhead jumpers.

I keep seeing things like wireless EVMs with surface mount SMA connectors for high strain items like antennae, and they are a disaster. Most surface mount connectors have robustness issues, but SMAs seem amongst the easiest to break off a board.

[Nikos A.]

EDIT: Is it a square wave or sine wave that you are outputting? If square, what is the edge time that you are trying to achieve?

This is a 10Mhz sine wave output