Keep in mind that soldermask will also change the field distributions
around a
microstrip line, and will somewhat mitigate the microstrip's dispersive
behavior
as well.

I once worked with some miccrostrip couplers at around 2-4 GHz and found
that
directivity was significantly improved by adding two layers of thin kapton
tape
on top of the coupled region, a solution that went into production.  I
expect that
the usual soldermask layer would have about the same effect.

Dana


On Thu, Feb 22, 2018 at 8:15 PM, jimlux <jim...@earthlink.net> wrote:

> On 2/22/18 12:29 PM, Leo Bodnar wrote:
>
>> Here is ENIG fact that is not widely known at the moment but which some
>> might find useful.
>>
>> I could not understand why I get better TDR and insertion loss results
>> from solder-mask covered microstrip transmission lines than from otherwise
>> identical microstrips on the same substrate with soldermask removed and,
>> therefore, covered with ENIG.
>>
>> Gold can't be bad, right? As it turns out, even gold coin has two sides
>> to it.
>>
>> I have found that Shlepnev and McMorrow conducted extensive research and
>> published data, some of which is presented here
>> http://www.simberian.com/Presentations/NickelCharacterizatio
>> nPresentation_emc2011.pdf
>>
>>
> the ever useful http://www.microwaves101.com/ site has an excellent
> discussion of this under the "skin effect" heading.
>
>
>
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