Oops !

Most of those meters are remarkably wimpy when it comes to test currents, i.e. 10mA. For this purpose I made a small 0.5A current source that's limited to 0.5V maximum output to avoid any possiblility of damaging a semiconductor. A 500mA current makes it easy to see the increasing drop along a conductor as you trace toward the short, using an ordinary handheld multimeter.

. 10 ohms, 5W . +5V o----/\\/\\---+----o test- . ,--|

And in the future, consider buying electrically tested boards for complex prototypes. There's a good possibility the short is hidden under a component body.

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

I just recently tracked down external layer power to ground short on a fully populated board. Just set your power supply max voltage below the circuits operational voltage in case the short accidentally clears. Set the current limit to few hundred mA. Probe the voltage drop across every decoupling cap. The lowest voltage drop will indicate that you are getting close to the location of the short and it will very likely not be the decoupling cap but a nearby IC pin. So far I have not had a single shorted ceramic cap nor a shorted board due to poor manufacturing.

--

    Boris Mohar

Standard prototype nightmare. You assemble a multi layer board with lots of high pin count SM devices, plus lots of decoupling caps and so on. Final check before power on - AAAAAAggghhh ! fractional ohm short between the ground and power planes !!. The pcb is (probably) ok, because you did some basic checks on it before you started assembling (and it was 'tested').

So, does anyone have any magic recipes for recovering from this ?

It may be a solder bridge or two, or a bad component (I've seen an 0603 cap that was a solid short.)

Does measuring resistances help ? - ground/power planes are pretty low impedance to start with

Even with reasonable tools, removing and replacing high pin count (100 -

200+) devices is somewhat risky.

I seem to remember that HP had a 'current sniffer' that enabled you to inject pulses and track them. Would that work with a plane/plane short ? Could you do something similar with a signal generator and a Spectrum Analyser with a suitable probe ?

Dave

[snip]

I had a 6V/100A supply. On bare screened thick film hybrid boards one little pulse was all it took to remove "invisible" shorts ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

Start with the keys-under-the-lamppost method. Goggle at it for ages with a jeweller's loop. Remove components that are easy to get off.

If that doesn't find it, try connecting a beefy power supply to the rails, at a suitably low voltage (below the normal supply) and see if you can blow the thing out, or at least see where it gets warm- assuming it's a solder blob and not a VCC/ground pair swapped somewhere.

But the best bet with these things is, at least the first time, assemble it incrementally and test as you go along.

Paul Burke

Pinhole faults only go "plink" ;-)

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

| 1962 | I love to cook with wine. Sometimes I even put it in the food.

I was being somewhat facetious... plink "adjustment" was made after ohming and microscope viewing ;-)

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

| 1962 | I love to cook with wine. Sometimes I even put it in the food.

Yes. I managed to locate a shorting whisker with a Keithley 197, which resolves to the milliohm. As you say it is painful, but when you are desperately desperate.......

--
Tony Williams.

Use a suitably limited power supply to put an ampere or so through the short and use a uV meter to trace the current flow. That should locate the short to a few mm. Don't try to blow it out with too much current. The short is not necessarily the weakest link.

It worked for me.

Jeroen Belleman

I recently helped one of our junior engineers troubleshoot this problem using this technique. Turns out the fellow neglected to read the part of the data sheet about the thermal pad on the bottom of the quad flat pack (many of the new ICs have them) - he had vias that were shorting out to this pad.

Tom

Hello Dave,

The "pcb is (probably) ok" statement makes me worry. What's the confidence level there? Take another bare PCB from the stack and measure it.

Ok, here are the tricks I used: Supply it with VCC at a few amps. Then take a millivoltmeter and solidly connect one probe to where VCC hits the board (board side, not on the wire or other side of a connector). Tap along the board with the other probe and look for a dropping gradient until you find the "valley". It'll be shallow so patience is required.

Another one that some consider a dowsing rod method: Take an EMCO near field probe for H-fields. If you don't have one make a few loops of 1/2" diameter at the end of a coax. Then ride a few hundred kHz on top of VCC, 10-20mVpp is enough. For feeding in use very good coax such as quad shield and lots of toroids slid onto it. Hook up to an analyzer or comm receiver and sniff along. Toroids on that coax, too. #77 material or something. Any short that isn't part of a plane but represents a small "trace" will light up.

Then there is the IR camera trick. Usually a bridge is the least conductive part of the whole scenario and tends to get warmer than its surroundings.

Don't do the car battery trick to burn it out. You or someone else can get hurt and if it was the PCB itself the stench can be awful.

Regards, Joerg

Hello Jim,

A tech who did that at one client needed a new shirt afterwards. Thank God he wore glasses.

Regards, Joerg

I've used this method with a current limited bench supply when all else has failed. Set the volts low and current to zero. Crank up the current a bit at a time looking/feeling & sniffing for a result. rob

Hello Jim,

If you happen to know it's a pinhole fault, that is. Non-pinhole faults have a tendency to go "KABOOM".

Regards, Joerg

Try reflowing, at least all the high-pin-count / low-pitch parts. Use plenty of liquid flux (e.g., an RA flux pen). If that doesn't work, repeat and poke around with some desoldering braid to see what you might wick out from behind the pins.

At least it worked for me. I brought up the first board of a proto AOK step by step, got cocky with the second one and populated it fully, complete with a power short. Spent a lot of time under a loupe, probing, scratching with picks, etc. until I was convinced it was a bad PCB. Reflowing the fine-pitch parts solved the problem. (Along with a little solder wicking above the foot, to draw away excess solder but not kill the joint under the foot.)

Not much risk, but the parts probably aren't cheap and labor isn't either. The air bath + air pencil system at

works really well. The board takes no damage, and with some care the parts can even be reused.

HTH, Richard

I pesume you have checked it with an ESR meter to eliminate the possibility of a reversed semiconductor?

Next step (after visual inspection, esp for bridged pins on any of the SM ICs) would be to isolate sections, if possible, to narrow the field a bit before you start removing bits

David - who is sympathetic

Dave wrote:

Pump a load of current through it and see where the smoke comes from?

--
Dirk

The Consensus:-
The political party for the new millenium
http://www.theconsensus.org