# RE: [SI-LIST] : parasitic value for via

From: Pierre-Luc Cantin ([email protected])
Date: Sat Sep 16 2000 - 16:43:20 PDT

Dear Chunho,

With the help of Ozgur, thank you Ozgur for your contribution,
here the revised demonstration of Johnson via capacitance

- picture the via being a cylinder diameter=D1
- picture a second cylinder wrapped around the first one D2>D1
- this second one represents power/gnd planes
- both cylinders are iso-potential
- un-fold both cylinders and you end up with two parallel plates.
- the separation between the plates is the via clearance (D2-D1)/2
- the height of both cylinders is the board thickness (T)
- the width of the unfold-cylinder is equal to the via circumference which
is it's diameter times pi=3.1416... (pi*D1).
Now let do the math using Johnson example as a reference:
- er=4.7
- T=63mil ---> 1600um
- D1=28mil ---> 711um
- D2=50mil ---> 1270um
....Cvia = 1.41*4.7*0.063*0.028/(0.050-0.028) = 0.53pF
Now using two parallel plates capacitance formula:
Cvia= er*eo*A/d = er*eo*T*D1*pi/(D2-D1)/2, eo=8.85aF/um
= 8.85aF/um*4.7*1600um*711um*pi/(1270um-711um)2
= 531589aF = 0.531pF !!!

Pierre-Luc Cantin
Sr. ASIC Designer /

[email protected]
www.Hyperchip.com

phone (514) 931-5335 x217
fax. (514) 931-9923

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