Finding a short on a prototype board

[Charlie E.]

Hi All,
Hoping ya'll will have some brilliant suggestion to help me out...
again!

Just received a batch of prototype boards. Added the through hole
parts to two, and tried to power up. No power!

After a bit of testing, determined that my regulated VCC is shorted to
ground! :-( So, I started looking for the obvious - solder bridges,
vias shorted to an adjacent pad, going over my design files in detail
for maybe an error on my part, etc. No joy, can't see where the error
is.

So, my problem, how can I locate this elusive short. Any ideas? It is
a populated board, so I can't just crank up current till it indicates
the problem 'visually' (and aromatically!) ;-)

Charlie

 

[hamilton]

Hi All,
Hoping ya'll will have some brilliant suggestion to help me out...
again!

Just received a batch of prototype boards. Added the through hole
parts to two, and tried to power up. No power!

After a bit of testing, determined that my regulated VCC is shorted to
ground! :-( So, I started looking for the obvious - solder bridges,
vias shorted to an adjacent pad, going over my design files in detail
for maybe an error on my part, etc. No joy, can't see where the error
is.

So, my problem, how can I locate this elusive short. Any ideas? It is
a populated board, so I can't just crank up current till it indicates
the problem 'visually' (and aromatically!) ;-)

Charlie

A on-destructive way to find a short is with a micro-ohm meter.

Google for "micro ohm meter", lots of hits to DIY or purchase.

This is a long and tedious process. (unless its an obvious short easily
cleared up)

If you are using thru hold parts, cut off the VCC leg on each chip one
at a time. If its a bad chip, you'll find it and repair the cut leads.

If its a PCB problem, go the uOHM meter route.

hamilton

 

[TTman]

Hi All,
Hoping ya'll will have some brilliant suggestion to help me out...
again!

Just received a batch of prototype boards. Added the through hole
parts to two, and tried to power up. No power!

After a bit of testing, determined that my regulated VCC is shorted to
ground! :-( So, I started looking for the obvious - solder bridges,
vias shorted to an adjacent pad, going over my design files in detail
for maybe an error on my part, etc. No joy, can't see where the error
is.

So, my problem, how can I locate this elusive short. Any ideas? It is
a populated board, so I can't just crank up current till it indicates
the problem 'visually' (and aromatically!) ;-)

Charlie

Remove any tantalum caps first, then check for chips the wrong way round...
protection diode the wrong way round?

 

[Nemo]

Is it a dead short, i.e. you put 1 volt on it and get just a couple of
mV across the power rails, or is it a diode drop? If a diode drop, it is
probably a component the wrong way round.

Have you used a type of regulator or transistor you've not used before?
Those 3 pin footprints are always the easiest ones to get the pins swapped.

About 20 years ago when multilayer boards were still considered high
tech, I got my first 4 layer PCB back... due to some fabrication error,
ALL vias were shorted to the 0V plane. Hah!

I have occasionally seen PCB's under-etched, leaving whiskers of copper
between tracks. This is why one pays to have boards "electronically tested".

 

[Ecnerwal]

Hi All,
Hoping ya'll will have some brilliant suggestion to help me out...
again!

Just received a batch of prototype boards. Added the through hole
parts to two, and tried to power up. No power!

After a bit of testing, determined that my regulated VCC is shorted to
ground! :-( So, I started looking for the obvious - solder bridges,
vias shorted to an adjacent pad, going over my design files in detail
for maybe an error on my part, etc. No joy, can't see where the error
is.

So, my problem, how can I locate this elusive short. Any ideas? It is
a populated board, so I can't just crank up current till it indicates
the problem 'visually' (and aromatically!) ;-)

Charlie

You say you have populated two. What about using one of the unpopulated
(you have more than two made?) and the brute force method? Or are all
the other boards populated with SMT parts but not with through-hole
parts? In any case, if they are hosed, the parts may be the last thing
to worry about (or the problem.)

Before visual and aromatic cues, warmth will indicate where to look as
you crank up the amperage. If you don't have a fancy thermal imager (or
a friend in the fire department who'd like an opportunity to play with
theirs for a good cause,) use your fingertips.

 

[Charlie E.]

You say you have populated two. What about using one of the unpopulated
(you have more than two made?) and the brute force method? Or are all
the other boards populated with SMT parts but not with through-hole
parts? In any case, if they are hosed, the parts may be the last thing
to worry about (or the problem.)

Before visual and aromatic cues, warmth will indicate where to look as
you crank up the amperage. If you don't have a fancy thermal imager (or
a friend in the fire department who'd like an opportunity to play with
theirs for a good cause,) use your fingertips.

Yes, they all have SMT parts populated. I have asked my board shop
for a copy of the unpopulated board, and they are sending me one.
Looks like I get to try and make a micro-ohmmeter... ;-)

Charlie

 

[Rich Webb]

Hi All,
Hoping ya'll will have some brilliant suggestion to help me out...
again!

Just received a batch of prototype boards. Added the through hole
parts to two, and tried to power up. No power!

After a bit of testing, determined that my regulated VCC is shorted to
ground! :-( So, I started looking for the obvious - solder bridges,
vias shorted to an adjacent pad, going over my design files in detail
for maybe an error on my part, etc. No joy, can't see where the error
is.

So, my problem, how can I locate this elusive short. Any ideas? It is
a populated board, so I can't just crank up current till it indicates
the problem 'visually' (and aromatically!) ;-)

This isn't a direct answer (probably!) but here's a wonderful video from
David L. captured live trying to find a short on a populated board:
<http://www.eevblog.com/2011/12/31/eevblog-230-arducopter-ardupilot-troubleshooting/>

Always good stuff over there (down there?).

 

[Spehro Pefhany]

Hi All,
Hoping ya'll will have some brilliant suggestion to help me out...
again!

Just received a batch of prototype boards. Added the through hole
parts to two, and tried to power up. No power!

After a bit of testing, determined that my regulated VCC is shorted to
ground! :-( So, I started looking for the obvious - solder bridges,
vias shorted to an adjacent pad, going over my design files in detail
for maybe an error on my part, etc. No joy, can't see where the error
is.

So, my problem, how can I locate this elusive short. Any ideas? It is
a populated board, so I can't just crank up current till it indicates
the problem 'visually' (and aromatically!) ;-)

Charlie

You've gotten some good suggestions.. when you're looking for stuff
(after you've absolved the bare boards), look at chips with pads under
them, parts with vias or traces under them, mechanical stuff.

Is it possible that a through-hole part is completing a circuit
between power nets?

I normally go searching with a bit of current and follow voltage
gradients to the fault, but it's not always easy. On one I found a
part that the manufacturer had moved from one side of the board to the
other without re-routing the board (extra tooling charge) but the legs
of the through-hole part were sitting on a trace connected to the
opposite supply, separated only by the solder resist. About 499 times
out of 500 that was okay. 8-(

 

[Charlie E.]

Use your standard power connector and wires, a variable supply and
resistor, and start with supply set to required operational voltage
level and adjust resistor for (say) 10mA.
Use a DVM (say) with low side on PCB at ground pad and poke on traces
with high side traveling toward supply trace while noting voltage.
Follow each physical path.
In some area there should be a much smaller change/increase -
indicating likely source of problem.

Thanks for all the great suggestions. Looks like I will need to
finally break down and get me a bench supply! A commercial
micro-ohmmeter is definitely not in the budget right now... ;-)

Charlie

 

[Charlie E.]

Use a D-cell battery and a 1 ohm resistor to force current, and a good
DVM to measure voltage drops in traces.

Of course, these are the power nodes, so I have copper pours on both
sides of the board... :-(

Charlie

 

[[email protected]]

Remove any tantalum caps first, then check for chips the wrong way round...
protection diode the wrong way round?

Also look for a hint from the I/V curve. Low resistive => short. Diode =>
backwards chip or protection diode. Odd diode voltage => backwards capacitor.
Etc.

 

[tm]

Hi All,
Hoping ya'll will have some brilliant suggestion to help me out...
again!

Just received a batch of prototype boards. Added the through hole
parts to two, and tried to power up. No power!

After a bit of testing, determined that my regulated VCC is shorted to
ground! :-( So, I started looking for the obvious - solder bridges,
vias shorted to an adjacent pad, going over my design files in detail
for maybe an error on my part, etc. No joy, can't see where the error
is.

So, my problem, how can I locate this elusive short. Any ideas? It is
a populated board, so I can't just crank up current till it indicates
the problem 'visually' (and aromatically!) ;-)

Charlie

Chill the board in the refrigerator or freezer. take it out and apply
power
look for the dry spot. The warmth of the short will dry out any
condensation
making the short easy to find.

regards,
al

____________________________

That works. First put the board in the freezer for say 20 minutes. Remove
the board to the bench and apply the limited current. The frost will melt
along the trace and you can take a quick picture for review.

tm

 

[Fred Abse]

Looks like I will need to finally
break down and get me a bench supply!

How on earth do you manage without one?

I have four on my bench.

 

[Fred Abse]

I really want an imaging magnetic field mapper for situations like this. I
have applied an AC current to a short, and probed with a magnetic pickup,
like a small drum core connected to an amp/speaker. You can *hear* the
short.

That might be an application for the fluxgate current probe discussed here
a while ago.

The question (still unanswered) about inner layers remains.

 

[hamilton]

How on earth do you manage without one?

I have four on my bench.

Why purchase a bench supply ?

Google for LM317.

h

 

[Charlie E.]

Of course, these are the power nodes, so I have copper pours on both
sides of the board... :-(

Charlie

Ok, found the problem.

I had to add the power pins to my symbol for the MCU. When adding the
pins, got one VDD-VSS pair reversed! As the boss says, Wrong Way
Charlie strikes again!

Now I have to see what I can do to fix this on these prototype. Can't
waste $1200 to do a re-spin... :-(

Charlie

 

[Charlie E.]

Blue wire ?

...Jim Thompson

Yep! have to lift the pins (Veerrry Carefully...) and then air-wire
them to the correct places. Well, I have ten boards to try and get it
right on... ;-)

Charlie

 

[Charlie E.]

So the Vdd-Vss short is internal to the CPU chip?

You can probably just cut open both bad connections, if it's not a
BGA; there are likely other pins that can do the job.

Your boss is being unkind.

Well, she is the boss... ;-)

It is a PIC24FJ256TQFP. I already found a problem with the routing on
one ground, don't want to lose another one. I always lift these pins
unintentionally when I try to hand solder them. Lets see if I can do
it when I want to!

Charlie

 

[qrk]

Hi All,
Hoping ya'll will have some brilliant suggestion to help me out...
again!

Just received a batch of prototype boards. Added the through hole
parts to two, and tried to power up. No power!

After a bit of testing, determined that my regulated VCC is shorted to
ground! :-( So, I started looking for the obvious - solder bridges,
vias shorted to an adjacent pad, going over my design files in detail
for maybe an error on my part, etc. No joy, can't see where the error
is.

So, my problem, how can I locate this elusive short. Any ideas? It is
a populated board, so I can't just crank up current till it indicates
the problem 'visually' (and aromatically!) ;-)

Charlie

Techniques I've used:
1. Use a 0.5V, 10amp adjustable current supply. Voltage is low enough
so it won't affect parts on the board. We have a Fluke TiS IR imager
which allows us to locate the warm spot. Sometimes this works fine,
other times not depending on the short.

2. Feed a few amps of current to the power plane and measure with a
microvolt meter at various bypass capacitors around the board. By
deduction, you can get close to the area. You may need to feed the
current in various areas to figure out what area may be shorted.

If your pcb house messed up, you may have many shorts all over your
board. If that's so, the above techniques won't work. I've come across
these on rare occasions, one just recently.

 

[Spehro Pefhany]

You forgot the most essential part..the elbow grease needed to put it
all together.

LOL. I think I'll send a summer student to order a jar of that stuff
from McMaster...