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
1. Silver Epoxy process 85%
2. Direct Eutectic process <1 3.="3." 4.="4." 4="4" 5.="5." 7="7" br="br" chip="chip" eutectic="eutectic" flip="flip" flux="flux" g2g="g2g" other="other" process="process">