PCB test — Power to Ground shorts
How the Toneohm 950 helps you find them,
and, how FTCam Pro can give you further help with plane short location on complex boards

This note explains how the Toneohm 950 works to help you repair many of those boards you may have considered a write off.

Secrets of VPS "Vector Plane Stimulus"

To maximise the number of boards you repair it will help if you understand about the proprietary Polar technology "VPS" employed by the 950 to find power to ground short circuits. Interested? Then read on...

VPS connections to a plane

How does VPS find plane to plane shorts?

VPS is a simple but powerful technique, and, though invented by Polar over 10 years ago this is the first time the technique has been described in full.

When ATE or BBT finds a power ground net shorted, you typically receive a long list of shorted components attached to the net.

Using a 950 you connect 4 Plane stimulus leads (VPS) to the 4 corners of the ground plane, a further lead is connected to reference the other side of the short, (whether another plane or a track).

The magic of VPS happens on the plane driven by the 4 stimulus leads. 

First current is passed from the top left corner of the plane to the bottom right corner, lets call this phase I, second current is passed from the top right corner to the bottom left (phase 2)

Current passes diagonally across the board in each of the two phases.

Lets think about what's happening in phase I, imagine for a while the plane is not a plane but a rectangular conductor, shrink this in your mind until it is thin — very thin, in fact just a piece of wire.

Now in phase I the VPS current flows from left to right along the wire.

Again imagine one point on the wire (the imaginary shrunk plane) where it is shorted to an adjacent plane.

Visualise connecting this point to the –Ve lead of a high impedance Voltmeter.

Connect now the +Ve lead of your imaginary voltmeter to a range of points on the wire, Move left and the further you go the Higher the positive voltage reading. Move right and the more –Ve, at the point where the leads are together you will see 0v. On a faulty board the Ov reading is on an equipotential line, the location of the short is at one point on this line.

So how do you find where the short is on the first equipotential line?  Now think in 2 dimensions, remember the first current is injected from the top left of the plane to the bottom right. In phase 2 the current is injected at right angles (top right to bottom left) to form a second set of 0v readings.
The short lies at the point where both readings are Ov.

Plane short location is quite simple on a board with a continuous plane on one layer. However if there are multiple planes, for example, analog and digital ground for example together on the same layer, you need to know the exact layout of the planes so you can connect the stimulus leads correctly.

By reading your cad data into FTCam you can display the copper areas of the ground planes. Treat each plane as if it is a separate PCB.

As described in the column on the left of this page, VPS drives current across the plane, and any breaks in the plane will either result in the direction arrows giving misleading information, or the plane stimulus not activating.

When you look at the data using FTCam you can clearly see the extents of the copper on inner layers. Use this graphical information to help you connect the Plane short stimulus (VPS) leads correctly.

Toneohm short direction
display

VPS not only looks at the magnitude of V1 and V2, it also monitors the polarity and uses this information to display a directional arrow on the front panel of the Toneohm 950, this in conjunction with a tone that rises as you approach the short lets you localise the fault into a small area.  You may find on very dense boards or boards with BGA it helps to X-ray in the locality identified by the 950.

Polar Instruments Ltd

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