Hi Luis

The dielectric constant of soldermask is slightly 
different to that of FR4 however in practice the 
variation is neglibible.

			(-1.55*H'/H)
>From IPC-D-317	e=er[1-e	    ]

I also noticed from the discussion that it has been 
mentioned that the frequency of operation will 
determine the impedance of the substrate. That is 
not exactly correct as it is in fact the rise time 
of the leading edge of the pulse that is the 
influencing factor.

	f=0.35/Tr
			where f is in GHz
			      Tr is in nS

At the end of the day what I do is just specify in 
the PCB Spec that the trace widths and prepreq 
thickness may be varied to obtain the desired 
impedance. And really who cares what material is 
used in the board manufacture as long as the 
impedance is what we want!

Cheers,

Barry Olney, In-circuit Design, Australia

Luis Rivera wrote:
> 
> Bob,
> 
> You are correct, the dielectric constant of a material does not change
> between microstrip, embedded microstrip or stripline configurations.
> 
> Avner,
> 
> Some general words of caution:
> 
> Dielectric constant does change depending on resin and reinforcement
> type, resin content, frequency, temperature and humidity. Typically,
> dielectric constant decreases as frequency increases.
> 
> On microstrips, the fact that there is air on one side of the
> transmission line lowers the effective dielectric constant. Solder
> mask coatings counteract this effect to some degree.
> 
> Please call off-line if you would like to discuss further.
> 
> Luis Rivera
> [log in to unmask]
> CompuRoute, Inc.
> Dallas, TX
> (214) 340-0543
> 
> >
> >
> >   Avner--
> >          If I am wrong, I hope someone out there will make me smarter, but
> >    the dielectric "CONSTANT" is just that--constant for that type of material.
> >    IMOP, when calculating impedance of microstrip or stripline, the dielectric
> >    constant of a material doesn't change, less the material's unique tolerance
> >    range REF: IPC-D-275 Table 3-2. Therefore, unless proven wrong, the constant
> >    DOES NOT change, but the impedance value WILL vary depending on which
> >    formula you are using depending on you pwb construction. Hope that helps.
> >    If wrong, please correct me via E-mail,
> >
> >                              Bob Vanech
> > E-Mail : [log in to unmask]
> >
> >
> >

-- 
___________________________________________________

Barry Olney				
Managing Director			

In-Circuit Design Pty Ltd   	Ph: +61 3 9205 9595
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Suite 211, Princess Tower	Mbl:+61 4 1117 0827
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