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.)
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chiming
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Date: 2018-02-20 01:53 am (UTC)From: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.)