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How to solve the heat dissipation problem of CSP package?


What is CSP?

The chip scale package (CSP) refers to a package technology in which the size of the package itself does not exceed 20% of the size of the chip itself (the next generation technology is a substrate level package, and the package size is the same as the chip). To achieve this goal, LED manufacturers reduce unnecessary structures as much as possible, such as using standard high-power LEDs, removing ceramic heat-dissipating substrates and connecting wires, metalizing the P and N poles, and directly covering the fluorescent layer above the LEDs.

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

Why is the CSP package facing thermal challenges?

The CSP package is designed to be directly soldered to a printed circuit board (PCB) via 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, since the CSP package removes the ceramic substrate as a heat sink member, this allows heat to be transferred directly from the LED substrate to the PCB board, thereby becoming a strong spot heat source. At this time, the heat-dissipation challenge for the CSP has changed from "level one (at the LED substrate level)" to "secondary level (at the entire module level)."

In response to this situation, the designers of the module started using metal-clad printed circuit boards (MCPCBs) to handle CSP packages.

The ideal heat dissipation model for MCPCB

Most of the MCPCB's structure is usually: The metal surface is plated with a layer of about 30 microns of surface copper. At the same time, this metal surface is covered with a layer of resin media containing thermally conductive ceramic particles. However, excessive thermal conductive ceramic particles can affect the performance and reliability of the entire MCPCB.

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

According to the researchers' analysis, 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 media itself, thickness is the only variable.

However, due to the limitation of the production process and the consideration of the service life, the thickness of the dielectric layer cannot be reduced. Therefore, researchers need a new material to solve this problem.

How do nano-ceramics become the best solution for MCPCB?

The researchers found that an electrochemical oxidation process (ECO) can generate a layer of tens of micrometers of alumina ceramic (Al2O3) on the aluminum surface, while the alumina ceramic has good strength and relatively low thermal conductivity (about 7.3 W/mK). However, since the oxide film is automatically bonded to 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 also has a high total thermal conductivity (about 115W/mK) at very low temperatures. Therefore, this material is well suited to the needs of CSP packaging.

In conclusion

As designers continue to explore the search for suitable CSP packaged materials, they often find that their needs have exceeded existing technologies. The problem of heat dissipation has led to the birth of nano-ceramic technology, which can fill the gap between traditional MCPCB and AlN ceramics. This will encourage designers to introduce more compact, clean and efficient light sources.