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What Is CSP?

CSP (chip scale package) package is a package of its own size does not exceed the size of the chip itself 20% of the packaging technology (next-generation technology for the substrate level package, the package size and chip the same). In order to achieve this, LED manufacturers as much as possible to reduce the unnecessary structure, such as the use of standard high-power LED, remove the ceramic heat dissipation substrate and cable, metal P and N pole and directly above the LED cover the fluorescent layer.

According to Yole Développement statistics, CSP package will account for 34% of high-power LED market in 2020.

Why does the CSP package face thermal challenges?

The CSP package is designed to be soldered directly to a printed circuit board (PCB) via metallized P and N poles. In a certain respect is indeed a good thing, this design reduces the thermal resistance between the LED substrate and the PCB.

However, since the CSP package removes the ceramic substrate as a heat sink, heat is transferred directly from the LED substrate to the PCB to become a strong point source. At this point, the cooling challenge for the CSP from the "level (LED base level)" into a "two (the entire module level)."

In this case, the module designer began to use the metal-covered printed circuit board (MCPCB) to deal with the CSP package.

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Figure 1, 1x1 mm CSP LED Thermal radiation model on 0.635 mm AlN ceramic substrate (170 W / mK).

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Figure 2, 1x1 mm CSP LED on the MCPCB (150 W / mK) on the thermal radiation model.

As can be seen from Figure 1 2, the researchers conducted a series of thermal radiation simulations for MCPCB and aluminum nitride (AlN) ceramics. Due to the structure of the CSP package, the heat flux was only transmitted through very 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 of MCPCB

Usually most of the MCPCB's structure: the metal surface coated with a layer of about 30 microns on the surface of copper. At the same time, the metal surface also has a layer of ceramic particles containing thermal conductivity of the resin layer covered. But too much thermal ceramic particles will affect the performance and reliability of the entire MCPCB.

At the same time, there is always a trade-off between performance and reliability for the thermally conductive media layer.

According to the researchers, the MCPCB needs to reduce the thickness of the dielectric layer for better heat dissipation. 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 determined only by the properties of the medium, the thickness is the only variable.

However, because the dielectric layer due to the limitations of the production process and the use of life considerations can not be unlimited to reduce the thickness, so the researchers need a new material to solve this problem.

How does nano-ceramic become the best solution for MCPCB?

The researchers found that an electrochemical oxidation process (ECO) could produce a layer of several tens of microns of alumina ceramic (Al2O3) on the aluminum surface, while the alumina ceramics had good strength and relatively low thermal conductivity 7.3 W / mK). However, because the oxide film in the electrochemical oxidation process with aluminum atoms automatically bonded, thereby reducing the thermal resistance between the two materials, but also has a certain structural strength.

At the same time, the researchers combined nano-ceramics with copper cladding, giving the overall thickness of the composite structure a very high total thermal conductivity (about 115 W / mK) at very low temperatures. Therefore, this material is suitable for CSP package requirements.

in conclusion

When designers continue to explore materials that look for suitable CSP packages, they often find that their needs are beyond the existing technology. The heat dissipation problem leads to the birth of nano-ceramic technology, which can fill the gap between the traditional MCPCB and AlN ceramics. Thus driving the designer to introduce a more compact, clean and efficient light source.