Troubleshooting 'shorted' power rails

On Monday, May 20, 2019 at 2:23:56 PM UTC-4, Martin Rid wrote:

dagmargoodboat@yahoo.com Wrote in message:
I had two cases in a row recently of many-layer power-planedboards with power supplies that wouldn't come up above a diodedrop, and I was able to use a trick devised for the first one tosolve the problem on the second one, too --I set a current-limited supply to ~30mA & left the boardto stabilize thermally. I then looked for the 'hot' partswith an IR thermometer. Worked like a charm.Temp. rise was about 1oC @ 20mW. The IR thermometer? $14well-spent.Cheers,James Arthur

I remember the Shortsqueak. Like a logic probe. Worked on
resistance, and changed pitch as you got closer to the short. I
think Nuts&volts had something similar too.
Cheers

--


----Android NewsGroup Reader----
http://usenet.sinaapp.com/

One of my most-treasured tools is a buzz-box I made ages ago from
an LM3909, that lets you easily hear the difference between a short,
an ohm, and tens of ohms.

But that wouldn't have been much help here.

Since it was acting like a diode, I considered using freeze spray
and watching the voltage until I hit a part that made the rail
wiggle appropriately.

Cheers,
James Arthur

 

On Sunday, May 19, 2019 at 5:30:58 PM UTC-4, bitrex wrote:

On 5/19/19 5:22 PM, dagmargoodboat@yahoo.com wrote:
I had two cases in a row recently of many-layer power-planed
boards with power supplies that wouldn't come up above a diode
drop, and I was able to use a trick devised for the first one to
solve the problem on the second one, too --

I set a current-limited supply to ~30mA & left the board
to stabilize thermally. I then looked for the 'hot' parts
with an IR thermometer. Worked like a charm.

Temp. rise was about 1oC @ 20mW. The IR thermometer? $14
well-spent.


Cheers,
James Arthur


You can get a full thermal camera attachment for Android from the state
store for $200, sometimes less like $15-20 off on sale from time to time:

https://www.amazon.com/dp/B00NYWAHHM/ref=twister_B07QVJVVGD?_encoding=UTF8&psc=1

I actually posted that here some time ago. But I've gotten along fine
without one. Besides, I don't really need Google-candy tracking me just
so that I can do electronic stuff.

A 'smart' phone and all that baggage seems like vast overkill when a
$14 thermometer does 85% of the same thing, and without even spying on
me <gasp>.

Cheers,
James Arthur

 

On Sunday, May 19, 2019 at 6:25:05 PM UTC-4, bloggs.fre...@gmail.com wrote:

On Sunday, May 19, 2019 at 5:23:02 PM UTC-4, dagmarg...@yahoo.com wrote:
I had two cases in a row recently of many-layer power-planed
boards with power supplies that wouldn't come up above a diode
drop, and I was able to use a trick devised for the first one to
solve the problem on the second one, too --

I set a current-limited supply to ~30mA & left the board
to stabilize thermally. I then looked for the 'hot' parts
with an IR thermometer. Worked like a charm.

Temp. rise was about 1oC @ 20mW. The IR thermometer? $14
well-spent.

What was it?



Cheers,
James Arthur

In the first case, wrong parts stuffed. Digikey switched labels
on the bags, and the assembly house missed it.

The second board had Vcc & ground swapped on an ICs. I'm not sure
if the designer did that, or the layout guy.

In both cases the result was a diode-drop from power to ground, and
power supplies that wouldn't power up.

Cheers,
James Arthur

 

[bitrex]

On 5/20/19 4:08 PM, bitrex wrote:

On 5/20/19 12:55 AM, tabbypurr@gmail.com wrote:
On Sunday, 19 May 2019 22:48:13 UTC+1, John Larkin  wrote:
On Sun, 19 May 2019 14:22:57 -0700 (PDT), dagmargoodboat@yahoo.com

wrote:

I had two cases in a row recently of many-layer power-planed
boards with power supplies that wouldn't come up above a diode
drop, and I was able to use a trick devised for the first one to
solve the problem on the second one, too --

I set a current-limited supply to ~30mA & left the board
to stabilize thermally. I then looked for the 'hot' parts
with an IR thermometer.  Worked like a charm.

Temp. rise was about 1oC @ 20mW.  The IR thermometer? $14
well-spent.


Cheers,
James Arthur

Thermal imagers are getting affordable and are great to have around.
Not only can they find shorts and bad parts, it's good to image a new
design and see if anything runs hot.

https://www.dropbox.com/s/mlaf2yce2z3xiv7/IR_0025.jpg?dl=0

https://www.dropbox.com/s/r4q2z8vnonwh4mc/IR_Board.jpg?dl=0

Our test people use a thermal imager too, to spot unusual stuff.

One eternal problem is electronics is "where is the current going?"

Sometimes you can measure microvolt drops in traces and planes to
answer that question. It would be great if we could see mag fields!

There is mag field viewing film. And you can make your own viewer by
dropping iron filings in a shampoo bottle. Crude but I've seen it
demoed and work.


NT


Many smartphones, even cheap ones, come with three-axis Hall effect
sensors built in nowatimes. With a free app you can always have a tool
for sensing e.g. hidden wires in walls or whether an outlet somewhere is
getting power

In fact it would probably be possible to write an app that lets you
sweep the phone thru an volume of space and generate a 3D graphical
representation of the field strength magnitude in that space. Hmm, maybe
there's a market for that.

 

[bitrex]

On 5/20/19 12:55 AM, tabbypurr@gmail.com wrote:

On Sunday, 19 May 2019 22:48:13 UTC+1, John Larkin wrote:
On Sun, 19 May 2019 14:22:57 -0700 (PDT), dagmargoodboat@yahoo.com

wrote:

I had two cases in a row recently of many-layer power-planed
boards with power supplies that wouldn't come up above a diode
drop, and I was able to use a trick devised for the first one to
solve the problem on the second one, too --

I set a current-limited supply to ~30mA & left the board
to stabilize thermally. I then looked for the 'hot' parts
with an IR thermometer. Worked like a charm.

Temp. rise was about 1oC @ 20mW. The IR thermometer? $14
well-spent.


Cheers,
James Arthur

Thermal imagers are getting affordable and are great to have around.
Not only can they find shorts and bad parts, it's good to image a new
design and see if anything runs hot.

https://www.dropbox.com/s/mlaf2yce2z3xiv7/IR_0025.jpg?dl=0

https://www.dropbox.com/s/r4q2z8vnonwh4mc/IR_Board.jpg?dl=0

Our test people use a thermal imager too, to spot unusual stuff.

One eternal problem is electronics is "where is the current going?"

Sometimes you can measure microvolt drops in traces and planes to
answer that question. It would be great if we could see mag fields!

There is mag field viewing film. And you can make your own viewer by dropping iron filings in a shampoo bottle. Crude but I've seen it demoed and work.


NT

Many smartphones, even cheap ones, come with three-axis Hall effect
sensors built in nowatimes. With a free app you can always have a tool
for sensing e.g. hidden wires in walls or whether an outlet somewhere is
getting power

 

On Sunday, May 19, 2019 at 5:48:13 PM UTC-4, John Larkin wrote:

On Sun, 19 May 2019 14:22:57 -0700 (PDT), dagmargoodboat@yahoo.com
wrote:

I had two cases in a row recently of many-layer power-planed
boards with power supplies that wouldn't come up above a diode
drop, and I was able to use a trick devised for the first one to
solve the problem on the second one, too --

I set a current-limited supply to ~30mA & left the board
to stabilize thermally. I then looked for the 'hot' parts
with an IR thermometer. Worked like a charm.

Temp. rise was about 1oC @ 20mW. The IR thermometer? $14
well-spent.


Cheers,
James Arthur

Thermal imagers are getting affordable and are great to have around.
Not only can they find shorts and bad parts, it's good to image a new
design and see if anything runs hot.

https://www.dropbox.com/s/mlaf2yce2z3xiv7/IR_0025.jpg?dl=0

https://www.dropbox.com/s/r4q2z8vnonwh4mc/IR_Board.jpg?dl=0

Our test people use a thermal imager too, to spot unusual stuff.

One eternal problem is electronics is "where is the current going?"

Sometimes you can measure microvolt drops in traces and planes to
answer that question. It would be great if we could see mag fields!



--

John Larkin Highland Technology, Inc

lunatic fringe electronics

I've got a pal who'd loan me his FLIR, but it was a lot faster
this time just to whip out the 1-pixel wonder.

TDR might be good for finding shorted planes. But boards and parts
are generally so good these days, I've never had that happen.

Cheers,
James Arthur

 

[Winfield Hill]

Bill Sloman wrote...

Microvolt sensitive DVM's are cheaper, if not as quick.

If you pump enough current into the short, the
clues from measured voltage drops are higher.
A potential problem with thermal debugging is,
the guilty short may not dissipate much heat.


--
Thanks,
- Win

 

[Tom Del Rosso]

dagmargoodboat@yahoo.com wrote:

TDR might be good for finding shorted planes. But boards and parts
are generally so good these days, I've never had that happen.

Yeah I'm pretty good at that.

But seriously how can a TDR find a short on a plane?

 

[Winfield Hill]

dagmargoodboat@yahoo.com wrote...

Bill Sloman wrote:

Microvolt sensitive DVM's are cheaper ...

But a microvolt DVM can't track down a diode-to-ground,
which was the problem.

Yes, it can. If starting from the V+ rail, use the
DVM to measure drops at each fork, and see which one
is carrying current. Voltages on the ones that aren't
don't change. Eventually this narrows down to the
offending item. As you progress past the item, the
voltage drops stay the same, revealing the spot.


--
Thanks,
- Win

 

[Bill Sloman]

On Tuesday, May 21, 2019 at 9:20:31 AM UTC+10, Tom Del Rosso wrote:

dagmargoodboat@yahoo.com wrote:

TDR might be good for finding shorted planes. But boards and parts
are generally so good these days, I've never had that happen.

Yeah I'm pretty good at that.

But seriously how can a TDR find a short on a plane?

Launch the pulse from two sides of the plane. The delay time to reflection lets you draw a circle around each of the launch points. Where the circles intersect should be the local of your short.

If the circles interesect at two points, launch a third pulse from yet another point on the edge.

--
Bill Sloman, Sydney

 

[Clifford Heath]

On 21/5/19 9:51 am, Bill Sloman wrote:

On Tuesday, May 21, 2019 at 9:20:31 AM UTC+10, Tom Del Rosso wrote:
dagmargoodboat@yahoo.com wrote:

TDR might be good for finding shorted planes. But boards and parts
are generally so good these days, I've never had that happen.

Yeah I'm pretty good at that.

But seriously how can a TDR find a short on a plane?

Launch the pulse from two sides of the plane. The delay time to reflection lets you draw a circle around each of the launch points. Where the circles intersect should be the local of your short.

If the circles interesect at two points, launch a third pulse from yet another point on the edge.

Or launch a pulse from one point, and look at the arrival at two places.
Like a Rotman Lens. I'd love to see some discussion of those here - has
anyone implemented one?

Clifford Heath.

 

[John Larkin]

On Tue, 21 May 2019 12:07:05 +1000, Clifford Heath
<no.spam@please.net> wrote:

On 21/5/19 9:51 am, Bill Sloman wrote:
On Tuesday, May 21, 2019 at 9:20:31 AM UTC+10, Tom Del Rosso wrote:
dagmargoodboat@yahoo.com wrote:

TDR might be good for finding shorted planes. But boards and parts
are generally so good these days, I've never had that happen.

Yeah I'm pretty good at that.

But seriously how can a TDR find a short on a plane?

Launch the pulse from two sides of the plane. The delay time to reflection lets you draw a circle around each of the launch points. Where the circles intersect should be the local of your short.

If the circles interesect at two points, launch a third pulse from yet another point on the edge.


Or launch a pulse from one point, and look at the arrival at two places.
Like a Rotman Lens. I'd love to see some discussion of those here - has
anyone implemented one?

Clifford Heath.

TDRing a plane wouldn't be useful to find a short. A pcb plane is a
very lossy transmission line/capacitor with some weak edge and corner
reflections.

https://www.dropbox.com/s/wkiehn6iowq3emf/TDR_3Vplane.JPG?dl=0

On a loaded board with parts and bypass caps, it would be even worse.

Besides, your board would have to have multiple SMA connectors or
equivalent into all its planes.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[Clifford Heath]

On 21/5/19 12:35 pm, John Larkin wrote:

On Tue, 21 May 2019 12:07:05 +1000, Clifford Heath
no.spam@please.net> wrote:

On 21/5/19 9:51 am, Bill Sloman wrote:
On Tuesday, May 21, 2019 at 9:20:31 AM UTC+10, Tom Del Rosso wrote:
dagmargoodboat@yahoo.com wrote:

TDR might be good for finding shorted planes. But boards and parts
are generally so good these days, I've never had that happen.

Yeah I'm pretty good at that.

But seriously how can a TDR find a short on a plane?

Launch the pulse from two sides of the plane. The delay time to reflection lets you draw a circle around each of the launch points. Where the circles intersect should be the local of your short.

If the circles interesect at two points, launch a third pulse from yet another point on the edge.


Or launch a pulse from one point, and look at the arrival at two places.
Like a Rotman Lens. I'd love to see some discussion of those here - has
anyone implemented one?

Clifford Heath.

TDRing a plane wouldn't be useful to find a short. A pcb plane is a
very lossy transmission line/capacitor with some weak edge and corner
reflections.

https://www.dropbox.com/s/wkiehn6iowq3emf/TDR_3Vplane.JPG?dl=0

On a loaded board with parts and bypass caps, it would be even worse.

Besides, your board would have to have multiple SMA connectors or
equivalent into all its planes.

Just leave some probe points, solder coax to them. The first 0R
reflection should be from the short to the two probe points, and the
time differential would allow some triangulation. You just need to zero
out the respective delays on two ports.

Have you tried a TDR setup with one Tx and more than one sampler?
I did a similar thing with ultrasound once, it was surprisingly effective.

 

On Monday, May 20, 2019 at 7:20:31 PM UTC-4, Tom Del Rosso wrote:

dagmargoodboat@yahoo.com wrote:

TDR might be good for finding shorted planes. But boards and parts
are generally so good these days, I've never had that happen.

Yeah I'm pretty good at that.

But seriously how can a TDR find a short on a plane?

I figured one could just bang the plane in X, then Y, then get the
fault's coordinates from timing the early reflections. Unfortunately
John Larkin, TDR maestro of the known universe, just dumped an ocean of
cold water on that notion.



Cheers,
James Arthur

 

On Monday, May 20, 2019 at 8:37:00 PM UTC-4, Winfield Hill wrote:

dagmargoodboat@yahoo.com wrote...

Bill Sloman wrote:

Microvolt sensitive DVM's are cheaper ...

But a microvolt DVM can't track down a diode-to-ground,
which was the problem.

Yes, it can. If starting from the V+ rail, use the
DVM to measure drops at each fork, and see which one
is carrying current. Voltages on the ones that aren't
don't change. Eventually this narrows down to the
offending item. As you progress past the item, the
voltage drops stay the same, revealing the spot.


--
Thanks,
- Win

I certainly understand the theory, I just don't see this application
of it.

Let's see -- my test current was 30mA, the most I dared without
risking a board full of pricey ICs. That'll make roughly 12uV
across the 400-ish micro ohms of a 100x70mm 1oz. Cu power plane.
Doesn't that sound like a bit of a handful to hand-probe?

It would help if the fault were at constant temperature, but it isn't.
Vf ramps down as the component warms up from the dissipation.

I suppose one could let the board thermally stabilize, then follow the
uV drops, being careful not to touch and warm one's test probes.

In my first case there were five faults (five wrong components
stuffed), which would be mighty fancy tracking / simultaneous
equation-solving with a voltmeter of any kind.

Hitting it with freeze spray while watching Vf would be fast and fun.
A -10C step would be easy to see (maybe even a good time to drag out
an old-school analog VOM .

Cheers,
James Arthur

 

[Bill Sloman]

On Tuesday, May 21, 2019 at 3:29:02 PM UTC+10, dagmarg...@yahoo.com wrote:

On Monday, May 20, 2019 at 7:20:31 PM UTC-4, Tom Del Rosso wrote:
dagmargoodboat@yahoo.com wrote:

TDR might be good for finding shorted planes. But boards and parts
are generally so good these days, I've never had that happen.

Yeah I'm pretty good at that.

But seriously how can a TDR find a short on a plane?

I figured one could just bang the plane in X, then Y, then get the
fault's coordinates from timing the early reflections. Unfortunately
John Larkin, TDR maestro of the known universe, just dumped an ocean of
cold water on that notion.

John Larkin has some interesting ideas about his place in the known universe, some of which suggest that he knows less about the known universe than he thinks, which is to say the universe he knows about may be smaller than the real one.

--
Bill Sloman, Sydney

 

[Bill Sloman]

On Tuesday, May 21, 2019 at 3:20:37 PM UTC+10, dagmarg...@yahoo.com wrote:

On Monday, May 20, 2019 at 8:37:00 PM UTC-4, Winfield Hill wrote:
dagmargoodboat@yahoo.com wrote...

Bill Sloman wrote:

Microvolt sensitive DVM's are cheaper ...

But a microvolt DVM can't track down a diode-to-ground,
which was the problem.

Yes, it can. If starting from the V+ rail, use the
DVM to measure drops at each fork, and see which one
is carrying current. Voltages on the ones that aren't
don't change. Eventually this narrows down to the
offending item. As you progress past the item, the
voltage drops stay the same, revealing the spot.


--
Thanks,
- Win

I certainly understand the theory, I just don't see this application
of it.

Let's see -- my test current was 30mA, the most I dared without
risking a board full of pricey ICs. That'll make roughly 12uV
across the 400-ish micro ohms of a 100x70mm 1oz. Cu power plane.
Doesn't that sound like a bit of a handful to hand-probe?

It's a lot easier if you use a proper four-terminal milli-ohm meter with a sinusoidal or chopper bidirectional current source and synchronously rectify the differential voltage signal between the potential leads.

Hook up the current leads where you like, and use the potential leads to see where the current is flowing. A shorting diode will only carry current one way, so you only see half the voltage swing, but that should be plenty

It would help if the fault were at constant temperature, but it isn't.
Vf ramps down as the component warms up from the dissipation.

Not a great deal.

I suppose one could let the board thermally stabilize, then follow the
uV drops, being careful not to touch and warm one's test probes.

That's where AC excitation and synchronous detection helps - thermal time constants are relatively long and the accompanying microvolt thermocouple voltages don't vary far enough to show up a signal.

In my first case there were five faults (five wrong components
stuffed), which would be mighty fancy tracking / simultaneous
equation-solving with a voltmeter of any kind.

Persistence would do it. There would be tracks/paths where current was flowing (in a measurable direction) and the rest.

Hitting it with freeze spray while watching Vf would be fast and fun.
A -10C step would be easy to see (maybe even a good time to drag out
an old-school analog VOM .

That would work to some extent. Ten degrees Celcius is 20mV on Vf.

Forward diode drops do tend to be all over the place, so one of your five "wrong component" diodes is likely to be carrying most of the current and you'd have to freeze that one to find it, then fix it and move on the next.

--
Bill Sloman, Sydney

 

[George Herold]

On Tuesday, May 21, 2019 at 1:20:37 AM UTC-4, dagmarg...@yahoo.com wrote:

On Monday, May 20, 2019 at 8:37:00 PM UTC-4, Winfield Hill wrote:
dagmargoodboat@yahoo.com wrote...

Bill Sloman wrote:

Microvolt sensitive DVM's are cheaper ...

But a microvolt DVM can't track down a diode-to-ground,
which was the problem.

Yes, it can. If starting from the V+ rail, use the
DVM to measure drops at each fork, and see which one
is carrying current. Voltages on the ones that aren't
don't change. Eventually this narrows down to the
offending item. As you progress past the item, the
voltage drops stay the same, revealing the spot.


--
Thanks,
- Win

I certainly understand the theory, I just don't see this application
of it.

Let's see -- my test current was 30mA, the most I dared without
risking a board full of pricey ICs. That'll make roughly 12uV
across the 400-ish micro ohms of a 100x70mm 1oz. Cu power plane.
Doesn't that sound like a bit of a handful to hand-probe?

It would help if the fault were at constant temperature, but it isn't.
Vf ramps down as the component warms up from the dissipation.

I suppose one could let the board thermally stabilize, then follow the
uV drops, being careful not to touch and warm one's test probes.

In my first case there were five faults (five wrong components
stuffed), which would be mighty fancy tracking / simultaneous
equation-solving with a voltmeter of any kind.

Hitting it with freeze spray while watching Vf would be fast and fun.
A -10C step would be easy to see (maybe even a good time to drag out
an old-school analog VOM .

Cheers,
James Arthur

Hi James I like your technique. It's too bad we don't have better
built in heat sensors (like a snake). (Our lips are fair...
but almost kissing a pcb is going to get one looks from colleagues. :^)

With the DC current idea, could you try and track it down by looking
at the voltage drop along the ground current return line?
(Right... maybe uV's as you guesstimate above.)

George H.

 

[Phil Hobbs]

On 5/21/19 9:07 AM, George Herold wrote:

On Tuesday, May 21, 2019 at 1:20:37 AM UTC-4, dagmarg...@yahoo.com wrote:
On Monday, May 20, 2019 at 8:37:00 PM UTC-4, Winfield Hill wrote:
dagmargoodboat@yahoo.com wrote...

Bill Sloman wrote:

Microvolt sensitive DVM's are cheaper ...

But a microvolt DVM can't track down a diode-to-ground,
which was the problem.

Yes, it can. If starting from the V+ rail, use the
DVM to measure drops at each fork, and see which one
is carrying current. Voltages on the ones that aren't
don't change. Eventually this narrows down to the
offending item. As you progress past the item, the
voltage drops stay the same, revealing the spot.


--
Thanks,
- Win

I certainly understand the theory, I just don't see this application
of it.

Let's see -- my test current was 30mA, the most I dared without
risking a board full of pricey ICs. That'll make roughly 12uV
across the 400-ish micro ohms of a 100x70mm 1oz. Cu power plane.
Doesn't that sound like a bit of a handful to hand-probe?

It would help if the fault were at constant temperature, but it isn't.
Vf ramps down as the component warms up from the dissipation.

I suppose one could let the board thermally stabilize, then follow the
uV drops, being careful not to touch and warm one's test probes.

In my first case there were five faults (five wrong components
stuffed), which would be mighty fancy tracking / simultaneous
equation-solving with a voltmeter of any kind.

Hitting it with freeze spray while watching Vf would be fast and fun.
A -10C step would be easy to see (maybe even a good time to drag out
an old-school analog VOM .

Cheers,
James Arthur

Hi James I like your technique. It's too bad we don't have better
built in heat sensors (like a snake). (Our lips are fair...
but almost kissing a pcb is going to get one looks from colleagues. :^)

Well, _my_ PCBs are almost that good looking. (But not quite.)

With the DC current idea, could you try and track it down by looking
at the voltage drop along the ground current return line?
(Right... maybe uV's as you guesstimate above.)

A chopamp running at some very large gain driving an audio VCO and
speaker might be useful. It would be more useful with some diodes
across the feedback resistor to extend the range to higher voltage
drops. Have to try that one of these times.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

 

[John Larkin]

On Mon, 20 May 2019 22:20:32 -0700 (PDT), dagmargoodboat@yahoo.com
wrote:

On Monday, May 20, 2019 at 8:37:00 PM UTC-4, Winfield Hill wrote:
dagmargoodboat@yahoo.com wrote...

Bill Sloman wrote:

Microvolt sensitive DVM's are cheaper ...

But a microvolt DVM can't track down a diode-to-ground,
which was the problem.

Yes, it can. If starting from the V+ rail, use the
DVM to measure drops at each fork, and see which one
is carrying current. Voltages on the ones that aren't
don't change. Eventually this narrows down to the
offending item. As you progress past the item, the
voltage drops stay the same, revealing the spot.


--
Thanks,
- Win

I certainly understand the theory, I just don't see this application
of it.

Let's see -- my test current was 30mA, the most I dared without
risking a board full of pricey ICs. That'll make roughly 12uV
across the 400-ish micro ohms of a 100x70mm 1oz. Cu power plane.
Doesn't that sound like a bit of a handful to hand-probe?

Don't be shy, pump in an amp or two from a power supply set to make
0.5 volts max. Then use a DVM with uV resolution. Probe any pairs of
points where the plane is accessable, parts or vias, and see which
direction the current is flowing.

But personally, first I'd crank in a lot of current and thermal image.

It would help if the fault were at constant temperature, but it isn't.
Vf ramps down as the component warms up from the dissipation.

I suppose one could let the board thermally stabilize, then follow the
uV drops, being careful not to touch and warm one's test probes.

In my first case there were five faults (five wrong components
stuffed), which would be mighty fancy tracking / simultaneous
equation-solving with a voltmeter of any kind.

Multiple shorts calls for amps and thermal imaging.

Fire your assembler!


--

John Larkin Highland Technology, Inc

lunatic fringe electronics