Just some further findings on this particular problem;

There are four through hole connector pads (A,B,C,D) in a row,  each pad
0.8 inches from the next pad.
There is a 6 thou, half ounce track (same internal layer) running in
parallel with the four pins, about 100 thou from the pads.
The four pins are connected to the pads with stubs. IE: the traces do
not go through the pads

The resistance measurements are as follows:
A to B =  1.2 ohms
B to C =  1.2 ohms
C to D =  1.2 ohms

A to C =  2.5 ohms
B to D =  2.5 ohms

A to D =  3.7 ohms

Which makes the trace stub and pad/via resistance approx 0.05 ohms
And the parallel trace resistance between any two adjacent pins approx
1.2 ohms

The other internal layer is the same, but the A to D resistance is 2.4
ohms

Some other PCBs show approx identical resistances
While others have different resistance but the same ratios
Some are OK

None of the external traces show this effect. Even those going through
vias from the top to the bottom of the PCB.

regards,

Phil.


-----Original Message-----
From: TechNet  Behalf Of Valerie St.Cyr
Sent: Tuesday, 14 December 2004 1:30 AM
To: [log in to unmask]
Subject: Re: [TN] Resistive internal layers


Here's a thought: if all the tracks on a particular layer exhibit high
resistance then likely that layer was either reworked or may have
excessive oxide treatment. If, however, it is only the one track, then
the
problem is with the track itself. In the past we have found that a
"dishdown" or nick or "mousebite", anything that reduces the volume of
copper in a spot on the track is now a "bottleneck" that increases the
line resistance. The other possibility is that a via in that net has low
copper or copper voiding, which would also show up as high resistance on
the net.

To really root cause the problem would require doing some cross-sections
...

Good luck and let us know what you find.

Regards, Valerie

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