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I am designimg a board with an SMA connector for 8 GHz. The trace is calculated as a coplanar waveguide. For the given stackup, it is 0.25 mm wide. The SMA connector pin is also 0.25 mm wide.

If I make the footprint size equal to the trace and SMA connector size can it be soldered without problems or should the footprint be bigger? 0.41 mm is recommended in the datasheet but must I then remove the gnd plane for a larger trace?

Theoretically the same size should be the best solution for a minimum impedance discontinuity or am I wrong?

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Theoretically the same size should be the best solution for a minimum impedance discontinuity or am I wrong?

Even if the track and pin are the same width, the increase in height will give you a discontinuity. This would still be true to some extent on microstrip, but you are using coplanar, where the effect would be more marked. You should pull back the grounds in the vicinity of the joint anyway to maintain an impedance match.

A pin and pad the same width can work, if your soldering process is well controlled. However a wider pad gives you the opportunity for an edge fillet of solder, which allows you to visually inspect the joint, and gives you larger tolerances around alignment and the amount of solder.

As you ought to relieve the grounds a little anyway, and always have to pay attention to ground currents around large features like connectors meeting boards (the signal is usually obvious, but the ground can bite unexpectedly), it would probably be wise to use the recommended track width under the pin (for solder quality), and then design the rest of the transition for good match. You can choose to make the transition to the wider pad anywhere you like, right at the pad, or a little removed from it if there's room. The latter should allow you to get an easier transition, with less discontinuity to compensate for at each change of dimensions.

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