modified: Saturday 18 June 2016
Cutting open red COB Leds
These are '10W red LED COBs' off eBay with a AA battery for scale:
They are less than 2 AUD each from China and do seem to be able to survive 10W, provided you heatsink them. The orange bit ontop seems to be some kind of silicone.
Last night a friend and I decided to chop some of them up.
COB stands for 'chip on board'. COBs are basically a bunch of LEDs in a single module, often wired in a combination of series and parallel.
If you gradually raise the voltage/current going through them, good COBs evenly turn on. Bad ones have mismatched diodes -- parts of them turn on first, then everything starts to appear more even as you up the current.
Cutting them open
Our first two victims:
Here you can see the 9 individual LED dies as well as the gold bond wires linking them. They sit in a transparent layer of silicone below the main orange silicone.
Unfortunately these two COBs were killed during their interrogation. We're not sure of exactly how we killed them. Possibly we cut through some bond wires, but later we found applying pressure in certain ways also kills the modules whilst you are cutting them.
For the next victim we kept a small amount of power applied during the operation. This allowed us to get feedback on when we were cutting too ferociously, as the LEDs on the COB would go out or dim. The results were very interesting.
These pictures have false/altered colour: in reality things looked a lot more red.
It seems like the LEDs are actually blue. Presumably the orange layer acts like a phosphor to convert the blue into red. We checked the spectrum using the reflection off a DVD (they act as a diffraction grating) and found only two primary colour components, so the LEDs are definitely not white.
My guess is that they use blue LEDs in these coloured COBs because that's what they use in their white LEDs (below the yellow phosphor). I expect that the majority of the market is for white COBs, not coloured ones. It's simpler to just change the phosphor than it is to use a different chemistry of LEDs to get the different colours.
Anyway, all just guessing.
The final victim, back in daylight:
...and shining purple (red + blue) onto a piece of paper (the colour is decently accurate in this photo):