Friday, October 9, 2009

Wafer Level Package LED

WLPLED Wafer level Package for LED is an emerging technology from Denmark Hymite and now Taiwan for Going to the new WLP LED, more and more people especially with IC foundry facilities and experience are moving onwards to WLP LED. Now others involved in LED WLP technology include the Industrial Technology Research Institute (ITRI), Solidlite Corp., VisEra Technologies Co., Ltd., Xintec Inc. and Silicon Base Development Inc. (SiDBI).

Why Silicon
The key factors for this trend is that there are the following advantages: Silicon’s CTE or coefficient of thermal expansion is below 4 ppm/C, TC or thermal conductivity is in the 150-180 W/m.k. range, Silicon can provide a good thermal condition for LED.

IC die Integration
WLP Silicon wafer can be made using the lithographing process, in which photomasks replace molds. The TVS die and or the LED invertors chip may be able to be integrated onto the substrates within the same LED footprint.

Base Material Selection
There are three major way for substrates election as for LED substrates material,
Ployphthalamide (PPA) is the major lead-frame material for low-power chips packaging while ceramic and liquid crystal polymer (LCP) are primarily used for high-power packages.

Cost Comparison
The cost of ceramic packaging is the highest whereas WLP is the cheapest, with PPA and LCP in the middle. If a ceramic package costs US$0.25, a PPA would be around US$0.08, an LCP would be around US$0.10 and WLP would be around US$0.04.

Ceramic has a good melting point, ceramic-based lead frame, like PPA and LCP lead frames, has to be made with tooling, hence limiting the substrate reduction to micron scale. A PPA lead frame, for instance, can be miniaturized to 3.2 mm long by 1.8 mm wide at best, holding a 9-mil LED chip and an 8 mil TVS Die in it. WLP process enabling reflective layers such as Aluminum to be grown on silicon lead frames in the burning-hot chemical vapor deposition chamber. "The layer can reflect some 5% more light from the chip relative to generic lead frames, thereby boosting efficacy of a lamp.

Bonding Process
Epoxy and flux Eutectic reflow process is not a concern, Epoxy can be using stamping epoxy which is best for small cavity like 30mil x 30 mil with the latest stamping epoxy process and sophisticated control it sis able to provide a production mode of epoxy spread of 4 mil or less at each side of the LED chip still leaving room for the second bond pad and TVS die Having an ultra high melting point also makes silicon suitable for eutectic bonding, which is noted for a high melting point of minimum 282 Deg C. "The gold-zinc alloy is used to affix LED chip to silicon lead frames. As epoxy glues used in most LED packages melt at 180 Deg C, eutectic bonding is much better in thermal resistance and does not capture light in the package during high temperatures as epoxy. However Direct Eutectic bonding process impose a high challenge on the bonding equipment, as there is not easy to control the whole substrates with 8” diameter at such a constant temperature say +/-3 deg C. The second concern is that the first bonded LED may suffer too much on the eutectic temperature that it might actually kill the LED’ take an example Cree EZ1000 die can only last in 335 deg C for 5 seconds. And as we know the LED is kept cooler say a Rebel is reduced from a temperature of 330 Deg to 325 Deg C, the Actual life time can be
Increased more 18,000 hours! So there is some new novel eutectic bonding processes are under development.

Future LED
Likely the WLP is one of emerging material for LED manufacturing especially there is tight LED bare die supply. The IC foundry in Taiwan would not leave this opportunity to convert their MOCVD facilities to manufacture LED on a WLP format!

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