hello
Can anyone explain this video?

 

When LED's are poorly manufactured, or improperly handled in assembly, they can experience bond wire failure.
Normally, they are just plain old dead, but under certain circumstances, they can form a "thermal oscillator" where the bond wire makes contact, then almost immediately, the minute heat generated causes the circuit to open, the die cools, then the cycle repeats.

There must be 3 in series here, one is bad, all three flash.

The dimensions in question are so small that the frequency of this blinking can be quite fast.

 

i work in trading company and we test a samples of each part. for example i test 2000 modules from 50000 smd modules.
in test process i drive modules with 1.5 times of normal current. for example if one module drive 0.02 ampere in 12 volt, i increase voltage and drive it with 0.03 ampere for 4 hour.
if 3 percent of led's goes blinking in 4 hour, i conclude this part has no quality.
but for led's in video i test a sample of them and has no problem.

 

When LED's are poorly manufactured, or improperly handled in assembly, they can experience bond wire failure.
Normally, they are just plain old dead, but under certain circumstances, they can form a "thermal oscillator" where the bond wire makes contact, then almost immediately, the minute heat generated causes the circuit to open, the die cools, then the cycle repeats.

There must be 3 in series here, one is bad, all three flash.

The dimensions in question are so small that the frequency of this blinking can be quite fast.

i test your idea about "thermal oscillator" and i think it's true. in this video when i close heater near led in the middle it start blinking.

but i have understood that in all case, the led in the middle has this problem.
do you have any idea about this?
also when i push tow side of pcb board, blinking become stop and when i pull it, led turns off.

have you any idea about this problem for middle led?

 

I'll bet the problem is caused during reflow soldering- the center LED is overheated and damaged.

The middle of the board gets hotter, or stays hot longer, due to the geometry of the PCB.
This can be confirmed by running the boards through the reflow with thermocouples installed to log the temperature.

Check the LED data sheet, the soldering temperature profile shows the time / temp the LED's can safely withstand.

The size and shape of a PCB can have a big influence on the maximum temperature that parts are subject to, LED's are very sensitive, a good manufacturer will run a board fully instrumented with temp loggers to validate the setup before running the production line.

 

do you know any way for testing modules for detecting this defect ? because when i implement 12 volt for turning module on all work well but after one week or more, blinking become start.

 

Burn them in for a week at elevated temperature.

 

Do you know any good sources for study about LEDs defects?