Saturday, August 22, 2009

Ceramic Base Package LED

There are numerous types of LED which was made with Ceramic as base material.With the Consideration of high Thermal Conductivity 180 W/m/K and 4.8 Coefficient of Thermal Expansion ppm/K which matches with the Blue LED Die material InGaN. As we know that the thermal performance of a LED is crucial in keeping the LED “Cool”.

Take for an instance, referring to the Data Sheet of Rebel; reducing the junction temperature from 135 deg C to 130 Deg C of Rebel will be able to increase the life time of 18,000 hours!

Silicon bas material probably is the best substrate material as Silicon’s Thermal Conductivity is 168 W/m/K and 4.2 Coefficient of Thermal Expansion ppm/K which are even better than AlN for die matching. Silicon Substrates might be the best base material for CTE matching.

There is always debatable that G2G Gold to Gold may be the better in term of heat transfer capability than Direct Eutectic. There is less LED guys practice in control the Bond line thickens of Silver epoxy as that is not easy for a small LED chip 0.3mmX 0.3mm or even 1mmx1mm. Some of the requirements include the control the BLT to be more than 5 microns and less than 10 microns that is already tough!

At the same time to control good die placement accuracy typically at a range of +/-20 microns and die rotation angel to be within +/-1 deg for same a LED chip of 1mmx1mm.
Is easy with some of the best LED die bonders in the market. The Challenge is how to ensure the solder has a consistence and high BLT. How to ensure that there is no overflow of the solder material onto the adjustment circuitry as the LED footprint is going to smaller and smaller.

How about Soft solder and Direct Eutectic process. Solder has a good CTE matching as we can control a thicker Bond Line thickness on Solder and may be too expensive for similar thickness for Direct Eutectic process.

There is novel process being discussed is to apply Soft solder as the bonding material for LED chip. As Soft solder is excellent in CTE with Copper as substrate material as Copper Thermal Conductivity is 300 and his CTE is 18 which is too high for LED chip material, so if we can have a successful thick bond line thickness such as to act as good cushion for the In GaN to Copper, may this is the futuristic perfect die bonding process. Some of the major LED companies are working closely with the bonder equipment supplier to develop this kind of process.

From the attached picture a few example of LED is displayed, they have a similar characteristic, Small footprint 3 mm x3 mm, thin as 2 mm, Dome lens and ceramic substrates. From the about packages shown, the major players has their own 1W LED package that has similar package size, more or less a standard has been formed. That is a good to the SSL industrial followers to apply the LED on their SSL display panel and or lamp fixtures.

I am interesting to see what will be the next successful package that can replaced the current one with the following outperforming factors in Cost, Thermal conductivity, and performance!

22 Aug 2009 1100 am

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