LED light strand mystery

[darkoshi]

Before xmas last year, I put up a new multi-colored xmas light strand at Qiao's house. It's the simple kind; it doesn't flash or blink or change colors. A few weeks ago, I noticed that 2 of the bulbs, fairly close to the end... the 9th and 10th ones from the plug, to be exact... were out. Every now and then, I see those same 2 bulbs turn back on for a while, or flicker in unison as if they are on a different circuit than all the other bulbs. The other 98 bulbs on the strand stay on steady. The strand itself is motionless the whole time. What could cause just 2 of the bulbs to flicker like that?

Comments

[randomdreams]

I can't give you a for-sure answer, but in many cases LED's have the die mounted on metal that attaches to one terminal, and then a wire about 1/3 the thickness of a human hair bonded to the die, and stretching over to the other terminal. The manufacturer then fills the whole thing up with clear epoxy that forms the lens the light goes through. Any tension/compression or even self-heating from operation and poorly bonded wires will pop loose from the die, and then intermittently contact depending on heat, air pressure, humidity, or anything else that makes the epoxy change shape.
A lot of this is because people used gold wire for bonding dies to carriers for decades, but they're increasingly using copper wire, and unless your die bonder environment is scrupulously clean, the copper picks up oxides and the die bond is, as a result, lousy. (They're formed by pressing the wire against the pad and then vibrating it at ultrasonic frequencies, and the friction produces enough heat to locally melt both surfaces, and also wipes oxides out of the area during the weld formation, but copper picks up a LOT of oxides.)

[darkoshi]

So maybe both bulbs just happen to have a bad connection inside, and are being affected by the same slight environmental stimuli. I should switch the bulbs to different positions, further apart from each other, and see how that affects them. They did seem to be very firmly positioned in their sockets when I checked that.

[randomdreams]

I bet they won't improve. Usually once they start flickering, contact resistance grows where the intermittent open lives, and they get worse.