Finding power - gnd shorts

The problem is that in 230 V countries, the mains fuses are only 10-16 A, so if there is a stubborn short on the PCB, the 50-100 A short circuit current may blow the mains fuse but leave the PCB short unaffected :-(.

For stubborn shorts, it might be a better idea to use a welding transformer.

Even if you do not try to evaporate the short, one idea might be to let some AC current (a few amps) flow through the PCB and use a miniature coil connected to the oscilloscope probe and try to locate in which PCB track branches the AC current flows to the short. By rotating the coil, you might even be able to determine if the current is flowing in a track along the long or short axis of the PCB.

Paul

Do you have a blank board to check? I hope so. Can you see the clearance where vias etc. pass through the power planes by holding the board up to a strong light?

As others said you can follow the gradient. If the gradient does not change then you've found a direction where the plane is not carrying current. But with two internal planes this won't be all that easy.

Hate to say it, but it kinda sounds like it might be internal to the board from what you're saying. Like maybe they made two gnd planes or something like that.

Just a couple of times I've taken almost every single component out of a board to find the problem with 100% certainty (failure analysis, just to ensure we didn't get more like that). Whiskery shorts on boards (sometimes intermittent so it could pass e-test) were the hardest to find, but you could see them under a microscope. OTOH, sometimes it's huge and right in front of your face.

Best regards, Spehro Pefhany

The mains fuse doesn't immediately blow at 16A. I've seen copper evaporate from the PCB with a short on the the mains side with the mains fuse still left intact.

Yes, this sounds like an excellent idea! Maybe I can even evaporate the entire power plane completely eliminating any possibility of a short!

Rick

Some 20 years ago I had a 28 slot, 8 layer backplane with this sort of problem - dust on the artworks during manufacture. I had to us a 10 amp power supply to vaporise all of the shorts. Most went at about an amp but there were a few that took more than 5 Amps before they opened. Fortunately no active components on that board.

Ian

For that purpose I can recommend one of our machines with 40,000uF caps at 350 Volts, they're ideal for evaporating copper from boards!

No, but I have over a hundred boards that are working just fine. Actually, I have 103 working boards... unfortunately, I need to deliver 104, no kidding! I have three other boards where I have identified the problems and can be fixed. One of those has at least two open vias. These boards are from the batch I got from Sunstone where they had a 25% X-out rate due to plating problems in the vias. I ordered 6 panels. They made 7 and I still didn't get enough boards. They had to make a seventh panel with 16 working boards to complete the order.

I was very concerned about vias opening in the field. So I am glad that I have only found one board with open vias after assembly. I just got 56 boards out of a four day bake which I will be retesting.

Rick

Many posts indicate that I should expect the something to be getting warm from the 1 Amp of current. Yes, the power supply is getting warm. The voltage on the board is only 5 mV max. That equates to 5 mW. I think it would take a very sensitive device to see the temperature rise from 5 mW of power. Handing the board wearing gloves would leave a larger thermal imprint than this.

But using a FLIR camera does sound like fun!

Rick

Hi Rick I've described this methode before. Most people seem to think that a method of finding the short requires feeding current through the short. My method uses a different idea. Put your supply across the 12v line, from one end to the other, in current limit. The line then has a small voltage drop across it. Place a DVM lead on the ground. The location isn't important since there is no current in the ground. Use the 200mv scale. This works best with a 4 or 5 digit meter. With the other lead, probe along the 12V line. When the meter reads closest to zero, you are at the location of the short. I have used this method many times when other methods seem to fail. The idea is to consider the 12 v line to be like a resistor with a tap on it. The ground it the tap. With the second lead as another tap, when both taps are at the same location along the resistor, there is 0 volts. Dwight

When you say you found various shorts, were they design flaws (which would be repeated on each board) or manufacturing flaws (which would be individual)? This is one board out of a run of 110. I have found one other board with a short on the 3 volt rail, but this one is on the 12 volt rail. So there is no design flaw.

I am currently suspecting a decoupling cap at this point. The gradient is small and points toward one end of my 4.5 x 0.85" board. The 12 volt plane is only on about half the board and power has been applied to the end near the middle of the board. The gradient points to the opposite end. When I get some more time to work on this, I will test with power on the "opposite" end and test toward the power connector. If I see the same gradient, I will start removing components. If the gradient slopes the other way, I will suspect multiple shorts which are likely an internal board problem (which I expect is unlikely).

BTW, thanks for all of the suggestions everyone! Some of the ideas were obvious to me, others at least made me think a bit. I appreciate the different viewpoints.

I may try the freeze spray thing. I don't have a supply that will put out 10 A, but I have an AC transformer that puts out 1 VAC at some huge current. That with a diode might actually do the job, who knows?

Rick

Yes, that's a clear advantage of the 110V (one can expect twice the current at 220 is needed to blow the fuses), but 220 is still more fun for this application - sparkles, smoke, danger etc. (I recently got 100n at 3kV discharged left to right hand fingertips... was making a new 3kV 60W power supply, would not have been a success without any of that :-). For practical purposes a car battery should be the best source to blow PCB areas.

Didi

------------------------------------------------------ Dimiter Popoff Transgalactic Instruments

------------------------------------------------------

Original message:

What was the size of your vias? I've been using 8 mil trace and space and 21 mil vias for many years without problems. However, some of the larger fine-pitch stuff I use now will soon force finer traces and spacing.

Mark Borgerson

By the way, can you make a hot sausage in a 110 V country by just sticking the neutral at one end of the sausage and live conductor into the other end ?

Paul

If I get to that point, would you like to provide a unit for evaluation and I will post the results. Maybe this will generate a lot of sales???

Rick

I used 6/6 and 10 mil vias in 24 mil pads. Sunstone originally said they could make 10 mil vias, but after I got the boards they said they used a 13 mil drill since that was within their spec of +- 3 mil. As long as they got the holes on the pads, I didn't care, but some of the holes are right at the edge of the pads. I've never been able to tell if this is a problem as long as the trace is not cut from the pad.

Rick

No, that operation here requires an adaptor and half-length sausages. :-)

Good idea! When I get some time I will try this. Thanks.

Rick

use car battery? ;)

-Lasse

No extra sales I'm affraid. It is not their intended use but was their self-destruct mechanism. :-)