Dr. Li Cheng, Deputy General Manager of Nationstar Optoelectronics
LED flip-chip currently mainly refers to the ability to directly attach the chip to a ceramic or other substrate or bracket without soldering wires and achieve electrical interconnection. We call it DA chip (Directly Attached Chip). This should be distinguished from early flip-chips that transferred the chip to a silicon or other material substrate and then still required wire bonding interconnections.
Flip-chip is a chip structure that existed a long time ago. It is juxtaposed with vertical structures and horizontal structures. It is characterized by the active layer facing downwards, and the transparent sapphire layer located above the active layer. The light emitted by the active layer needs to pass through the sapphire substrate to reach the outside of the chip.
Compared with traditional horizontal or formal chips, the electrical side of the traditional chip connected to the substrate through metal wire bonding faces upward, while the electrical side of the flip-chip chip faces down, which is equivalent to turning the traditional chip over. In terms of manufacturing difficulty, flip-chips are generally between horizontal structure chips and vertical structure chips. Horizontal chips are the least difficult to manufacture, followed by flip-chips. Vertical structure chips are the most difficult to manufacture. The most direct reflection is the yield rate of the chip. The more difficult the chip is to make, the lower the yield rate.
The traditional ball-mounting process is actually a type of flip-chip packaging process, called the BGA die-bonding method. Philips and Nichia's early flip-chip packaging products all used this process. It is in a parallel relationship with the DA packaging process that we often talk about now. The DA packaging process is simpler and easier to operate.
Nationstar Optoelectronics started flip-chip packaging research in 2011, which was an expansion and extension based on our existing eutectic process at that time.
Advantages of flip chip
1. No need to dissipate heat through sapphire, and the heat dissipation performance is good. The flip-chip structure shortens the heat flow path from the heat source to the substrate because the active layer is closer to the substrate. The flip-chip has lower thermal resistance. This feature makes the performance of the flip-chip decrease very little from lighting to thermal stability.
2. In terms of luminous performance, driven by large current, the luminous efficiency is higher. Under low current density conditions, the brightness of flip-chip chips is not far different from that of horizontal chips, both of which are lower than those of vertical-structure chips. However, flip-chip chips have superior current expansion performance and ohmic contact performance because of their design with multiple via holes, silver emitter mirrors, etc. The voltage drop of flip-chip structure chips is generally lower than that of traditional, vertical structure chips. This makes flip-chip chips very advantageous when driven by large currents, showing higher light efficiency. It should be noted that the high reliability and high brightness of flip-chip chips can generally only be demonstrated when driven by large currents. In fact, driven by low currents, they are not much different from horizontal chips.
3. Under high-power conditions, flip-chip chips are more secure and reliable than formal chips. In LED devices, especially in high-power packages with Lens (except for traditional anti-lumen structures with protective shells), more than half of the dead-light phenomena are related to damage to gold wires. Flip-chips can be packaged without gold wires, which greatly reduces the probability of device dead-light from the source.
4. The size can be smaller, which reduces product maintenance costs and makes optical matching easier; it also lays the foundation for the development of subsequent packaging processes; currently, small and medium-sized on-board chips have begun to receive attention and development.
From the perspective of the above product performance, flip-chip packaging products have very outstanding advantages in certain performances. For example: vibration resistance, hot and cold shock resistance, which shows superior reliability, can extend product life and reduce product maintenance costs.
From a market perspective, flip-chip is currently an important branch of high-reliability products and occupies a certain market in high-power packaging devices. With the development of flip-chips, some small-sized flip-chips have appeared on the market. Flip-chips have a tendency to penetrate into small and medium-sized power applications. Under this trend, it is believed that flip-chips will be more useful in the future.
Of course, there are still some problems with flip-chip, the most prominent of which is that the active layer of the chip faces downwards. During the chip preparation and packaging process, if the process is not handled properly, it is easy to cause large stress damage. This is a big challenge for chip factories and packaging factories. It requires optimization from chip splitting, sorting, crystal expansion, die bonding, eutectic and other aspects to make truly satisfactory products. This is an important challenge to corporate technology and innovation capabilities.
Therefore, flip-chip technology can also be regarded as a manifestation of corporate competitiveness. Strengthening the development of flip-chip and related products is of great significance to strengthening corporate competitiveness.
However, the future application prospects of flip-chips in the lighting industry are still very optimistic. Flip-chips improve the reliability of lighting products; they are small in size and high in power, which means that lamps can be made more compact.
For flip-chip chips with solder paste solidification that may be used on a large scale in the future, there will be certain advantages in cost, which will change the current dominance of horizontal structure chips.
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