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PAD size for Edge Mount SMA Connector - RF Cafe Forums

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ashishbondia

Post subject: PAD size for Edge Mount SMA Connector Posted: Mon Dec 29, 2008 7:44 am

Captain

Joined: Wed Aug 16, 2006 8:20 am

Posts: 23

Location: India

I am using an Edge mount SMA connector to feed CPWG lines on my PCB. The Application notes form The SMA manufacturer recommends 70 Mils wide pad for the central conductor pin which is 50 mil diameter and 150 mil long.

While making the footprint I used the same dimensions mentioned by the manufacturer.

On simulating the PCB uisng a 2D solver, I found that the return loss was very poor. My GND plane was 20 Mils below the Signal trace. Since I am using a multilayer PCB the stackup gave a total thickness of 64 mils as required to fit in the SMA connector.

I cleared the GND on all layers below the SMA signal PAD and used the lowest layer (at 64 mils form the signal layer) as the GND layer, in order to reduce the parasitic capacitance. the performance did not improve much.

Please share your comments and suggestions to improve the return loss

_________________

Ashish Bondia,

Design Engineer- RF

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IR

Post subject: Posted: Sat Jan 10, 2009 12:42 pm

Site Admin

Joined: Mon Jun 27, 2005 2:02 pm

Posts: 373

Location: Germany

Hello Ashish,

First of all your CPWG thickness should create 50-ohm impedance and should be calculated accordingly. There are different types of SMA connectos, some have 2 GND pins aligned with the central conductor and some have 4 GND pins. From your description I assume you use the 4 pins type.

In addition, try to see if there are also connectors with different width of the central conductor.

As for the GND conection - you should connect the GND on the top with vial holes to the other layers for formation of a continuous GND plane.

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ashishbondia

Post subject: Posted: Tue Jan 13, 2009 2:37 am

Captain

Joined: Wed Aug 16, 2006 8:20 am

Posts: 23

Location: India

Hi IR,

I am using the 4 GND pin SMA connector. I cannot change to a different width SMA connector now as the part has been already ordered. The 70 Mil wide pad suggested by the vendor for SMA center pin gives an impedance as high as 75 Ohms (VSWR of 1.5) at 7 GHz. I tried to reduce the pad width to 50 ohm (exactly the same as the center conductor diameter), and got a VSWR of less than 1.2

I am afraid that reducing the PAD to 50 mil width would create some problem during Soldering

_________________

Ashish Bondia,

Design Engineer- RF

Top

IR

Post subject: Posted: Tue Jan 13, 2009 2:52 pm

Site Admin

Joined: Mon Jun 27, 2005 2:02 pm

Posts: 373

Location: Germany

From my experience, I do not expect any problems with the soldering of the SMA connector. ..

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