LEDs are typically mounted on a submount wafer that is later diced to separate out the individual LEDs/submounts. Each submount portion of the wafer has top electrodes that are bonded to electrodes on the LED, such as by ultrasonic bonding. An underfill material, such as epoxy or silicone, is then injected under the LED to provide mechanical support and protect the LED from contaminants. Any underfill material substantially outside of the LED footprint (e.g., outside of 20 microns) is removed so that the submount surface is clean. One reason to remove the underfill material that extends beyond the LED die footprint is that, if the underfill is epoxy (starts off yellow) and is exposed to UV light, the epoxy turns black and absorbs light.
The submount also has a set of more robust electrodes, electrically connected by a metal pattern to the LED electrodes, that are typically bonded to a printed circuit board (after the submount wafer is diced) using conventional solder reflow or other means.
It is known to provide reflective metal electrodes on the bottom surface of the LEDs so that light emitted downward by the LED active layer is reflected upward rather than being absorbed by the submount. Some of the LED's emitted light also impinges on the submount surface surrounding the LED die footprint. To reflect that light, it is known to deposit a reflective metal ring around the LED, such as silver or aluminum.
Forming a metal reflector takes additional steps, and the metal must be insulated from the top metal pattern on the submount.
What is needed is a better way to reflect light upwards from the surface of a submount or other LED mounting surface.