Re: Alumina Substrate

On Thu, 05 Jun 2008 20:18:48 GMT, Rich Grise <rich@xxxxxxxxxxx> wrote:

On Wed, 04 Jun 2008 12:46:27 -0700, John Larkin wrote:

We have trouble keeping impedances reasonable on 8-layer boards with
Er=4.6.

Forgive the dumb question, but what's "Er"?


Dielectric constant, relative to vacuum. FR-4 runs about 4.6, which is
about 15 pF per square inch of 0.062 board, side-to-side. That makes a
50 ohm microstrip (surface) trace about 120 mils wide.

Make the dielectric thinner, and capacitance goes up, so a 50 ohm
trace has to get skinnier. Sandwich the trace inside between planes,
and you have stripline, even lower impedance. At 8 or so layers,
inner-layer traces start to get too skinny to have reasonable
impedances. 100 layers of ceramic, with Er=10 maybe, boggle my tiny
mind.

John


.

Relevant Pages

  • Re: Alumina Substrate
    ... ohm microstrip trace about 120 mils wide. ... Make the dielectric thinner, and capacitance goes up, so a 50 ohm trace ... have stripline, even lower impedance. ... At 8 or so layers, ...
    (sci.electronics.design)
  • Re: Alumina Substrate
    ... ohm microstrip trace about 120 mils wide. ... Make the dielectric thinner, and capacitance goes up, so a 50 ohm trace ... have stripline, even lower impedance. ... At 8 or so layers, ...
    (sci.electronics.design)
  • Re: Reverse engineering
    ... >no more than 2 layers. ... Mark each component with a label of some sort ... Make a photo of both sides and markup the ... Then start at one end of the board and draw each trace, ...
    (sci.electronics.design)
  • Re: Reverse engineering
    ... >>no more than 2 layers. ... Then start at one end of the board and draw each trace, ... >>mark off each trace that you have drawn out. ...
    (sci.electronics.design)
  • Re: PCB Power Trace Widths, Ground Planes, and Routing
    ... >> layers. ... >> with achievable trace geometry. ... all cards, regardless of whether they are master cards (like the ones ... use 4 mil traces on all the inner layers, IIRC. ...
    (sci.electronics.design)
Quantcast