Trend of LED Manufacturing

Trend of Package Size
The Trend is going for high performance Ceramic base small footprint LED at 3mmx 3mm. Cree Xlamp is 3.45mm x 3.45mm, Not sure if Lumileds Rebel will set a new standard in future. Rebel package size is 3.62mm x4.61mm. Oslon of Osram 3 mm x 3mm x 1.6 mm (Smallest).

Trend of Wafer Size
The 6” wafer was recently been an hot topics and seems that many US, Japanese and European LED players are ready to go for it. As the large wafer will definite produce more LED and at lower cost of 30% estimated!

The industry requires the equipment suppliers to synchronize with the LED Development. We have been saying the front end is developing faster than the back end. I think that is true.

Trend of Package
Traditionally that individual company has it own design and that has been gone through 10 years market test. There are so many people who partly are copy cats and partly are LED makers with good performance chip. Though the thermal performance may not as good as the original but they are selling much cheaper, so the market has been segregated into two sectors: one is high end that has strength on output and thermal performance and naturally higher in price, the other is the rest major users that may not require higher thermal performance but the cost is just 40% or less than the original. So thee two market segments have been well defined now. The ratio is about 80:20. For those High End Packages from EU and USA, if the cost is not able to further driven down to an intermediate level, they will be facing tough competitons for Similar package made by Taiwanese and Chinese.

So many of these companies are focusing for new development on ceramic small foot print LED.

Trend of the Material
The use of larger panel of Ceramic Substrates has a better cost advantage as the larger the panel say form 4” to 6 there is an added cost but not in direct proportional to the area of the material. The advantage of larger than has a more significant saving on the machine utilization time as output indexing does lower the output of the unit. More saving will be the Process time for Silicone Liquid mounding as typical mould time is in 5-10 mins, and if the LED panel is 4” would be saving a lot of time for the Machine by comparing with 2” which has only 25% of the LED of 4” Panel.

Trend of process
There is a need for high thermal performance on LED as to minimize the thermal drift, as this drift causes instability of the colour Wave Length, and Efficacy. Now 85% of the LED is now manufactured by Epoxy process, and there are numerous ways for trying to define itself for a better process. Solder is one of them and through studies that there will be more benefit by using Solder.

Let's watch out for a couple of years for a better die bonding process that can provide better, cheaper and brighter LED. The End users will be beneficiary.

Soft Solder Process for HB LED manufacturing

There is numerous ways to mount a die onto the substrates either made of Copper, ceramic, Leadframe, PCB, and Alumina, or a Pre-mould PLCC leadframe material has to go through a series of consideration. Refer to the attached tables. It is self-explanatory!
We can see that we have to first decide what kind of package we want to build, and for what application. Say if that is made for out door illumination, or For AFL (automobile Front lamp) that will be very harsh for the LED package as we require to pass a stringent Stress cycle requirement. The way how we can cool down the chip is the prime consideration.

Of course, the temperature can be made by force air cooling in an automobile, but in the case of a car running in dessert high way, the LED will be very hot. The junction temperature may go up to 125 Deg C. if the heat sink is not ale to carry out the heat, their performance of the chip will deteriorate and or soon burn out.

Seoul Semi new SMD LED package
12th May, there is a novel design of LED from Seoul Semiconductor to launch of its 120lm/W
high-efficiency and SMD type LED (Part No: LCW100Z1) for general purpose lighting.

As an ultra-thin LED with dimensions of 3.5mm x 2.8mm x 1.6mm, the LCW100Z1 is a high efficiency product that delivers up to 7.8lm (@0.06W) at the low current of 20mA and 14.3lm at the current of 40mA, and its viewing angle is 120 degrees.
This is a silver epoxy on Copper substrates. The heat dissipation should be more superior than the other type on ceramic and PCB. However, they can not achieve a very high BLT more than 1.5 mil, so there is still room for certain performance improvement.

Die Bonding Process
Direct Eutectic process is expensive for the 80/20 AuSn layer which has to be thicker than the surface roughness of base material.

Flux Eutectic process has it drawback as the flux will be the trouble they give up in the years to come for Lens Contamination and Delimination.

High Silver Content Epoxy Bonding
Having limitation in void control and not perfect for CTE matching as it is not easy to product a high enough BLT without any epoxy tailing issues with zero void.

Soft Solder Process
On the other hand Soft solder has been widely used for automobile and power device for more than 40 years. Soft solder has excellent heat-dissipation properties, of typically 35W/mK. high ductility, exhibited outstanding robustness against declamation. During thermal stress condition the solder can withstand a great thermal stress caused by asymmetrical thermal expansion of LED material SIC and Copper (SiC CTE is 3.1~4.7W/k-m and copper is 16.5ppm/K). These extraordinary characterizers lead us to consider an ultimate process for LED.

The remain four challenges are to maintain an Solder coverage, zero void, good die bonding placement and die tilting.

There is some challenge on the solder coverage as the LED chip is thin as 6 mil for EZ1000, and that will be some challenge to have both 100% Void free and good wetting for four edges. However that is research that one company has the solution, and they have actually can achieve +/-20 micron die placement with practical Zero void!

Soft Solder LED process
Basically this is a novel bonding process that requires precision, good temperature profile set-up and hest tunnel design with consideration of the formic gas flow, with CAE and proper het insulation material a standard solder dispensing system with accurate amount of solder to be place on the substrates. A high speed bonding head which can sustain a high temperature with minimum controllable thermal expansion or active alignment on every start of bonds using patent optical alignment method. Also there is a Reflow concern, if after two to three time reflow the solder will be softened and cause the die to be lift up, That will kill the process, If using solder haveing lead, that might not be good for the Product. So this is a kind of delimma, yet to be solved!

Solder Void
Since the die tilting and void will create uneven distribution of solder underneath the die, this void will reduce thermal and electrical conductivity and hence heat dissipation will be reduced These will create a higher electronics resistance. Moreover, larger void are often the beginning of an elimination of the die from the solder, which mean life time reduction. The Void has to be close to zero of more than 2% of the total area under the die.

BLT and Die tilting issue
A consistent BLT requires the Centre of the solder dispense dot to be properly aligned with each other. That is to have the vision of other mechanical way to properly align their two centres to be on point. As the Solder will be flowed back a few milliseconds after the collet has placed the die onto the solder, the solder will be squeezed out and once after the collet is move up, the capillary effect would work from four sides of the die bottom to pull in the solder. The smaller the die , the smaller the capillary force and more critical to properly align the die by the molten solder as there will be having insufficient or no flow back of solder material.

If the die is not placed at the centre of the solder lump, one the edge having minimum solder will not be having the same amount if pulling force as their other three edges and hence a tilted die will occur.

Vacuum Delay and timing
The proper control of the vacuum release is critical for controlling the BLT and Tilting of the Die.
The wrong timing for vacuum release will create die tilting issues, either too early or too late of the timing , especially for smaller dies. If the delay is too early the die will lose control as die falls onto the solder without control. I the vacuum release is too late, the collect will suck back the die.

Mechanical alignment
There is perfect coplanarity for the Collet to be level with the bonding pad, as well between the solder wire dispensing head with the bonding pad.

Purge Air
On the other hand there is used to have certain Purge air at the collet such as to break the vacuum aster. That amount of purge air can cause die tilting as well, in any sense, can not set too much, and that purge air should be so adjusted by software to be controller in proportional to the die size.

Conclusion
Soft solder process has been running in the power device for more than 40 years. That is a mature process. However High Power LED using lead free Solder is a novel process, there is a lot of challenge such as reflow issue for major reliability concern. How to control the amount of Solder is the key point. The advantage is cost and zero Void, and highly reliable LED product could be made! Another step jumping into the Replacement light bulb application.

Who can manage to use the low cost process for best final product reliability will be the one who can win the market.

LED Die bonding Manufacturing Process

There is continuous seeking for most suitable process with commercial balance of perfomance, cost and reliability for HBLED manufacturing. As from years expierence in this LED manufacturing environment and equipement development , we have a lot understanding that there is minimum 5 common practice for LED die bonding.

Die bonding Process for LED manufacturing
1. Silver Epoxy process
2. Direct Eutectic process
3. Flux Eutectic process
4. Flip Chip G2G process
5. Soft solder process


Silver Epoxy LED Die bonding
This is the most common process as I can see 95% of the World’s LED was made by this process. As for High power LED, most of the applications are epoxy with high silver content, but the silver conent can not go up to 70%, silver epoxy can be applied by either dispensing or stamping.
Hi Power LED can have certain period of life time without degrading on th epoxy or causing epoxy delimination. However, if the LED is running at high current the silver epoxy process may not be the best.

The concern with epoxy is reliability. We have not found epoxy material that does not crack under severe temperature cycle condition for more than 1,000 hours. Although in terms of thermal conductivity, it could be quite similar to Au/Sn solder. Some LED vendors do use epoxy for attaching LED substrate onto heat sink, but they will suffer as thermal performance may not be the best due to thermal conductivity is not the highest amongst all the processes. This process is simple, cheap and easy for manufacturing. The UPH is 10,000 - 18,000. An hour for certain package.

Direct Eutectic process
The main attractions of Au/Sn process are in its excellent performance in reliability and its high thermal conductivity. The drawback is of course the comparatively high cost. Applictaion such as LED based automotive forward lighting or high quality exteriro light fixtures. This LEDs consumption volume is still limited in the luxury automobile market segment, quality and performnace are everything. That is the main reason we the industry requires highest quality standard of performance reliability will use Au/Sn Direct Euetctic Process.

The bonding time is one of the bottle neck for machine UPH typical value is 3,000 to 5,000 depending on die size, void requirement, surface roughness and the thickness of the Gold Tin layer. On the other hand, the use of Forming gas will inevitably add on the cost.

Flux Eutectic process
The problem with flux eutectic Au Sn process is the possibility of contamination of the LED lens as flux evaporates gradually. There are so called "low-flux eutectic paste" in the market place claimed by various vendors. We are still having queries on such claim. This can have a simpler process than Eutectic process machine. UPH can be up to 18,000 as Epoxy process.


Flip Chip process
A gold bump has to be made on the substrates and special plated bump pad has to be made at LED side. Flip Chip machine requires high accurcay +/- 10 microns for alignment between the die and the substrates. Due to the process may requires pick and place the chip twice for ellimilate the pick up ejection error and or using down look camera then re-pick up agan by a bond arm. This two way pick ups will slow down the machine bonding cycle time and hence lower the UPH.
Typical UPH is around 3,000-6,000. So this is an expensive way for LED manufacturing.
Only less than handful of company using this process for LED manufacturing, Typical sucessful company is Lumileds.

Soft solder process
Traditional the soft solder can be used for high power device application. Why there is no one working on LED so far? This is my question today!

The main concern with soft solder is the placement accuracy. +/-100 um seems to be normal for such process. One of the European supplier claims that their soft solder process can achieve +/- 38um @3 Sigma. This is still not good enough for the LED lighting application. Silver is much more expensive than soft solder and there is not many people using this process. As there is Blue ray absorption by the soft solder. The grey solder will not be reflecting the LED light as good as silver, its luminous efficiency is lower than Silver Epoxy,

ASM is the leader in this Soft solder process and they have and process report showing they can well control the process and able to achieve a die placement accuracy of +/-25 microns.

Cost Comparison on Die Bonding process
1. Silver Epoxy process as 1
2. Direct Eutectic process 3
3. Flux Eutectic process 1.2
4. Flip Chip GGI process 4
5. Soft solder process 0.8

Apllication comparison
1. Silver Epoxy process 85%
2. Direct Eutectic process <1% 3. Flux Eutectic process 7% 4. Flip Chip G2G process 4% 5. Other process






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