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ENGLISH[Small Pitch] How to choose the right LED display packaging material in the MINI-LED small pitch era
Published:2020-01-10 10:23:06
Judging from the development trend of LED display technology, both the device-based EMC lamp beads and COB module technology have entered the "0.X" era on the pixel pitch index. Who can become the mainstream of the two technology routes in the future, more is From the industrial chain, whether it is a more efficient division of labor cooperation model or a through-out platform model.
The former relies on mature packaging and display industry division of labor, which is conducive to creating more stable and lower cost display products; the latter hopes that the display screen will be used as an entrance to integrate more information-based platform products.
The two display package solutions correspond to packaging materials suitable for their packaging requirements. The small-pitch EMC lamp beads of the front-mounted and vertical chip packages are encapsulated with solid epoxy resin. The name of the EMC lamp beads is also derived from the nature of the packaging material (Epoxy Molding Compound). Solid epoxy resin with its superior air tightness, adhesion and hardness can ensure the reliability of PCT of EMC lamp beads and the convenience of cutting and processing.
The large-scale array COB form Mini RGB using flip chip can be packaged with silicone resin. High-refractive index phenyl silicone resin can fully release the stress after curing of large-size substrate packages with its excellent low-stress performance, avoiding substrate warping. The high hardness above ShoreD65 can ensure that the display surface does not stick to dust, and the precise cutting dimensions are conducive to no Stitching.
In terms of product and market maturity, no doubt deviceized RGB EMC lamp beads are the mainstream in the market, and EMC epoxy resin is their preferred packaging material. This discussion takes JCDecaux products as an example to talk about EMC lamp beads and epoxy resin packaging materials.
In general, in EMC lamp bead packaging, to ensure the consistency of ink color and optical function, we add three materials: "B, D, F" to achieve: Black Pigment, light scattering microbeads ( Diffuser) and Filler with High Transparency.
The ink color of the display is the primary performance to ensure the contrast. At the same time, the uniformity of the ink color of the display is the core quality criterion that directly affects the user experience. Therefore, the "ink color" is the biggest technical challenge faced by EMC packaging and resin material suppliers.
The higher the resin blackness, the better the display contrast; however, the decrease in the transmittance of the packaging material leads to an increase in chip power consumption and melanin will absorb the accumulated heat, which is ultimately detrimental to the long-term reliability of the display. Therefore, seeking the unity of the contradiction between blackness (ink color) and transmittance, and using the smallest amount of melanin to achieve the highest contrast is the first design priority of display packaging materials.
JCDecaux optimized and controlled the original particle size and aggregated particle size of the melanin, and strengthened the process of mixing and dispersing with the resin to optimize the amount of melanin added to less than five ten thousandths (weight ratio) to achieve contrast and transmittance. The best balance.
The uniformity of the dispersion of melanin in EMC resin is the most concerned factor for the yield of lamp bead packaging manufacturers, and it is also the quality of the consistency of the ink color after the screen manufacturers assemble the screen. Poor uniformity of the resin-end black mix will cause the finished product of the packaging plant to have to be divided into 3-5BIN according to the brightness of the lamp beads, and the resin with good black control can help the packaging plant to improve the yield in the direction of a BIN.
Of course, the uniformity of ink color is related to many factors such as the ink color control of the substrate, the uniformity of the substrate thickness, the precision of the packaging mold, and other packaging management factors. Therefore, RGB EMC lamp bead packaging plants often have comprehensive quality control capabilities. Strong business.
The red light chip and the blue-green light chip in the RGB EMC lamp beads have different sizes and different light emitting angles. Therefore, it is necessary to add light-scattering microbeads in the resin to fully mix the light emitted by the three RGB chips inside the lamp beads and emit light at 140o Achieve consistent white balance with the same light intensity distribution within the angle. Display screens that use EMC lamp beads for improper light mixing often have a reddish preset white balance at wide viewing angles.
The addition of light-scattering microbeads has a similar optical contradiction with the melanin addition, that is, the better the light mixing effect, the greater the light loss, and the poor combination of the scattering microbeads and the resin will lead to the degradation of the package PCT performance.
Light-scattering microbeads are generally made of organic resin, and common materials are PMMA, silicone and other materials. The choice of microbead material, the particle size distribution of the microbeads, and the amount of formulation of the scattering powder in the resin as much as possible are important factors for the overall optical design and reliability design of EMC beads.
The RGB EMC lamp bead package specification sequence has been continuously reduced in size from the standard EMC1010 along the direction of small pitches, and it has advanced in the direction of 0808 and 0606. The newly developed 4IN1 module also successfully achieved the "mini-scale" of 0.7mm pitch. Regardless of the miniaturization of EMC independent lamp beads or 4IN1, the package design tends to use a thinner substrate.
After the thin substrate is combined with epoxy resin, the package warpage is more prominent than that of the ordinary thickness substrate, which is a "difficult" problem that affects the cutting efficiency of the device. In order to reduce the curing shrinkage and thermal deformation shrinkage of epoxy resins, adding inorganic fillers to the resin is a common practice for EMC sealing resins. However, LED packaging materials are different from IC packaging. Universal Silica fillers can make optical packaging materials lose transparency.
In response to this problem, JCDecaux has developed a 100% spheroidized transparent inorganic microbead material specially used for RGB EMC packaging, which can maintain a level of transparency equivalent to pure resin, and the spheroidized microbeads can replace some organic light scattering microspheres. The light mixing function of beads enhances the PCT resistance of resin EMC composites. In the range of 20-50% (wt%), the warpage of the substrate after packaging can be greatly reduced, and high-efficiency device cutting can be smoothly realized.
The mixing of melanin-based BDF functional materials and epoxy resins can be collectively referred to as the blackening process, which can be divided into dry and black.
The dry blackening process is mainly based on the finished transparent EMC resin, which is pulverized and mixed into BDF functional materials. After resin pulverization, functional powder dispersion, and cake forming, the EMC resin required by the packaging plant is made; wet blackening In the process, the resin brand manufacturer must add BDF functional materials during the resin formulation mixing stage, mix according to the resin formulation, resin smash, and cake molding to complete the EMC finished product production at one time.
"Dry method and black" is a widely used method in current packaging factories. The packaging factory can flexibly adjust the appropriate blackness according to changes in substrates and chips. However, the powder operation itself is a relatively complicated chemical material production process, the stability of the ink batch is difficult to control, and it faces many constraints such as environmental protection and safe production. It is difficult for the packaging plant to achieve large-scale self-control.
In addition, in the dry process of blackening, the powder state resin is easy to absorb moisture, and there are potential disadvantages such as mold sticking, increased cavity in the package, and reduced airtightness of the device after packaging. The wet blackening process is conducive to product quality control, but resin manufacturers need to have the ability to quickly adjust the material formula according to the needs of the packaging plant, as well as the ability to flexibly and flexibly produce and deliver in batches.
Package structure
In order to solve the heat dissipation problem in LED packaging, industry insiders at home and abroad have developed a variety of packaging structures.
Flip chip structure
For traditional form-mounted chips, the electrodes are located on the light-emitting surface of the chip, which will block part of the light and reduce the light-emitting efficiency of the chip. At the same time, the heat generated by the PN junction of this structure is conducted out through the sapphire substrate. The thermal conductivity of sapphire is low and the heat transfer path is long. Therefore, the chip of this structure has large thermal resistance and it is not easy to radiate heat. From an optical and thermal point of view, this structure has some disadvantages. In order to overcome the shortcomings of chip mounting, Lumileds Lighting developed a flip chip in 2001. In this type of chip, the light is taken out from the sapphire on the top, which eliminates the shading of the electrodes and leads and improves the light output efficiency. At the same time, the substrate uses silicon with high thermal conductivity, which greatly improves the heat dissipation effect of the chip.
Micro spray structure
Sheng Liu et al. Proposed a micro-spray structure system to solve the heat dissipation problem of high-power LEDs. In this sealed system, the fluid in the fluid cavity forms a strong jet at the series of micro nozzles under a certain pressure. The jet directly hits the lower surface of the LED chip substrate and takes away the heat generated by the LED chip. Under the action, the heated fluid enters the small fluid cavity to release heat to the external environment, which causes its temperature to drop, and flows into the micropump again to start a new cycle. This micro-spray structure has the advantages of high heat dissipation efficiency and uniform temperature distribution of the LED chip substrate. However, the reliability and stability of the micro-pump have a great impact on the system. At the same time, the system structure is more complicated and the operating cost is increased.
Thermoelectric refrigeration structure
A thermoelectric cooler is a semiconductor device whose PN junction is composed of two different conductive materials, one carrying a positive charge and the other carrying a negative charge. When a current passes through the node, the two charges leave the junction area and simultaneously carry There is heat to achieve the purpose of cooling. Its working principle is shown in the figure below.
Compared with other high-power LED cooling structures, thermoelectric cooling structures have the advantages of energy saving, small size, and easy integration with LED modules. At present, some scholars at home and abroad have conducted related research on the application of thermoelectric cooling structures on high-power LED modules. Tian Dalei and others applied the thermoelectric cooling structure to the LED heat dissipation system, and experimentally studied the cooling conditions of the LED and the thermoelectric cooler under different currents, and measured the LED junction temperature. The results show that the thermoelectric cooling structure is used. The high-power LED display module can greatly reduce the operating temperature of the device, and the substrate temperature can be reduced by more than 36% compared with the structure without the use of this structure. This data shows that the use of thermoelectric cooling structures on high-power LED modules is a good Cooling method.
Zheng Tongchang et al. Conducted a heat dissipation simulation on a 50W high-power LED module system using a thermoelectric cooling structure. The structure of the LED module system is shown in the following figure. The results show that the LED module system using thermoelectric cooling can make The LED junction temperature is reduced and the use of multi-stage semiconductor cooling to dissipate high-power LED modules has broader research value.
Chun Kai Liu et al. Also studied the use of thermoelectric cooling structures on high-power LEDs, and the results showed that the use of thermoelectric cooling structures can effectively reduce the thermal resistance of the entire LED system to 0. In addition, members of the research group also used thermoelectric cooling The related research of 1W LED system is carried out. The research results show that the luminous efficiency of the thermoelectric cooling system structure in the LED system is 1.3 times that of the non-thermoelectric cooling system structure. It can be seen that the thermoelectric cooling system has an important impact on the thermal resistance and luminous efficiency of the LED.
Packaging materials
According to the analysis of the above formula, it can be seen that the interface thermal resistance has a great impact on the total thermal resistance of high-power LEDs. The main point of reducing the total thermal resistance of the LED is how to reduce the interface thermal resistance. Therefore, it is very important to choose the appropriate thermal interface material and substrate material.
Thermal interface materials
At present, thermal interface materials commonly used in LED packaging include thermally conductive adhesives, conductive silver adhesives, and the like.
(a) Thermal conductive adhesive
The main component of the commonly used thermally conductive adhesive is epoxy resin, so its thermal conductivity is small, its thermal conductivity is poor, and its thermal resistance is large. In order to improve its thermal conductivity, high thermal conductivity materials such as aluminum oxide, boron nitride, and silicon carbide are usually filled inside the substrate. Thermally conductive adhesive has the advantages of insulation, thermal conductivity, shock resistance, easy installation, and simple process, but its thermal conductivity is very low (generally less than 1w / mk), so it can only be applied to LED packaging devices that do not require high heat dissipation.
(b) Conductive silver glue
Conductive silver glue is a GeAs, SiC conductive substrate LED. It has red, yellow, yellow-green chips with back electrodes. It is a key packaging material in the packaging or dispensing process of LED packaging. It has a fixed bonding chip, conductive and thermally conductive, conductive. The effect of heat has an important impact on the heat dissipation, light reflectivity, and VF characteristics of LED devices. As a thermal interface material, conductive silver glue is currently widely used in the LED industry. At the same time, some scholars have conducted related research on the application of conductive silver glue in LEDs. Hao Xiaoguang draws from the requirements of the conductive mechanism of conductive silver glue, the performance indicators of high-reliability conductive silver glue for LED packaging, and testing technology. Single-component solvent-free, room-temperature storage and heat-dissipating conductive silver paste is the current development direction of LED packaging. Have good prospects.
Substrate material
Through the above analysis, a certain heat dissipation path of the LED package device is from the LED chip to the bonding layer to the internal heat sink to the heat dissipation substrate and finally to the external environment. It can be seen that the heat dissipation substrate is important for the heat dissipation of the LED package. Therefore, the heat dissipation substrate must be Has the following characteristics: high thermal conductivity, insulation, stability, flatness and high strength.
(a) MCPCB
The metal-based printed circuit board (MCPCB) is bonded to a metal (copper, aluminum, etc.) with a high thermal conductivity on the original printed circuit board to improve the heat dissipation effect of the electronic device. MCPCB is the key link between internal and external heat dissipation paths, and it has the following functions: ① heat dissipation channels for LED chips; ② electrical connections of LED chips; ③ physical support of LED chips.
The advantages of MCPCB are relatively low cost and large-scale production, but there are certain disadvantages: ① The thermal conductivity is low, and the thermal conductivity of MCPCB can reach 1-2.2W / (m · K). ② The thickness of the insulation layer in the MCPCB structure should be moderate, neither too thick nor too thin. Too thick an insulating layer increases the thermal resistance of the entire MCPCB and affects the heat dissipation effect; the insulating layer is too thin. If the voltage applied to the MCPCB is too high, it will break through the insulating layer and cause a short circuit. In order to improve the thermal conductivity of MCPCB, Li Huaping and others optimized the process parameters of the plasma micro-arc oxidation (MAO) film from 20um to 40um through a series of experiments to optimize its thermal conductivity to 2.3 W / (m · K). The thermal resistance of the MAO-MCPCB substrate is lower (less than 10K / W), so that this type of heat-dissipating substrate will show up in the LED industry and even the general lighting industry.
(b) Ceramic substrate
The sintered ceramic substrate has good heat dissipation, high temperature resistance, humidity resistance, breakdown voltage, and breakdown voltage. It also has good thermal expansion coefficient matching, and has the characteristics of reducing thermal stress and thermal deformation. Therefore, ceramics are expected to become important substrate materials in high-power LED packages in the future. At present, the most commonly used ceramic materials are alumina, aluminum nitride, beryllium oxide, silicon carbide, and the like. The properties of these common ceramic materials are shown in the table below.
From the data in the table above, it can be seen that the thermal conductivity of the three materials, aluminum nitride, beryllium oxide, and silicon carbide, is relatively good, and the thermal conductivity of aluminum oxide is poor, about one-seventh that of aluminum nitride. However, among these three materials with high thermal conductivity, BeO is toxic. If they are accidentally sucked into the lungs, it will cause pulmonary beryllium disease. At present, some countries in the world have banned this material. Although AlN has a high thermal conductivity, but The technical threshold is relatively high, so the price
The former relies on mature packaging and display industry division of labor, which is conducive to creating more stable and lower cost display products; the latter hopes that the display screen will be used as an entrance to integrate more information-based platform products.
The two display package solutions correspond to packaging materials suitable for their packaging requirements. The small-pitch EMC lamp beads of the front-mounted and vertical chip packages are encapsulated with solid epoxy resin. The name of the EMC lamp beads is also derived from the nature of the packaging material (Epoxy Molding Compound). Solid epoxy resin with its superior air tightness, adhesion and hardness can ensure the reliability of PCT of EMC lamp beads and the convenience of cutting and processing.
The large-scale array COB form Mini RGB using flip chip can be packaged with silicone resin. High-refractive index phenyl silicone resin can fully release the stress after curing of large-size substrate packages with its excellent low-stress performance, avoiding substrate warping. The high hardness above ShoreD65 can ensure that the display surface does not stick to dust, and the precise cutting dimensions are conducive to no Stitching.
In terms of product and market maturity, no doubt deviceized RGB EMC lamp beads are the mainstream in the market, and EMC epoxy resin is their preferred packaging material. This discussion takes JCDecaux products as an example to talk about EMC lamp beads and epoxy resin packaging materials.
In general, in EMC lamp bead packaging, to ensure the consistency of ink color and optical function, we add three materials: "B, D, F" to achieve: Black Pigment, light scattering microbeads ( Diffuser) and Filler with High Transparency.
The ink color of the display is the primary performance to ensure the contrast. At the same time, the uniformity of the ink color of the display is the core quality criterion that directly affects the user experience. Therefore, the "ink color" is the biggest technical challenge faced by EMC packaging and resin material suppliers.
The higher the resin blackness, the better the display contrast; however, the decrease in the transmittance of the packaging material leads to an increase in chip power consumption and melanin will absorb the accumulated heat, which is ultimately detrimental to the long-term reliability of the display. Therefore, seeking the unity of the contradiction between blackness (ink color) and transmittance, and using the smallest amount of melanin to achieve the highest contrast is the first design priority of display packaging materials.
JCDecaux optimized and controlled the original particle size and aggregated particle size of the melanin, and strengthened the process of mixing and dispersing with the resin to optimize the amount of melanin added to less than five ten thousandths (weight ratio) to achieve contrast and transmittance. The best balance.
The uniformity of the dispersion of melanin in EMC resin is the most concerned factor for the yield of lamp bead packaging manufacturers, and it is also the quality of the consistency of the ink color after the screen manufacturers assemble the screen. Poor uniformity of the resin-end black mix will cause the finished product of the packaging plant to have to be divided into 3-5BIN according to the brightness of the lamp beads, and the resin with good black control can help the packaging plant to improve the yield in the direction of a BIN.
Of course, the uniformity of ink color is related to many factors such as the ink color control of the substrate, the uniformity of the substrate thickness, the precision of the packaging mold, and other packaging management factors. Therefore, RGB EMC lamp bead packaging plants often have comprehensive quality control capabilities. Strong business.
The red light chip and the blue-green light chip in the RGB EMC lamp beads have different sizes and different light emitting angles. Therefore, it is necessary to add light-scattering microbeads in the resin to fully mix the light emitted by the three RGB chips inside the lamp beads and emit light at 140o Achieve consistent white balance with the same light intensity distribution within the angle. Display screens that use EMC lamp beads for improper light mixing often have a reddish preset white balance at wide viewing angles.
The addition of light-scattering microbeads has a similar optical contradiction with the melanin addition, that is, the better the light mixing effect, the greater the light loss, and the poor combination of the scattering microbeads and the resin will lead to the degradation of the package PCT performance.
Light-scattering microbeads are generally made of organic resin, and common materials are PMMA, silicone and other materials. The choice of microbead material, the particle size distribution of the microbeads, and the amount of formulation of the scattering powder in the resin as much as possible are important factors for the overall optical design and reliability design of EMC beads.
The RGB EMC lamp bead package specification sequence has been continuously reduced in size from the standard EMC1010 along the direction of small pitches, and it has advanced in the direction of 0808 and 0606. The newly developed 4IN1 module also successfully achieved the "mini-scale" of 0.7mm pitch. Regardless of the miniaturization of EMC independent lamp beads or 4IN1, the package design tends to use a thinner substrate.
After the thin substrate is combined with epoxy resin, the package warpage is more prominent than that of the ordinary thickness substrate, which is a "difficult" problem that affects the cutting efficiency of the device. In order to reduce the curing shrinkage and thermal deformation shrinkage of epoxy resins, adding inorganic fillers to the resin is a common practice for EMC sealing resins. However, LED packaging materials are different from IC packaging. Universal Silica fillers can make optical packaging materials lose transparency.
In response to this problem, JCDecaux has developed a 100% spheroidized transparent inorganic microbead material specially used for RGB EMC packaging, which can maintain a level of transparency equivalent to pure resin, and the spheroidized microbeads can replace some organic light scattering microspheres. The light mixing function of beads enhances the PCT resistance of resin EMC composites. In the range of 20-50% (wt%), the warpage of the substrate after packaging can be greatly reduced, and high-efficiency device cutting can be smoothly realized.
The mixing of melanin-based BDF functional materials and epoxy resins can be collectively referred to as the blackening process, which can be divided into dry and black.
The dry blackening process is mainly based on the finished transparent EMC resin, which is pulverized and mixed into BDF functional materials. After resin pulverization, functional powder dispersion, and cake forming, the EMC resin required by the packaging plant is made; wet blackening In the process, the resin brand manufacturer must add BDF functional materials during the resin formulation mixing stage, mix according to the resin formulation, resin smash, and cake molding to complete the EMC finished product production at one time.
"Dry method and black" is a widely used method in current packaging factories. The packaging factory can flexibly adjust the appropriate blackness according to changes in substrates and chips. However, the powder operation itself is a relatively complicated chemical material production process, the stability of the ink batch is difficult to control, and it faces many constraints such as environmental protection and safe production. It is difficult for the packaging plant to achieve large-scale self-control.
In addition, in the dry process of blackening, the powder state resin is easy to absorb moisture, and there are potential disadvantages such as mold sticking, increased cavity in the package, and reduced airtightness of the device after packaging. The wet blackening process is conducive to product quality control, but resin manufacturers need to have the ability to quickly adjust the material formula according to the needs of the packaging plant, as well as the ability to flexibly and flexibly produce and deliver in batches.
Package structure
In order to solve the heat dissipation problem in LED packaging, industry insiders at home and abroad have developed a variety of packaging structures.
Flip chip structure
For traditional form-mounted chips, the electrodes are located on the light-emitting surface of the chip, which will block part of the light and reduce the light-emitting efficiency of the chip. At the same time, the heat generated by the PN junction of this structure is conducted out through the sapphire substrate. The thermal conductivity of sapphire is low and the heat transfer path is long. Therefore, the chip of this structure has large thermal resistance and it is not easy to radiate heat. From an optical and thermal point of view, this structure has some disadvantages. In order to overcome the shortcomings of chip mounting, Lumileds Lighting developed a flip chip in 2001. In this type of chip, the light is taken out from the sapphire on the top, which eliminates the shading of the electrodes and leads and improves the light output efficiency. At the same time, the substrate uses silicon with high thermal conductivity, which greatly improves the heat dissipation effect of the chip.
Micro spray structure
Sheng Liu et al. Proposed a micro-spray structure system to solve the heat dissipation problem of high-power LEDs. In this sealed system, the fluid in the fluid cavity forms a strong jet at the series of micro nozzles under a certain pressure. The jet directly hits the lower surface of the LED chip substrate and takes away the heat generated by the LED chip. Under the action, the heated fluid enters the small fluid cavity to release heat to the external environment, which causes its temperature to drop, and flows into the micropump again to start a new cycle. This micro-spray structure has the advantages of high heat dissipation efficiency and uniform temperature distribution of the LED chip substrate. However, the reliability and stability of the micro-pump have a great impact on the system. At the same time, the system structure is more complicated and the operating cost is increased.
Thermoelectric refrigeration structure
A thermoelectric cooler is a semiconductor device whose PN junction is composed of two different conductive materials, one carrying a positive charge and the other carrying a negative charge. When a current passes through the node, the two charges leave the junction area and simultaneously carry There is heat to achieve the purpose of cooling. Its working principle is shown in the figure below.
Compared with other high-power LED cooling structures, thermoelectric cooling structures have the advantages of energy saving, small size, and easy integration with LED modules. At present, some scholars at home and abroad have conducted related research on the application of thermoelectric cooling structures on high-power LED modules. Tian Dalei and others applied the thermoelectric cooling structure to the LED heat dissipation system, and experimentally studied the cooling conditions of the LED and the thermoelectric cooler under different currents, and measured the LED junction temperature. The results show that the thermoelectric cooling structure is used. The high-power LED display module can greatly reduce the operating temperature of the device, and the substrate temperature can be reduced by more than 36% compared with the structure without the use of this structure. This data shows that the use of thermoelectric cooling structures on high-power LED modules is a good Cooling method.
Zheng Tongchang et al. Conducted a heat dissipation simulation on a 50W high-power LED module system using a thermoelectric cooling structure. The structure of the LED module system is shown in the following figure. The results show that the LED module system using thermoelectric cooling can make The LED junction temperature is reduced and the use of multi-stage semiconductor cooling to dissipate high-power LED modules has broader research value.
Chun Kai Liu et al. Also studied the use of thermoelectric cooling structures on high-power LEDs, and the results showed that the use of thermoelectric cooling structures can effectively reduce the thermal resistance of the entire LED system to 0. In addition, members of the research group also used thermoelectric cooling The related research of 1W LED system is carried out. The research results show that the luminous efficiency of the thermoelectric cooling system structure in the LED system is 1.3 times that of the non-thermoelectric cooling system structure. It can be seen that the thermoelectric cooling system has an important impact on the thermal resistance and luminous efficiency of the LED.
Packaging materials
According to the analysis of the above formula, it can be seen that the interface thermal resistance has a great impact on the total thermal resistance of high-power LEDs. The main point of reducing the total thermal resistance of the LED is how to reduce the interface thermal resistance. Therefore, it is very important to choose the appropriate thermal interface material and substrate material.
Thermal interface materials
At present, thermal interface materials commonly used in LED packaging include thermally conductive adhesives, conductive silver adhesives, and the like.
(a) Thermal conductive adhesive
The main component of the commonly used thermally conductive adhesive is epoxy resin, so its thermal conductivity is small, its thermal conductivity is poor, and its thermal resistance is large. In order to improve its thermal conductivity, high thermal conductivity materials such as aluminum oxide, boron nitride, and silicon carbide are usually filled inside the substrate. Thermally conductive adhesive has the advantages of insulation, thermal conductivity, shock resistance, easy installation, and simple process, but its thermal conductivity is very low (generally less than 1w / mk), so it can only be applied to LED packaging devices that do not require high heat dissipation.
(b) Conductive silver glue
Conductive silver glue is a GeAs, SiC conductive substrate LED. It has red, yellow, yellow-green chips with back electrodes. It is a key packaging material in the packaging or dispensing process of LED packaging. It has a fixed bonding chip, conductive and thermally conductive, conductive. The effect of heat has an important impact on the heat dissipation, light reflectivity, and VF characteristics of LED devices. As a thermal interface material, conductive silver glue is currently widely used in the LED industry. At the same time, some scholars have conducted related research on the application of conductive silver glue in LEDs. Hao Xiaoguang draws from the requirements of the conductive mechanism of conductive silver glue, the performance indicators of high-reliability conductive silver glue for LED packaging, and testing technology. Single-component solvent-free, room-temperature storage and heat-dissipating conductive silver paste is the current development direction of LED packaging. Have good prospects.
Substrate material
Through the above analysis, a certain heat dissipation path of the LED package device is from the LED chip to the bonding layer to the internal heat sink to the heat dissipation substrate and finally to the external environment. It can be seen that the heat dissipation substrate is important for the heat dissipation of the LED package. Therefore, the heat dissipation substrate must be Has the following characteristics: high thermal conductivity, insulation, stability, flatness and high strength.
(a) MCPCB
The metal-based printed circuit board (MCPCB) is bonded to a metal (copper, aluminum, etc.) with a high thermal conductivity on the original printed circuit board to improve the heat dissipation effect of the electronic device. MCPCB is the key link between internal and external heat dissipation paths, and it has the following functions: ① heat dissipation channels for LED chips; ② electrical connections of LED chips; ③ physical support of LED chips.
The advantages of MCPCB are relatively low cost and large-scale production, but there are certain disadvantages: ① The thermal conductivity is low, and the thermal conductivity of MCPCB can reach 1-2.2W / (m · K). ② The thickness of the insulation layer in the MCPCB structure should be moderate, neither too thick nor too thin. Too thick an insulating layer increases the thermal resistance of the entire MCPCB and affects the heat dissipation effect; the insulating layer is too thin. If the voltage applied to the MCPCB is too high, it will break through the insulating layer and cause a short circuit. In order to improve the thermal conductivity of MCPCB, Li Huaping and others optimized the process parameters of the plasma micro-arc oxidation (MAO) film from 20um to 40um through a series of experiments to optimize its thermal conductivity to 2.3 W / (m · K). The thermal resistance of the MAO-MCPCB substrate is lower (less than 10K / W), so that this type of heat-dissipating substrate will show up in the LED industry and even the general lighting industry.
(b) Ceramic substrate
The sintered ceramic substrate has good heat dissipation, high temperature resistance, humidity resistance, breakdown voltage, and breakdown voltage. It also has good thermal expansion coefficient matching, and has the characteristics of reducing thermal stress and thermal deformation. Therefore, ceramics are expected to become important substrate materials in high-power LED packages in the future. At present, the most commonly used ceramic materials are alumina, aluminum nitride, beryllium oxide, silicon carbide, and the like. The properties of these common ceramic materials are shown in the table below.
From the data in the table above, it can be seen that the thermal conductivity of the three materials, aluminum nitride, beryllium oxide, and silicon carbide, is relatively good, and the thermal conductivity of aluminum oxide is poor, about one-seventh that of aluminum nitride. However, among these three materials with high thermal conductivity, BeO is toxic. If they are accidentally sucked into the lungs, it will cause pulmonary beryllium disease. At present, some countries in the world have banned this material. Although AlN has a high thermal conductivity, but The technical threshold is relatively high, so the price
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