Dead LEDs

Getting White LEDs with long lifetimes is easy. Getting them
at a low cost is another matter.

I've been testing various white LEDs for longevity and been
getting some rather bad results from Chinese sourced
products. Reports from others testing LEDs for the same
purpose are similar. This is not a total surprise as the
general received wisdom is that this is the case. It is
however a somewhat surprise as it is not obvious why the
Chinese products should tend to be so bad. A Chinese
supplier (name will not be stated) even made some changes to
try and meet my spec and the results were no better.

By Chinese I mean companies that are Chinese based - NOT
known internationals who are domiciled elsewhere but may use
Chinese manufacturing (eg Avago, Cree, Nichia, ...) - such
companies are demonstrably more liable to get it right.

The universal claim is that white LEDs last 100,000 hours. I
can assure you that many don't come anywhere close (by 2+
orders of magnitude in some cases).

My questions are:

- What mechanism makes Chinese LEDs so bad?

- Why is this allowed to be? ie why don't they do whatever
it takes to fix it.

If anyone feels that my statements are a generalisation and
that some Chinese white LEDs do have the sort of lifetimes
one would expect then *PLEASE* do tell me the brands!. I'd
be extremely happy to be wrong and to be able to source LEDs
at non-market-leader prices.

For white phosphor LEDs (blue radiator and yellow phosphor
re-radiator) the degradation mechanism seems to be actual
LED die output level. A possible mechanism in some cases MAY
be die over-temperature due to excessive over-rating of die
current capabilities. Phosphor death does not seem to be an
issue in what I have seen. (it is in some other cases). Die
bonding adhesive to the LED structure cup is claimed to be a
problem in some cases but when this was changed in the LEDs
I was getting to a Japanese sourced bonding product of good
parentage it made about zero difference.

Interestingly, and not directly related, some name brand
LEDs give atrocious spectral results at very low currents
while others are about as good across a wide range of
currents.



            Russell



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