[SI-LIST] Re: DC resistance measurement

  • From: Istvan Novak <istvan.novak@xxxxxxxxxxx>
  • To: zhang_kun@xxxxxxxxxx
  • Date: Tue, 22 Aug 2006 08:38:54 -0400

Dear Zhangkun,

DC measurements of PDNs can sometimes be almost as tricky as 
high-frequency measurements.  When it comes to the DC resistance of the 
planes, first you have to decide what you want to measure.

If we want to measure the sheet resistance of the planes, we need to 
keep in mind that the definition of sheet resistance assumes a uniform 
square (or rectangular, if we do scaling) sheet of material with UNIFORM 
current entering and exiting two opposite sides.  So unless the rest of 
the plane is taken into account by correction factors, the current entry 
and exit must be along the full opposite edges of the planes.  On a bare 
laminate we can approximate this condition very well with some work, but 
on production boards we seldom have the means to make the connections, 
which satisfy these conditions.

If we want to measure the DC resistance between actual connection points 
on a real PCB, we need to remember that the connection geometry may make 
a big difference.  With only one entry and one exit connection, the 
resistance approaches infinite as the size of connections approaches 
zero.  In practical applications we may have multiple entry and exit 
points: high-current connections to DC-DC converters and other power 
sources (should) have multiple vias to the planes, and high-current 
chips (should) have multiple vias/pins connecting to the package.  Under 
these circumstances it is better to measure and plot the potential 
surface at any and all points that you can measure.  If you have a 
stable DC current source with high internal resistance, the source 
current can be measured separately  through a shunt.  Once you know the 
source current, it  is connected to the DUT and the voltage drops at 
various points can be measured with an accurate DC voltmeter. 


Istvan Novak
SUN Microsystems

Zhangkun wrote:

>Dear all
>Now, we are measure DC resistance of power plane in PCB. The presision should 
>be 0.1 mohm at least. However, because of the clamp of the meter, the error is 
>about 1m ohm. In another word, the difference between two measurement for the 
>same DUT is about 1 mohm. Is there any advice about this topic? Anything will 
>be help. In your measurement, how to connect the probe to the plane?
>Best Regards

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