Finding a faulty chip that draws too much current

Question

Note that this is a theoretical question - there is no schematic I can show. I will show some schematic, but it will be a very simplified version of an actual circuit, only for illustration purposes.

Assume I have a voltage converter that takes as an input my main voltage (from a power supply) and outputs a certain voltage, for example 1.8V. It would look something like this:

^{simulate this circuit – Schematic created using CircuitLab}

When connecting my circuit to the P.S, I notice it draws too much current (the P.S shows that).

Since I have multiple voltage converters in my circuit (not shown here), I check resistance between each output of each converter to ground. I see that the resistance between 1.8V to ground is almost 0 Ohms. Now I know the fault is either in the voltage converter or one (or more) of the other components drawing power from that 1.8V.

I desolder the resistor shown in the image to disconnect the converter from the other components and see that the converter is fine, but checking resistance from the point connected to all those components still shows 0 Ohms.

My question is - how would you check which component is the faulty one, without desoldering each suspicious component? As you can see in the image, the 1.8V supply is connected directly to the components, without a resistor/bead.

For the sake of this question, assume I have access to whatever equipment needed (no matter how pricey). I wouldn't want solutions to be limited due to availability of equipment.

Thank you!

Answer 1

A thermal imager is very useful in this situation. They are not terribly expensive these days. If you don't have one, a bare finger can be substituted for a sensor.

ADDITION: There are also Thermochromic Paints for different temperature ranges that can be used to identify hot spots.

Answer 2

You could use a PCB current probe. A search showed up the following.

Figure 1. A TTi current probe.

The probe-head is held on the PCB trace under investigation and the output can be monitored on an oscilloscope and, presumably, in the case of DC on a multimeter.

Figure 2. The probe head.

I have never heard of a "Fluxgate Magnetometer" before and I doubt they'll give away too much detail. Good old Wikipedia says the following:

A fluxgate magnetometer consists of a small, magnetically susceptible core wrapped by two coils of wire. An alternating electric current is passed through one coil, driving the core through an alternating cycle of magnetic saturation; i.e., magnetised, unmagnetised, inversely magnetised, unmagnetised, magnetised, and so forth. This constantly changing field induces an electric current in the second coil, and this output current is measured by a detector. In a magnetically neutral background, the input and output currents match. However, when the core is exposed to a background field, it is more easily saturated in alignment with that field and less easily saturated in opposition to it. Hence the alternating magnetic field, and the induced output current, are out of step with the input current. The extent to which this is the case depends on the strength of the background magnetic field. Often, the current in the output coil is integrated, yielding an output analog voltage, proportional to the magnetic field. Source: Magnetometer.

Answer 3

Assuming the supply is putting out a large current (eg. hundreds of mA) you can follow the voltage gradient from the supply using a voltmeter on its most sensitive range. When you find the minima on the net (or plane) you've found the sink (Vcc) or the maxima on the ground net.

Kind of a manual implementation of a steepest descent optimization algorithm.

Answer 4

Ghetto FLIR:

Squirt some low boiling point liquid (like flux cleaner) on the board. See where it boils.

https://www.youtube.com/watch?v=t5fICjcaJ3E#t=13m19

Answer 5

The fastest and cheapest way I learn from Youtube.

Power up your board and pour some alcohol. See which area dries up first.

Youtube link: https://www.youtube.com/user/rossmanngroup

Answer 6

There is a spray for that.

Google "cold spray electronics" and you will find many hits, like this one

Spray the stuff on and watch where it disappears most quickly. That is the point generating the heat - ergo drawing too much current.

This stuff has other troubleshooting uses - should be standard in any well equipped electronics lab.

I found a video on YouTube where this method is demonstrated. It is rather slow moving but gives the idea - the short is found around 4 minutes in. Incidentally they used a dusting spray with the can held upside down - even easier than buying freezing spray.

Answer 7

So you have a rail shorted hard to ground. In my experience this is usually a soldering problem.

My technique is to hook the rail in question up to a bench PSU. Set the voltage limit at the normal operating voltage of the rail and the current limit to about 1 amp. The current is something of a compromise, too low and the volt drops will be difficult to measure, too high and you risk burning things up. 1 amp seems a reasonable compromise for most boards.

I then use a multimeter on a sensitive voltage range to trace the flow of current around the board.

Answer 8

You haven't specifically mentioned that you can rule out the traces or visible solder points. So first thing I would do is take a microscope and check the traces (especially in homemade boards) and the solder points for shorts.

I have found many solder shorts (because I'm obviously bad at soldering) but also many copper shorts between traces on self made boards.

This method doesn't take long but won't help you find all the possible faults.

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As you mentioned price is not an issue, I'd say this is another worthy method:

As another real high tech solution, you can use an X-Ray machine. With that you even have the possibility to see shorts under chips which is especially useful with BGA chips.

So that would look something like this:

By X-Ray_Circuit_Board_Zoom.jpg: SecretDiscderivative work: Emdee (X-Ray_Circuit_Board_Zoom.jpg) [CC BY-SA 3.0 or GFDL], via Wikimedia Commons

X-Ray images can be a bit misleading at times, but you get used to interpret what you see, much like a doctor does.

If the machines supports it, you can also look at different angles and do a full 3D-Scan, which is pretty impressive but often not necessary.

And it being X-Ray you have quite a bit of paperwork ahead of you getting it all set up.

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Another method, which is related to the voltage drop method, might be using a Milli-Ohm-Meter and measuring all the Vcc to GND nodes near the chips.

While your normal meter might read 0 Ohm, a Milli-Ohm-Meter might show a value, the node with the least resistance would be the most interesting one.

Answer 9

Put some thermosensitive paper (like from a shopping receipt) on the circuit. Here is a Youtube video.

Power up. Wait. Check for discoloration. Of course, a really solid short circuit has a voltage of zero across and will not produce significant heat. But most faulty circuits with a large current draw will have enough resistance to be trackable with heat other than only at the voltage regulator.

Answer 10

Inject a square wave and scope the (tiny - obviously) ringing at the driven-end and then "walk" the scope's earth (and probe of course) along each path (toward each IC). The ringing will lessen until you arrive at the short itself (with both scope's earth return and probe-tip on either side of it).

Answer 11

Your problem is the result of management malpractice by the creators of the circuit board: they failed to design for testability. This is a common problem in automatic test engineering.

The answers above using thermal imaging or some other way to find the hot chip are your best bet. Note, however, that if the chip is an absolute short, it will not dissipate any power and will appear cool because ALL of the power is heating the internal resistance of the power supply. In that case, the current probe shown in the previous answer might work... if your circuit board traces are large enough and spaced far enough to isolate their magnetic fields.

Alas, if you have a modern circuit board with 17 layers and super tiny SMT chips, you are probably out of luck. Logistic support analysis generally designates such devices as disposable.

Welcome to the ATE world.

Answer 12

This is just a thought experiment.

Using a current source pulsing at about 1 kHz DC square wave at about 0.9 µS rise or fall time: This will make an audible tone at the start of frequency range of a standart AM receiver. The ground plane junction of the fault path must be distinguishable at the most. You may adjust the antenna length to adjust sensitivity.

I get the idea after seeing this answer about EMC: https://electronics.stackexchange.com/a/30684/62403

Answer 13

Techniques that rely on detecting heat dissipated at the short will be of limited use when you have bga packages. The package will hide the short. A 10 mil trace is good for about 1/2 amp. Go up to 1 amp and you risk fusing the trace (not necessarily a power trace but what is it shorted to?). I would de-solder the chips one at a time until the short is eliminated or becomes apparent.

Answer 14

Another option is to measure (with no power applied) the ohms at each IC, between V++ & GND. Assuming there is a short, the ohms will be lower than the rest. I've used this technique before to isolate, but I confess never on a PCB. Still, it is one more available option. With these digital meters you can measure the ohms so precisely. And where the ohms are lowest, is where the short is.