October 26, 2015: Lumileds has released an application note on assembling its Luxeon FlipChip white lighting LEDs.
Although intended to cover handling and assembly, it has some interesting snippets on light extraction – stuff you really need to know if you don’t want to be throwing a lot of light away.
It transpires that “more than 28% of the total light is emitted downward or behind the package” according to section 1.2, and that a reflective white solder mask right up to the package will help get some of this, but does not start to touch the fraction that comes out the back.
To get this, the solder mask has to be “highly reflective” and get right under the package – and accurately define solder areas for the pads – 3.6% of light is wasted if the aperture rises from 1 x 1mm to 1.2 x 1.2mm (section 2.9, 85% reflective mask).
As well as all this, solder and mask height have to be such that the package is lifted clear of the mask (see diagram). Achieving this is also covered in great detail, along with a great deal of thermal design information.
So, it is a neat tiny package, but you are wasting a quarter of emitted light (and think how much hard-won efficacy that is) if your pcb is not top-notch.
This is a thorough app note written in clear language – top marks Lumileds.
Maybe the title should include a reference to the pcb design information within.