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78 PCB007 MAGAZINE I MARCH 2022 e traditional FR-4 material's thermal expansion rate is around 0.04%. As we brought in the polyimide resin, the heat resistence and size stability has been improved. Together with carefully selecting the filler materials and readjusting the proportion, Wazam was able to reduce the material's thermal expansion rate to about 0.015%. Its CTE (coefficient of thermal expansion) decreased to 11.88 μm/(m·°C). From the Optical Dimension MBLU application requires materials with higher reflectivity. Materials must go through high temperature procedures such as reflow soldering. LED chips and driving IC will pro- duce heat while working, therefore, not only do we have high requirements for the materi- al's heat resistence, but we need it to maintain outstanding reflectivity. e traditional FR-4 material's thermosetting resin's color is yellow; with long term usage and heating, its color will change even more and therefore cause a reduction in reflectivity. Generally, CCL design has three resin sys- tems: phenol formaldehyde resin, epoxy resin, and polyimide resin. e traditional design gen- erally applies to the epoxy resin system. Regu- lar epoxy resin is deteriorated and discolored by heating. However, lipid ring clan epoxy resin is excellent in weather fastness, pellucidity, and heat resistence, which can improve the color- changing problem. Lipid ring resin has a rela- tively low melting viscosity, which can cause the board to be uneven when pressure mold- ing under heating. To solve this problem, some regular epoxy resin can be added to adjust the material's fluxility, and therefore improve the accuracy of board thickness. When the mixture of regular epoxy resin and polyimide resin is less than 50%, it won't affect the lipid ring res- in's function in resisting color changing. As we know, titanium dioxide can be used as a filler during the copper-clad laminate (CCL) fabrication process. Studies have shown that titanium dioxide has two types: anatase titanium dioxide and rutile titanium dioxide. Anatase titanium has good reflectivity within visible light shorter wavelength area, while rutile titanium dioxide has long term durability and great color fastness. Aer con- ducting hundreds of experiments, constantly adjusting the ratio of lipid ring resin, poly- imide resin, epoxy resin, anatase titanium, and rutile titanium dioxide, we finally found an appropriate proportion. Because of that, we have successfully produced a CCL that has high reflectivity within the visible light zone, great color fastness, high heat resistance, good Figure 2: Comparsion of dimensional stability of HW20.