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



-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist