3. 4 substrate technology
3.4.1, substrate material and wiring process thereof
The substrate of the flip-chip LED mainly serves as a support and connection. At present, the substrates used for flip-chip LEDs mainly include a silicon substrate and a ceramic substrate. The silicon substrate is mainly used as the substrate for the early flip chip, in order to be able to be used in the same package as the package.
The design of the silicon substrate is based on the electrode layout of the flip-chip LED chip , and the electrode on the silicon substrate is designed to match the electrode on the chip.
At the same time, in order to ensure that the surface wiring layer of the silicon wafer is not damaged by the external water vapor and corrosive environment, a passivation protective layer needs to be formed on the surface of the metal wiring layer on the surface of the silicon wafer. After the surface wiring and the passivation layer of the silicon wafer are fabricated, in order to solder the LED chip and the silicon substrate, it is necessary to form a bump metal corresponding to the LED chip electrode on the surface of the silicon substrate.
The advantage of the silicon substrate is that it facilitates integration, and ESD, power control IC, etc. can be integrated and integrated at the stage of substrate fabrication. The disadvantage is that it is fragile, can not be used as a package substrate, and requires an additional external package bracket, which is costly.
The ceramic substrate is now the most popular flip-chip LED substrate, and the metal wiring for flipping the surface of the LED ceramic substrate is mainly completed by the DPC (Direct Plate Copper) process.
The ceramic substrate made by DPC can achieve a thin line width and line spacing, which can meet the precision requirements of flip chip LED chips. Compared with plastic materials such as PPA and PCT used in traditional brackets, ceramic substrates have the advantages of high thermal conductivity, high temperature resistance and good stability.
Therefore, ceramics have great advantages in high-power applications, which is more conducive to the characteristics of flip-chip high current and high reliability.
3.4.2, substrate metal bump process
Since electrical connection is required, metal bumps need to be formed on the substrate, and metal bumps are connected to the metal pads of the LED chip; of course, metal bumps can be formed on the metal pads of the LED chip, but less Use, because the current LED wafer is still mainly 2 inches, the processing of bump metal on the LED wafer is not cost-effective.
The choice of bump material usually requires good remelting performance, which facilitates the formation of bumps and the soldering process due to its self-alignment and shrinkage during remelting.
Depending on the material and application, there are several ways to make metal bumps, mainly as follows:
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