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.
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