[SI-LIST] Re: Loss Tangent of Solder Mask
- From: "Scott McMorrow" <scott@xxxxxxxxxxxxx>
- To: ray_waugh@xxxxxxxxxxx
- Date: Thu, 06 Mar 2003 20:49:24 -0800
Ray,
Actually with coplanar waveguide over ground the loss effect of
soldermask should be higher than even microstrip, due to the field
intensity within the gap. I suspect that you see little difference due
to your relatively short length.
When I was with another company, we made some measurements of microstrip
and stripline traces on a very long test backplane structure (>20").
The microstrip traces were wider than the stripline traces and our
field solver predictions with Ansoft 2D showed that we would have
marginally lower loss with our microstrip traces embedded in soldermask.
In this case we assumed that the loss tangent was about the same as the
FR406 we were using, about 0.02 in the 1 to 20 GHz range, or so. In
fact, when we performed side by side comparisons, we found that the
microstrip conductors had significantly higher loss by about 50%,
consistent with a higher loss tangent for the soldermask. At this point
we discontinued our investigation and decided that for long traces it
would not be a good idea to use microstrip routing.
Some day I always thought it would be a interesting idea to investigate
the loss properties of different soldermask materials on a series of
test boards with a substrate material that has well controlled
properties, like the Rogers 4000 series.
best regards,
scott
--
Scott McMorrow
Teraspeed Consulting Group LLC
2926 SE Yamhill St.
Portland, OR 97214
(503) 239-5536
http://www.teraspeed.com
ray_waugh@xxxxxxxxxxx wrote:
>Neeraj...
>
>What we use is coplanar waveguide over a groundplane, on the top of a
>single-layer board 0.032 inch thick (ground plane on the bottom). This makes
>for nice, easy-to-etch dimensions for line and gap. We put the solder mask on
>the top, and it goes into the gap. However, I must admit that our boards are
>short, with lines that are generally two inches or less in length.
>
>Ray
>
>Raymond W. Waugh
>Agilent Technologies
>
>-----Original Message-----
>From: Neeraj Pendse [mailto:cnepsc@xxxxxxxxxxx]
>Sent: Thursday, March 06, 2003 4:12 PM
>To: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: Loss Tangent of Solder Mask
>
>
>
>Ray,
>
>Regarding coplanar wavegiudes, I have a question: are these buried or on
>the top layer of the PCB? If they are on the top layer, then wouldn't
>the deposition of the solder mask affect the area between the traces?
>
>- Neeraj
>National Semiconductor Corporation
>
>
>
>>ray_waugh@xxxxxxxxxxx wrote:
>>
>>Michael...
>>
>>Unless you are forming a transmission line using a conductor over a ground
>>plane with solder mask as the dielectric, it does not much matter. If you
>>are concerned about solder mask over the top of a microstrip line (for
>>example), the e-fields above the line is so weak that you'll never be able to
>>tell (by measurement) if there is solder mask there or not. We commonly use
>>solder mask on our 2 GHz CDMA test boards (on top of microstrip or coplanar
>>waveguide lines) and it has no effect whatsoever.
>>
>>Ray
>>
>>Raymond W. Waugh
>>Agilent Technologies
>>
>>
>>
>
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