The following is a research on the industrial technology of flip-chip soldering of power type GaN-based LED optoelectronic devices, introducing the development history, product application, research methods, technical routes and key problems of LED optoelectronics.
Domestic and foreign technology development and status quo
As a new generation of environmentally-friendly solid-state light source, GaN-based light-emitting diodes (LEDs) have become the focus of attention in the industry.
In 1992, Nakamura Shuji, who is known as the "father of blue light", successfully prepared Mg-doped p-type GaN. Subsequently, in 1993 and 1995, a high-brightness blue LED was successfully fabricated using an InGaN/GaN heterojunction structure. And thus won the 2014 Nobel Prize in Physics.
At present, high-power, high-brightness white LED has become a hot spot in the field of lighting.
Although the luminous efficiency of white LED has reached 170lm/W, there is still a certain gap from its theoretical value of 250lm/W. Therefore, further improving its luminous efficiency becomes a key technical problem of power type white LED.
In general, there are two ways to improve the luminous efficiency of LEDs, which are to improve their internal quantum efficiency and light extraction efficiency. On the other hand, how to improve the heat dissipation capacity has become another key to the development of power LED devices.
With the increase of LED power, especially the development of solid-state lighting technology, new and higher requirements have been put forward for the optical, thermal, electrical and mechanical structures of LED packages.
It can be seen that the research on packaging technology with high light extraction efficiency, low thermal resistance and high reliability is the only way for high-power LED to be practical and industrialized.
Flip-Chip technology, also known as flip-chip encapsulation, is a chip packaging process that is well established in the field of IC packaging technology.
Because it can meet the requirements of the above high-performance package, the power LED package based on flip chip technology is considered to be the key technology and development trend of packaging power type high brightness LED.
In the traditional horizontal and vertical wafer structures in the past, the absorption of the front electrode and the critical angle of the total reflection of the GaN-Air interface will greatly affect the light extraction efficiency;
On the other hand, in the conventional package structure, the heat of the LED chip needs to be conducted to the heat conductive substrate via the substrate sapphire (the thermal conductivity is only 38 W/mK), and the heat conduction path is long, so that the thermal resistance of the chip is large.
The flip-chip structure formed by flip chip technology inverts the sapphire substrate chip, and the chip is directly soldered on the heat conductive substrate, and the electrodes are connected to the substrate at the bottom, thereby avoiding the wire bonding caused by the chip height difference in the conventional package. Difficult problem. At this point, the exiting light exits from the transparent sapphire substrate at the top of the chip.
On the one hand, the flip-chip structure avoids the metal electrode blocking the emitted light, and increases the critical angle of total reflection at the light exit interface, thereby effectively improving the light extraction efficiency.
On the other hand, the metal electrode microbumps are in direct contact with a substrate such as silicon, metal or ceramic having a high thermal conductivity, so that the distance through which the current flows is shortened, the resistance is reduced, and the amount of heat generation is lowered, and such a combination makes the thermal resistance lower. Improve heat dissipation.
In addition, due to the absence of the gold wire at the front exit, the phosphor coating process of the white LED product is relatively easy to implement, especially in the process of phosphor coating, and the color consistency of the product will be greatly improved.
Compared with the traditional package, the flip-chip structure has the advantages of simpler packaging process, lower packaging cost and higher package yield.
The flip chip structure is composed of a substrate, a UBM, a solder ball, and a chip. Eutectic welding is often used to connect the chip to the substrate.
Eutectic welding, also known as low melting point alloy welding, has many advantages such as high thermal conductivity, small connection resistance, uniform heat dissipation, high welding strength, good process consistency, etc., so it is especially suitable for power devices with high power and high heat dissipation requirements. Welding.
Its basic feature is that two different metals can form alloys at a certain temperature at temperatures well below their respective melting points.
The eutectic metal layer of the common flip-chip LED is generally Au/Sn alloy (Au80Sn20), and the eutectic temperature of Au80Sn20 is 282 °C.
Eutectic soldering is divided into direct soldering and flux soldering. Direct soldering is a process in which a chip having a eutectic alloy at the bottom is directly hot-pressed and eutectic under a eutectic machine, and the pressure during eutectic does not exceed 50 g. This method has no flux, no cleaning process, and high output, but it has a large one-time investment.
Another eutectic method—flux eutectic, according to the electrode size of the flip-chip LED chip, pre-plating the Au/Sn alloy layer on the substrate, and then spotting the flux on the substrate to fix the LED chip on the corresponding alloy of the substrate. On the layer, in the industrial production process, an ordinary solid crystal machine can be used instead of a dispensing head, and then placed in a reflow furnace to form a eutectic welded joint.
The amount of eutectic flux in this process is relatively difficult to control, and the reflow profile is explored according to different reflow furnaces, and it is difficult to control its stability. The advantage is that the process investment is small.
Both eutectic methods require the stent to withstand the melting temperature of gold and tin (greater than 320 ° C), and the thickness of the gold plating on the surface of the substrate is required to be less than 2 μm. Otherwise, the molten eutectic material cannot completely fill the uneven interface, not only Increasing the thermal resistance of the device even makes the combination of the chip and the substrate unstable, affecting the package quality and the like.
In addition, new solid crystal materials have also appeared. In January 2014, Dexirals showed conductive adhesives. The conductive particles were only 5 microns in size. Using conductive adhesive, the P/N pole was completely insulated after being strongly applied to the substrate. Finally, the conductive particles were broken and the current conduction was completed.
The current Eu-Sn alloy eutectic requires an operating temperature of 300 ° C or higher, while the LEP conductive paste is used, and the operating temperature is controlled at around 180 ° C. Therefore, the heat-conductive substrate has more selectivity, and a glass substrate and a PET substrate can be used.
Therefore, cost savings can be achieved from all stages of wafer, substrate and equipment, and LED manufacturers only need to purchase a hot press with LEP conductive adhesive. The overall cost is estimated to be about 30% lower than that of Au/Sn eutectic.
In 2001, flip-chip LEDs were first proposed by Wierer et al., and their light extraction efficiency was increased to 1.6 times that of the formal structure.
In 2006, Shchekin et al. fabricated a thin-film flip-chip LED chip based on flip-chip AlGaInN LED chip. The structure uses laser lift-off technology to remove the sapphire substrate and thin the intrinsic GaN material under n-GaN. LED chip The optical output power is doubled compared to the conventional flip-chip structure, and the external quantum efficiency of the structure is 36% driven by a current of 350 mA.
Around the aspects of improving light extraction efficiency, improving heat dissipation performance, flip-chip bonding technology, etc., GaN-based flip-chip LEDs have done a lot of academic research work.
At the same time, the industry is also closely following up. Some manufacturers have introduced wafer-level CSP packages based on flip-chip technology.
For example, Taiwan Jingdian has launched the latest unpackaged wafer technology ELC, Taiwan's TSMC solid-state lighting unpackaged PoD module, Philips Lumileds' LUXEO NFlip Chip, LUXEONQ, CREE XQ-B, XQ-ELED products. Samsung recently introduced the latest flip chip products, including the medium power LM131A, high power LH141A and downlight modules.
Research methods and technical routes
The basic research ideas are: LED model design - software simulation, analysis and optimization of heat dissipation capability - LED optical design - flip chip soldering - coating phosphor - LED product molding.
(1) Selection criteria for materials.
The selection of related materials shall meet certain physical properties as required: such as welding stability, thermal conductivity and electrical conductivity of the soldering material, transparency of the packaging material, thermal stability, resistance to external forces, hardness, density, and refractive index uniformity. And stability, water absorption, turbidity, maximum long-term working temperature, anti-static and so on.
(2) Design and preparation of package brackets.
1 High thermal conductivity thermal conductive substrate material, available in aluminum plate (thermal conductivity 231W/mK), copper plate (385W/mK), ceramic material aluminum nitride (320W/mK), silicon (191W/mK), etc. ;
2 In order to avoid the fusion of the two electrodes in the eutectic, an insulating intermediate barrier layer of suitable height and size is designed according to the size of the chip and the position of the electrodes.
(3) Software simulation, analysis and optimization of structural parameters of LED product model structure heat dissipation.
According to the designed sample model, the simulation results are analyzed by using relevant simulation software, and the LED model is optimized by changing the structural parameters to obtain an optimized LED product model with excellent heat dissipation capability.
(4) Perform an optical design on the LED.
Designed for flip-chips, package holders, dies, exit lenses, etc. using optical design simulation software (such as TracePro). The aim is to achieve optimum light extraction efficiency.
(5) eutectic soldering technology for flip chip.
1 At the bottom of the package holder, solder paste is bonded to the positive and negative electrodes of the external circuit, and a certain height of insulation layer is adhered at the position of the barrier layer to avoid fusion of the two, and the height is higher than the height of the solder paste.
2 Align the positive and negative poles of the flip chip precisely with the circuit of the package holder and bond it to the bottom of the bracket. Through the eutectic soldering process, the temperature is controlled and the flip chip is firmly soldered to the bracket.
(6) Spray coating technology of phosphor.
The controllability of the concentration, thickness and shape of the phosphor layer can be achieved in consideration of the planar phosphor coating process. The uniformity of the spatial distribution of the light spot and the chromaticity between the tubes and the uniformity of the brightness are achieved.
(7) Molding or lens molding and product formation.
According to the optical design structure, a transparent epoxy resin or a silicone resin is used to mold the optical lens above the light output of the chip to increase the light extraction efficiency.
Key issues to be solved
(1) It is one of the key technologies to firmly solder the flip chip to the substrate by eutectic soldering technology.
Specifically, it relates to alignment of electrodes, temperature control during eutectic process, solidification force, roughness of chip and substrate surface, and the like.
(2) It is guaranteed that the positive and negative electrodes will not melt into a key problem during the process.
Specifically, it relates to how to design and prepare an intermediate barrier layer to achieve effective barrier of positive and negative electrodes.
(3) Spraying technology of phosphors.
Mainly the uniformity, thickness and shape of the phosphor coating.
(4) Light extraction efficiency is a key technical point.
Avoid damage to the flip chip quantum well during the manufacturing process, especially the soldering temperature, resulting in a decrease in light extraction efficiency.
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