11 finding power to ground plane shorts how vps* helps find ground plane shorts (*vector plane...

11

Finding power to ground plane shorts

How VPS* helps find ground plane shorts

(*Vector plane stimulus)

PolarInstruments.com

22


Overview

• Toneohm help locate shorts - After ATE or flying probe test has isolateda faulty net.

• Short circuit and bus fault location- Simple trace to trace or- Multilayer plane / trace or plane / plane

33


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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For simple track shorts

• Milliohm measurement

• For bus leakage non invasive track current measurement...– Contact– Or non contact

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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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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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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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How? – The next slides tell you how VPS works

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First imagine a track to ground short

Ground Plane

1010


VPS also works for plane to plane shorts

Ground Plane

Track

1111


The unknown short circuit is a position

Ground Plane

Track

Short

1212


VPS Injects a current across the plane

Ground Plane

Track

Short

I+

I-

1313


This sets up a potential gradient across the PCB

Ground Plane

Track

Short

I+

I-

0+1

+2

+3

-1 -2

1414


Measure voltage between short and plane position.The track simply acts as an extension of the meter lead

Ground Plane

Track

Short

I+

I-

0+1

+2

+3

-1 -2V1

1515


The short is somewhere along the line where V=0.

Ground Plane

Track

Short

I+

I-

0+1

+2

+3

-1 -2V1

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This has isolated the short in one dimension.

• Inject the current at right angles to locate the second dimension.

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By injecting at right angles..

Ground Plane

Track

Short

I+

I-0+1

+2

+3

-1 -2V2

1818


Now find where V2 = 0

Ground Plane

Track

Short

I+

I-0+1

+2

+3

-1 -2V2

1919


Ground Plane

Track

Short

I+

I-0+1

+2

+3

-1 -2V

The short is located where V1 + V2 = 0

2020


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

Ground Plane

Track

I+

I-0+1

+2

+3

-1 -2V

Short

+ +

- ++ -

- -

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The 950 Plane shorts mode uses this VPS technique to helpyou 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

Ground Plane

Track

I+

I-0+1

+2

+3

-1 -2V

Short

+ +

- ++ -

- -

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Your Toneohms pay for your investment very quickly

2323


Toneohm short and bus fault locators• USA / Canada - Polar Instruments Inc

(800) 328 0817 - Richard Smith

• UK Polar Instruments (UK) Ltd023 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

11 finding power to ground plane shorts how vps* helps find ground plane shorts (*vector plane...

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