Re: [SI-LIST] : stripline PCB board shrinkage?

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From: C Deibele (deibele@fnal.gov)
Date: Wed Feb 23 2000 - 14:15:23 PST


Ray Anderson wrote:
>
> Have you put your boards under a scope and measured the critical
> dimensions on your filters?

I know that the line widths are correct (75 mils)...
as for other dimensions, it gets difficult to determine with great
accuracy
due to physical length - I want to measure 460 mils with any great
accuracy is
difficult.

> If the dimensions look OK then you need to investigate material issues
> (error in Er assumption, anistropy, delta in substrate thickness etc.)
> and design issues (is Touchstone giving you the right answer, etc.).

i thought about anisotropy, but I am sort of ruling this out since the
stripline ground spacing is small and the fields are really confined to
be vertical. This could be a major assumption, but I can't imagine that
the anisotropy would be that big of an effect in this case. I measured
the dielectric thickness with a scope and put this into my model.
Additionally, I used a MoM tool to verify Touchstone's results. Since
the boards are made on a Teflon substrate, it seems believable that
there might be some shrinkage/expansion.

> Question: besides the center frequency being off, how is the bandwidth,
> shape factor and ripple ? Are they what you expected??

The bandwidth is what I expected, and the ripple was bigger than I had
expected. This was due to a manufacturing problem (they had a problem
making the mitered bends). I tried to work backwards with touchstone to
see if this would cause a frequency shift. No luck...I could reproduce
the ripples without a frequency shift by using the measured mitered
bends.

> Are the stripline filters being made as a multi-layer board, or are you
> etching as microstrip and then attaching the top layer afterwards?

The process goes as follows:

The board house takes one sheet of material, and cuts it into a top
piece and a bottom piece. the two pieces are routed for size, and then
they are etched, pressed together, drilled, plated...

I took great pain to ensure that I have a good launch into the boards...

At this point, I want to know if it is worth it to perhaps use a
different method of board fabrication besides etch.

What do you think??

-Craig

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