LED Flip Chip Technology and Development Trend

  • 2022-03-03

The "flip" of flip chip is compared with the traditional way of wire bonding connection from the perspective of the process and technology. Traditionally, the electrical surface of the chip is up towards, but flip chip is down towards, so called "flip chip". The essence of flip chip is on the basis of traditional craft, design chip light-emitting area and electrode area into different places, and at this time install it from the electrode area to the bottom of the cup for SMT. The process can eliminate welding line, but has high accuracy requirement for the solid crystal process, which is generally difficult to achieve high yield.

 

1. The material is silicon; 2. Electrical surface and welding should be done under the components; 3. Bottom filling is needed after substrate installment.

 

Advantages: 

Grow GaN LED structure layer by MOCVD technology on sapphire substrate, and P/N luminous zone emits light through the P shape area. Due to the poor p-type GaN conduction performance, in order to obtain good current extension, you need to form a layer on the surface of vapor deposition technology in P area of Ni - Au metal electrode layer. P area lead extracts through the layer of metal film. To obtain good current extension, Ni - Au metal electrode layer cannot be too thin. Therefore, the luminous efficiency of the device will be largely affected, which usually need to consider both current expanding and the light efficiency. But no matter in what situation, the existence of metal film always results in poor light transmittance performance. In addition, the presence of lead welding spot also makes the light efficiency of the device affected. Using GaN LED flip chip structure can fundamentally eliminate the above problems.

 

Improving luminous efficiency of LED chip

LED chip light-emitting efficiency determines the future of the LED street lamp energy saving ability and with the epitaxial growth technology and the development of multiple quantum wells structure, internal quantum efficiency of epitaxial wafer has been greatly improved. How to meet the standard of street lamp application standard depends largely on how to use the least amount of power from the chip to extract the most light, which means to reduce the driving voltage and increase the light intensity. Traditional structure of the LED chip general needs to plate a translucent layer of conductive layer on the GaN to make current distribution more uniform, and the conductive layer can absorb part of LED light and p electrode will cover part of the light, which limits the LED chip light efficiency. And the inversion structure LED chip not only can avoid the these problems above, but also can up the light by setting ohmic contact reflect light layer on p-Gah surface, which can reduce the driving pressure and improve light intensity. On the other hand, graphical sapphire substrate technology and chip surface roughness can also increase the light efficiency of LED chips about more than 50%. PSS structure is mainly to reduce the total reflection within the device and increase the efficiency of light photons, but surface roughness and chip technology can reduce the light loss when it is transmitted within the chip to the external area. At present, the LED chip uses the inversion structure and graphical technology, 1W white light power chip after encapsulation can reach a maximum of 134 lm/W photosynthetic efficiency under 5000k color temperature.

 

Chip junction temperature and heat dissipation

Heat dissipation problem is the most important technical problem of white LED power type which needs to be solved, and the cooling effect directly relates to the service life of street lamps and its energy saving effect. LED produces light by the electrons transition between the bands, which does not contain the infrared part of the spectrum, so the LED heat cannot emit by radiation. If LED chips cannot timely emit heat, it will accelerate aging of the device. Once LED temperature is more than highest critical temperature (according to different denotation and technology, the chip temperature is about 150 ℃), it often causes LED a permanent failure. Effectively solve the problem of heat dissipation of LED chips plays an important role in improving the reliability of LED street light and its service life. To do this, the most direct way is to provide a good heat conduction path for heat to spread out. Compared with the traditional formal structure which takes sapphire substrate as the path the vertical and the flip chip bonded chip structure has better heat dissipation capability. Vertical structure chip directly uses copper alloy as substrate, which effectively improve the cooling capacity of the chip. The Flip Chip technology put the LED chips to a higher thermal conductivity silicon substrate through eutectic solder, and golden bumps between the chip and the silicon substrate at the same time increase the cooling capacity of the LED Chip, ensuring the LED heat can quickly derived from the Chip.

 

LED chip packaging requirements

Improving light effect

Formal structure and vertical structure of the chip is the contact of GaN with fluorescent powder and silica gel, while the inversion structure is the contact of sapphire with fluorescent powder and silica gel. GaN refractive index is about 2.4, sapphire is 1.8, phosphor refractive index is 1.7, the silicone refractive index is usually 1.4-1.5. Sapphire / (silica gel + phosphor) and GaN/(silicone + phosphors) the total reflection of critical Angle are respectively 51.1°to 70.8°and 36.7°to 45.1°. At the same time, the structure of the chip design is different, which leads to the different current density and voltage and has obvious influence on LED luminous efficacy. For example, the voltage of traditional chips is usually above 3.5V, and flip chip structure, due to the design of electrode structure, current distribution is more even, its voltage reduces to 2.8V to 3.0V. Therefore, in the case of the same luminous flux, luminous efficacy of flip chip is about 16-25% higher than formal chip.

 

Improving reliability

Reliability of LED chip depends on LED phosphors, fluorescent powder, silica gel, bracket, gold thread and other materials. If LED chip cannot quickly export the heat, it will directly affect the LED chip and moderate the reliability of the phosphor powder and silica gel. The phosphor powder according to different system has various high temperature resistant ability, which usually decays under 100-120 ℃, so how to reduce the surface temperature of the LED chip become a key factor to enhance the reliability of LED. Vertical structure can export heat rapidly through the metal substrate to bracket, due to the low thermal conductivity of sapphire (about 20w/mK), heat cannot quickly export, and gradually accumulated, which has larger influence on the reliability of the phosphor powder. LED chips with inversion structure heat mostly pass through golden bumps to the silicon substrate (thermal conductivity is about 120 w/mK), and then lead to the bracket by a silicon substrate. Upward due to its low thermal conductivity, there is only a small quantity of heat accumulation in sapphire, realizing the separation of light and heat design, and at the same time, the sapphire surface temperature is low, which can prolong the aging period of phosphors, greatly improve the reliability of the LED and longevity. As a result of the good heat dissipation design of inversion structure, 1W flip chip has better L-I linear relationship, the saturation current tolerance and large current capacity. It can support current aging for a long time at 780mA.

 

LED chip developing trend in the future

At present high power LED street lamp is mainly achieved through "multiple chip parallel gold thread" and "PCB series-parallel". The former due to the need of photoelectric parameters matching between chips, and multiple gold series-parallel encapsulation process unreliability and low encapsulation, has not been widely used. While the latter need to strictly match many LEDs photoelectric parameters, and its optical design has difficulty. Therefore, "chip" modularization product is an important development direction of LED chip in the future. LED chip module greatly reduces the cost of the whole packaging production, strictly control the different parameters of integration module chip, and guarantee the reliability of the module chip application for a long time, and at the same time the module chip can be used as a unit to form a more powerful module. Using inversion technology can be realize on the chip level of different size, color, shape, power chip integration and achieve the super power module products, which is the advantage that any other chip technology cannot achieve.

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