Transcript Beyond the ATE printout..

Finding power to ground plane shorts
How VPS* helps find
ground plane shorts
(*Vector plane stimulus)
PolarInstruments.com
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Finding power to ground plane shorts
Overview
• Toneohm help locate shorts
- After ATE or flying probe test has isolated
a faulty net.
• Short circuit and bus fault location
- Simple trace to trace or
- Multilayer plane / trace or plane / plane
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Finding power to ground plane shorts
Tools for post ATE faultfinding
• How do you deal with an ATE printout that lists
50 components on one node all short to 50
components on another?
• E.G. a Vcc +5V short to Gnd?
• Or a Trace to plane short?
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Finding power to ground plane shorts
For simple track shorts
• Milliohm measurement
• For bus leakage non invasive track current
measurement...
– Contact
– Or non contact
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
Finding power to ground plane shorts
Plane to plane shorts...
• Present a new set of challenges
• The short (or bus fault) has to be located in
two dimensions
• So what how can a VPS equipped Toneohm
help?
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Finding power to ground plane shorts
Are plane shorts worth repairing?
• Surprisingly most plane shorts occur during
assembly....
– Without a 950 they can be difficult to find they are
assumed to be “internal”
• However 90% are assembly related and Easy to
repair
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Finding power to ground plane shorts
Why not throw away boards with
plane shorts?
• From a quality standpoint it is important to
know why the plane short has occurred...
– Is it a thermal cause?
– a layout problem?
– a latent defect with the PCB?
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Finding power to ground plane shorts
How? – The next slides tell you how VPS works
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Finding power to ground plane shorts
First imagine a track to ground short
Ground Plane
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Finding power to ground plane shorts
VPS also works for plane to plane shorts
Track
Ground Plane
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Finding power to ground plane shorts
The unknown short circuit is a position 
Track
Ground Plane
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
Short
Finding power to ground plane shorts
VPS Injects a current across the plane
I+
Track
Ground Plane

Short
I12
Finding power to ground plane shorts
This sets up a potential gradient across the PCB
I+
Track
+3
+2

Ground Plane
Short
I-
+1
13
0
-1
-2
Finding power to ground plane shorts
Measure voltage between short and plane position.
The track simply acts as an extension of the meter lead
I+
Track
+3
+2

Ground Plane
Short
I-
V1
14
+1
0
-1
-2
Finding power to ground plane shorts
The short is somewhere along the line where V=0.
I+
Track
+3
+2

Ground Plane
Short
I-
V1
15
+1
0
-1
-2
Finding power to ground plane shorts
This has isolated the short in one dimension.
• Inject the current at right angles to locate the
second dimension.
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Finding power to ground plane shorts
By injecting at right angles..
I+
Track
+3
Ground Plane

+2
Short
I-
V2
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+1
0
-1
-2
Finding power to ground plane shorts
Now find where V2 = 0
I+
Track
+3
Ground Plane

+2
Short
I-
V2
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+1
0
-1
-2
Finding power to ground plane shorts
The short is located where
V1 + V2 = 0
I+
Track
+3
Ground Plane

+2
Short
I-
V
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+1
0
-1
-2
Finding power to ground plane shorts
By looking at the polarity of V1 and V2 the 950
can display directional arrows on the front
panel that guide you to the area of the short
Track
I+
++
+3
+-
Ground Plane
-+
+2

Short
-I-
V
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+1
0
-1
-2
Finding power to ground plane shorts
The 950 Plane shorts mode uses this VPS technique to help
you rapidly locate plane to plane shorts, A 950 gives
a continuous display of short direction while you probe and
the current injection is switched automatically
Track
I+
++
+3
+-
Ground Plane
-+
+2

Short
-I-
V
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+1
0
-1
-2
Finding power to ground plane shorts
Your Toneohms pay for your investment very quickly
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Finding power to ground plane shorts
Toneohm short and bus fault locators
• USA / Canada - Polar Instruments Inc
(800) 328 0817 - Richard Smith
• UK Polar Instruments (UK) Ltd
023 9226 9113 - Paul Norrington
• Asia / Pacific Polar (Singapore)
+65 873 7470 - Amit Bhardwaj
• R.O.W. - Polar Instruments Ltd
Tel: +44 1481 253081- Martyn Gaudion
Email all offices: [email protected]
 Polar Instruments 2001 All trademarks recognized
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