Saturday, September 5, 2009

Small and Big Chip Design and efficacy

Blue represents the optical power and Purple indicates the Electrical power

LED efficacy is 25%, 75% of the electrical energy will be converted to heat energy, however when the smaller LED is having smaller power with say 20mA of current passing through, they efficacy could be as higher 55%.

There will be 55% of optical power Lumens output and 45% of energy will be dissipated as heat energy. however when the current increased say to 350mA, the efficacy can be about 25%. 75% of the electrical energy will be dissipated as heat energy which is a waste!

So we can see that there is a big different in LED design on the East and the West.
As most of the LED in Japanese Maker are using comparative small chip 0.2mm x 0.2 mm to 0.29mm X 0.29mm current such as 20-50mA range by using a number of small Chip to create more optical Lumens. Such design in fact as much a higher energy conversion than driving the LED at much a high current say 350mA or more, as when chip is bigger current .

But most of the US and or European design will be using 350mA as standard current reference for driving their LED. Such as K2, Rebel, XRE, XP, Oslon...

As for Japanese design such as Citizen, Sharp, we can see that they ar using a lot of LED from 10-20 pcs each of them are driving at smaller current, there will be a lot of manufacturing process as there will be multiple bonding on Die bonding and Multiple wires.

But the advantage is better energy saving, an some of them are connected in parallel, if there is one strand of LED chain is failure, there are still having some of the LED strands continue to light up. So as a reliability sense that is better. Whereas the other European Design will be black out of the LED is failed! So there thermal management of the current UHBLED will be an important engineering challenge for most of the LED players using big chip say 1mm x 1mm or bigger.

As we know if we have the best chip on hand, the final product reliability will be much depending on how low we can maintain the Junction Temperature. In any case we should design such a way to have a lowest Junction temperature of the Chip. Say if you can have a room temperature junction temperature, that is a dream case. And if you can maintain the Junction temperature at 85% m that is very good already, in worse case the junction temperature such not be higher than 125 Deg C.

As what highlight before if the temperature of the Chip is lower the life tine of the LED will be increased. Say form Rebel specification sheet if we can reduce 5 degrees from Junction temperate of 135 Degree C to 130 Degrees C, the life time of the Reel will be increased by 18,000 hours!!

If one day we can have a say Copper bas material and a very good of bonding connection material such as Solder (lead free is preferred) the Bond Line thickness is properly optimized for best CTE between the chip say EZ1000 and the Copper material. I think that is a very good design for low Junction temperature, as Copper is very good at heat energy conduction, and it is of low cost as comparing with AIN or Al2O3 ceramic substrates.

That can be a powerful LED… and if we can have very high efficacy Phosphor material and a good design of Lens, we may be able to create s very efficient LED with High Efficacy, good thermal conductivity, low cost as Solder is cheaper than Epoxy, or gold tin Eutectic process. However, the only concern is how good that it could sustain the reflow temperature say 260 deg C for two times. Will the Solder cause any delamination of the LED and the Substrates?

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