What is CSP?
CSP (chip scale package) packaging refers to a packaging technology in which the volume of the package itself does not exceed 20% of the size of the chip itself (the next-generation technology is substrate-level packaging, and the package size is the same as that of the chip). In order to achieve this goal, LED manufacturers reduce unnecessary structures as much as possible, such as using standard high-power LEDs, removing ceramic heat dissipation substrates and connecting wires, metalizing P and N poles, and covering the fluorescent layer directly above the LED.
According to Yole Développement statistics, CSP packaging will account for 34% of the high-power LED market in 2020.
Why do CSP packages face heat dissipation challenges?
The CSP package is designed to be directly soldered on a printed circuit board (PCB) through metalized P and N poles. In one respect, it is indeed a good thing. This design reduces the thermal resistance between the LED substrate and the PCB.
However, because the CSP package removes the ceramic substrate as a heat sink, this makes the heat transfer directly from the LED substrate to the PCB board and thus becomes a strong point heat source. At this time, the heat dissipation challenge for CSP has changed from "level one (LED substrate level)" to "level two (the entire module level)".
In response to this situation, module designers began to use metal-covered printed circuit boards (MCPCB) to cope with CSP packaging.
Figure 1. Thermal radiation model of 1x1 mm CSP LED on 0.635 mm AlN ceramic substrate (170 W/mK)
It can be seen from Figures 1 and 2 that the researchers conducted a series of heat radiation simulation tests on MCPCB and aluminum nitride (AlN) ceramics. Due to the structure of the CSP package, the heat flux is only transferred through the small solder joints. , Most of the heat is concentrated in the central part, which will lead to reduced service life, reduced light quality, and even LED failure.
Ideal heat dissipation model for MCPCB
Usually the structure of most MCPCBs: the metal surface is plated with a layer of copper on the surface of about 30 microns. At the same time, the metal surface is covered by a resin medium layer containing thermally conductive ceramic particles. However, too many thermally conductive ceramic particles will affect the performance and reliability of the entire MCPCB.
At the same time, for the thermally conductive medium layer, there is always a trade-off between performance and reliability.
According to the researcher's analysis, in order to achieve better heat dissipation, MCPCB needs to reduce the thickness of the dielectric layer. Since the thermal resistance (R) is equal to the thickness (L) divided by the thermal conductivity (k) (R=L/(kA)), and the thermal conductivity is only determined by the properties of the medium, the thickness is the only variable.
However, the thickness of the dielectric layer cannot be reduced indefinitely due to production process limitations and service life considerations, so researchers need a new material to solve this problem.
How can nano-ceramics become the best solution for MCPCB?
Researchers have found that an electrochemical oxidation process (ECO) can produce a layer of alumina ceramic (Al2O3) of tens of microns on the surface of aluminum. At the same time, this alumina ceramic has good strength and relatively low thermal conductivity (approximately 7.3 W/mK). However, since the oxide film automatically bonds with aluminum atoms during the electrochemical oxidation process, the thermal resistance between the two materials is reduced, and it also has a certain structural strength.
At the same time, the researchers combined nano-ceramics with copper clad so that the overall thickness of this composite structure has a high total thermal conductivity (approximately 115W/mK) at a very low level. Therefore, this material is very suitable for the needs of CSP packaging.
In conclusion
When designers continue to explore and find suitable CSP packaging materials, they often find that their needs have exceeded the existing technology. The heat dissipation problem has led to the birth of nano-ceramic technology. This nano-material dielectric layer can fill the gap between traditional MCPCB and AlN ceramics. So as to promote designers to introduce more compact, clean and efficient light sources.
