While trying to get inside one of the Cree LED light bulbs, I came to bear quite a bit of pressure on the inner metal column that houses the Cree XB-G LEDs. Apparently the small pry-bar I was using pushed on the LED packages at an angle. (I’m not sure because I only noticed the damage afterwards.)
[SAFETY WARNING: Do NOT remove the bulb cover and operate the light. This is a non-isolated bulb: You will be exposed to the ac house power.]
My first clue that I had inadvertently begun an LED package tear-down was when I noticed some tiny rectangles of orange plastic on the workbench. Note that these are not the entire LED that has come off from the pc board, but just the phosphor and lens. Apparently the phosphor coating adheres more tightly to the plastic lens over the LED package than to the LED chips and substrate.
This is a pretty neat opportunity to look inside these LED packages.
(I’m trying to consistently use the term “LED package” for the XB-G rather than just LED, because each XB-G is actually a pre-packaged component comprising eight tiny LED chips, which are sometimes called emitters.)
The advantage of packing up all these LED chips into one single package is that you now have a light source that’s much brighter and operates at a higher voltage than a single LED.
A solitary LED usually has a forward voltage of a bit less than 3V. The XB-G has all eight of its LED chips connected together in a string so that their forward voltages all add up. To the LED bulb designer, it looks like a single high-power LED with a forward voltage of 22.5V; 22.5V is a much easier voltage to work with than something less than 3V. Those 8 tiny LED chips fit into a package that is just a tenth of an inch square.
When I noticed the little rectangles of orange lying on the bench I gathered them up and looked at them under the microscope camera.
The photo shows them on my workbench mat with a toothpick for scale. What we’re looking at here is not the entire XB-G package, but just the lens with the phosphor stuck to the bottom. Sure enough, we can see the eight indentations in the yellow stuff showing where the eight LED chips were.
The yellow stuff is the phosphor that covers the LED chips, and is necessary to make a white light LED. The clear stuff that covers the phosphor is the lens, which serves both the protect the LED, and also to shape the light so that it doesn’t go straight out from the LED like a spot light.
When we look at a picture of a nice, whole XB-G package (above), we see the gray substrate on the bottom. The substrate supports the eight LEDs and their interconnects. Its other very important job is that it conducts the heat from the LED chips away from the LEDs and their phosphor and out to the heat sink. Heat is perhaps the biggest culprit in early failure and light degradation for LEDs.
In the photo above the 8 LED chips are faintly visible as yellow bumps in the center of the LED package.
When I inadvertently knocked the lens/phosphor combination off of the substrate, the substrate stayed behind on the metal column. The metal column supports the metal-core pc board that conducts the heat away from the LEDs chips, through the substrate, down the column and out to the heat sink collar.
Here’s a close up of the substrate with the LED chips still stuck to it, mounted on the bulb’s metal-core pc board:
Unfortunately, the LED package was no longer functional at this point: It would have been neat to see the LEDs’ blue light before the phosphor conversion. Apparently the rough treatment they received broke some of the package interconnects.