Why does an LED act like this?

[Falcon69]

Okay, I've experienced this numerous times, and I don't know what's happening.

I have an 0603 LED soldered to a circuit board I am working on. When I place my multi-meter on the diode setting and test the LED when it is NOT soldered on the board, it lights up.

However, when I solder it to the board, it does not. So I unsolder it from the board and connect to the multi-meter.  It works again.

Why does it not light on the circuit board when the multi-meter's leads are connected directly to the cathode and anode of the LED?

Thanks in advance for your input in clearing up this mystery.

[Mr.B]

You would need to show us more of the circuit.
What other components are having an influence on the test conditions...

[SeanB]

Possible you have a pad extending under the LED and shorting it out. Check resistance across the device in circuit to see.

[Falcon69]

well, it is connected to a rather large set of traces (1mm then branches off to rest of circuit at .6 and .4 and .4 and .4).   then is connected to a voltage regulators output.  Could it be reading resistance off all that?

It lights up perfectly fine if I connected 5 volts and ground across it.  But doesn't light when a multi-meter is connected (using it's diode output)

[jlmoon]

Have a schematic?
Alternate component loading on diode test function of your meter? 

[Zero999]

Have a schematic?
Alternate component loading on diode test function of your meter?

I'd suspect that's the most likely reason why this is happening.

[Falcon69]

okay, that makes sense.

Maybe the added resistance of the traces and the other components in-line are effecting the current through the Multi-Meter and it isn't supplying enough current to light the thing.

I have 16 other led's in the circuit, and they light up with the multi-meter, however, they are not connected to as many components are such large traces.

[SL4P]

Multimeter red-black is often the opposite voltage polarity in diode test mode (electron flows vs current flow).

But providing a circuit would be a good starting place for us to comment.
Remember your meter may also source more current than the LED wants to see - so it may work (for a little while) but not in the future.