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Date: Thu, 1 Sep 2016 09:51:18 +0200
From: "Gabriel Paubert (paubert AT iram DOT es) [via geda-user AT delorie DOT com]" <geda-user AT delorie DOT com>
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Subject: Re: [geda-user] Microwave PCB layout simulation or How to eat all
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On Wed, Aug 31, 2016 at 09:25:23PM -0700, Larry Doolittle wrote:
> Evan -
> 
> On Thu, Sep 01, 2016 at 12:07:28AM +0000, Evan Foss (evanfoss AT gmail DOT com) [via geda-user AT delorie DOT com] wrote:
> > On Wed, Aug 31, 2016 at 9:39 PM, Larry Doolittle <ldoolitt AT recycle DOT lbl DOT gov> wrote:
> > > On Wed, Aug 31, 2016 at 03:53:25PM -0400, Evan Foss (evanfoss AT gmail DOT com) [via geda-user AT delorie DOT com] wrote:
> > >> I thought soldermask issues were why most of the microwave and other
> > >> RF hardware I have seen has no soldermask.
> > > For the long lines, yes, depending on the designer, but eventually
> > > those lines usually touch chips.  And high-speed serial lanes (with
> > > the interesting frequency range now going up to 40 GHz) usually don't
> > > have soldermask stripped off.
> > At those frequencies I would actually think the chip would be supplied
> > as bare die and wire bonded.
> 
> These days Xilinx BGAs have I/O rated for 25 Gb/s, with pretty broad spectral
> content up to 40 GHz.  I'm used to microwave circuits mostly ranging from
> 1 - 3 GHz, and all the Mini-Circuits and Hittite chips that cover that range
> (and higher) are in relatively conventional packages.

Even to higher frequencies, we've been building here amplifiers with
chips like Hittite's HMC462LP5 and using them up to 12GHz. They are
specified to up 20GHz, but I can't test that high because we reach the
limits of the connectors/launchers on the boards we've built. 

At these frequencies, the choice of the substrate is important, we use
20mil thick Rogers RO4350 (or RO4003, can't remember) but the transition 
from coaxial cable to the controlled impedance track is the most critical
part: I've had once an enclosure with a mistake of 0.5 or 0.6 mm in the 
depth of the hole in which a glass bead is inserted. The effect on the
SWR was enormous, catastrophic at 10GHz and higher.

For the controlled impedance tracks, we typically use conductor backed
coplanar waveguide, taking into account the effect of the cover of the
shielded enclosure on the impedance.

And no these boards don't have solder masks, for other boards, where I
also had to wire bond a diode, the solder mask was mostly removed along
the tracks and the edges of the ground plane. Some was left as solder
stops to limit solder flow along the track.

    Gabriel