[SI-LIST] Re: Basic question on power plane capacitance

Steve,

I am measuring on a non-populated board with a simple capacitance meter, 
i.e. at some low frequency.  I might be off in thickness by 1 mil (4 instead 
of 5), but that won't explain doubling the capacitance. For the dielectric 
constant of FR-4 I used 4.3. I doubt it can get much bigger than that.


/Mikhail




----- Original Message ----- 
From: "steve weir" <weirsi@xxxxxxxxxx>
To: "Mikhail Matusov" <matusov@xxxxxxxxxxxx>
Cc: <si-list@xxxxxxxxxxxxx>
Sent: Thursday, September 09, 2010 2:21 PM
Subject: Re: [SI-LIST] Basic question on power plane capacitance


> Assuming that you are measuring while the board is unpowered and 
> unpopulated, the variables are the actual thickness, and composition of 
> the dielectric layer.  If you have some IC attached to the polygon all 
> bets are off.
>
> Steve.
>
> Mikhail Matusov wrote:
>> Hi all,
>>
>> I have a small polygon in a plane layer of a multi-layer PCB. There is a 
>> solid ground plane underneath it and the top signal layer above. I have 
>> calculated from the basic plane capacitance equation that the plane 
>> capacitance for this polygon should be in the range of 420 pF. However, I 
>> am measuring 890 pF with one meter and 1 nF with another. I was wondering 
>> what am I doing wrong?
>>
>>
>> Thanks,
>> /Mikhail
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>
>
> -- 
> Steve Weir
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