[volt-nuts] PCBs with ceramic substrates

[cheater00 cheater00]

Indeed, someone I asked elsewhere hinted that ceramic filled PTFE is a good
idea for curbing hook.

On Mon, 17 Apr 2017 06:20 Bruce Griffiths, <bruce.griffiths at xtra.co.nz>
wrote:

> When the glass fibre bunch periodicity approaches a significant fraction
> of a wavelength the periodic spatial modulation of the substrate dielectric
> constant under or surrounding transmission lines has interesting  effects
> on the properties of the transmission line. At even higher frequencies the
> surface texture of the copper also becomes an issue. Ditching glass fibre
> in favour of small ceramic particles eliminates the effect of the periodic
> spatial modulation of the substrate dielectric constant. Just as with
> dielectric absorption it should be possible to fit a Cole-Coles or similar
> model to the variation of dielectric properties (including loss) with
> frequency.
>
> Bruce
>
> On 17 April 2017 at 15:54 cheater00 cheater00 <cheater00 at gmail.com> wrote:
>
> Thanks, that second article had exactly what I had in mind. I would love
> to see that permittivity vs freq graph in more detail around one of the
> flat crossings of the saw graph with the smooth fitted curve, maybe +/- 10
> MHz around that crossing (not around the steep jump)
>
> If ceramic is fragile and FR4 can be hook less then I assume FR4 is
> better, testing anyways doesn't hurt.
>
> The periodicity of the "saw" graph is telling. Could this have to do with
> the size of layers created by the fibers, or the size of fibers themselves
> or their spacing? Maybe a random fiber dimension (diameter, spacing) would
> be better here than using a constant dimension of fibers.
>
> On Mon, 17 Apr 2017 05:35 Bruce Griffiths, <bruce.griffiths at xtra.co.nz>
> wrote:
>
> There's little out there on the frequency dependence of dielectric
> constant at audio and sub audio frequencies.
>
> A bit more for 10MHz and above like:
>
>
> http://www.edn.com/design/analog/4398951/2/What-PCB-material-do-I-need-to-use-for-RF-
>
>
> http://ac.els-cdn.com/S1474667015373043/1-s2.0-S1474667015373043-main.pdf?_tid=2e1d6d30-231a-11e7-84ab-00000aab0f26&acdnat=1492398280_a0295abea04d2b08db9f227ee52506fc
>
>
>  There's also the article in The October 8 1978 issue of electronics on
>
> Getting rid of hook: The hidden pc-board capacitance by authors from
> Tektronix and Norplex.
>
> This article is in the files section of the Tekscopes yahoo group.
>
> Otherwise there are various values given in the literature and on the web
> for the low frequency relative permittivity of FR4 which are all noticeably
> higher than the value at 10MHz.
>
> Bruce
>
> On 17 April 2017 at 14:31 cheater00 cheater00 <cheater00 at gmail.com> wrote:
>
> Thank you. Do you have a plot of the effect on impedance due to hook out
> into high frequencies, measured on a real world material? It would tell me
> a lot.
>
> On Mon, 17 Apr 2017 04:19 Bruce Griffiths, <bruce.griffiths at xtra.co.nz>
> wrote:
>
> Hook is merely a manifestation of the variation of dielectric constant
> with frequency.
>
> It affects the frequency response and transient of resistive attenuators
> by requiring more complex compensation than merely adjusting a trimmer cap
> to equalise the low frequency and high frequency attenuation. Since the
> dielectric "constant" (relative permittivity) of all dielectrics is
> frequency dependent all dielectrics will exhibit hook to some extent. Some
> PCB substrates like some versions of FR4 and G10 exhibit a significant
> variation in the dielectric constant from the dc value to a somewhat lower
> value for frequencies even in he audio range let alone frequencies of
> several MHz. Achieving a flat frequency response where the dielectric
> associated with circuit board capacitances exhibits significant hook is a
> complex task. Circuit board hook even affects the impedance of printed
> transmission lines (eg stripline, microstrip, CPW etc).
>
> Bruce
>
> On 17 April 2017 at 13:54 cheater00 cheater00 <cheater00 at gmail.com> wrote:
>
> Are conformal coatings the right way to handle this?
>
> I understand there are kinds of FR4 and G10 that don't have hook. What does
> one do about hook - how are those substrates improved? How does hook
> manifest in circuits?
>
> On Mon, 17 Apr 2017 03:48 David, <davidwhess at gmail.com> wrote:
>
> FR4 has problems with consistency. Samples can have problems with
> hook, dielectric absorption, leakage, and sensitivity to humidity.
>
> On Sun, 16 Apr 2017 12:08:07 +0100, you wrote:
>
> Hi Chuck
>
> But the context is "PCBs with ceramic substrates". Are any of *those*
> tough? They may well be, perhaps you know of some? It does not help us
> with the subject much if there are ceramics with these amazing
> properties if they are not available as PCBs.
>
> There is also the question of exactly what properties of FR4 are
> limiting for "metrology" use.
>
> John
>
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