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Solder Mask on RF PCB - RF Cafe Forums

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Post subject: Solder Mask on RF PCB Posted: Fri Jan 23, 2009 10:55 am


Joined: Wed Aug 16, 2006 8:20 am
Posts: 23
Location: India
I am designing a PCB to work from 100KHz to 7 GHz. I was almost done with my Layout and the gerber was ready for fabrication when someone mentioned to me that I should consider the effect of Solder mask.
He suggested that the Grounded CPW trace width and separation I used were valid only when I had air as the dielectric over my traces. Using a Solder mask would alter the impedance as it has a dielectric greater than air.
I quickly added a layer of Solder mask in my simulation (using Er for Solder Mask as 3.3 and loss tangent as 0.04
I found that The VSWR of my trace improved, however the trace was now more lossy.

I don't want to get rid of the solder mask as the Coplanar GND plane is just 20 mil away from the RF trace and many (0402) size components need to be soldered on them. Since I am not very good at soldering I am afraid that I would short the traces. My board is fairly large (10 inches by 7 inches) and has more than 500 components. Few traces are 1 inches long.

Has anyone faced a similar problem or has any suggestions...

Ashish Bondia,
Design Engineer- RF


Post subject: Posted: Fri Jan 23, 2009 5:36 pm

Site Admin

Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Solder mask at frequencies higher than 4GHz is not recommended because it adds insertion loss. Do not worry about soldering, once you start you will improve with time. But if you will miss 1-2dB gain in your Tx chain it will be much harder to compensate...


Post subject: Posted: Sat Jan 24, 2009 4:44 am


Joined: Wed Aug 16, 2006 8:20 am
Posts: 23
Location: India
Thanks IR
I am exploring the possibility of using Solder Dams to restrict the flow of solder on the bare traces.
However for a board having more than 500 component, the time required to create Solder dams along each pad would be large.

Along with the Solder mask another issue that is driving me crazy is that of Board Surface finish. I have a choice of HASL and ENIG finish. I have learnt that ENIG causes higher insertion loss and also degrades IMD performance.

Ashish Bondia,
Design Engineer- RF

Posted  11/12/2012
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