The LED module of the COB package has a plurality of LED chiplets mounted on the substrate. The use of multiple chips can not only improve the brightness of the module, but also help to achieve reasonable configuration of the LED chip , and reduce the input current of the single LED chip to ensure high efficiency. From a cost and application perspective, COB is one of the mainstream directions for future luminaire design.
The power density increases with the number of chips in the module. If the heat dissipation method is improper, the heat accumulated rapidly not only affects the electronic performance of the LED, but also affects the brightness and color of the LED. As the temperature of the chip increases, the spectrum appears red. Shift, the luminous efficiency is reduced. Studies have shown that the thermal conductivity of a mixture of polymer composites increases as the concentration of doped particles increases. Increasing the concentration of the fluorescent glue can effectively improve the thermal conductivity of the colloid.
In addition, different package structures have a significant impact on the light extraction efficiency of the module. In particular, since the refractive index of the encapsulant colloids is too different from the refractive index of air, the full emission occurring at the interface will seriously affect the light extraction of the device as a whole. Therefore, compared with the commonly used planar structure, the outer sealed free-form lens structure can significantly improve the light extraction of the module and improve the light extraction efficiency. At the same time, the COB package can also achieve a specific optical distribution.
This paper presents a new COB package structure based on conformal coating technology and free-form lens array. A layer of phosphor is coated on the surface of the chip by conformal coating technology, and a properly designed free-form lens is encapsulated for each chip mold to improve the light extraction of the module and achieve a specific optical distribution.
New COB package structure
Nowadays, the structure of COB on the market is complete, mainly filling the fluorescent glue on the substrate for completing the core placement and the gold wire bonding, and forming mechanical protection for the die and the gold wire. At the same time, by adjusting the amount of fluorescent glue, an optical structure of a plane or a circular arc surface is formed. Figure 1 is a schematic diagram of the mainstream COB package structure on the market. As can be seen from Fig. 1, this type of COB package structure covers the fluorescent glue over a large area to form a surface light source and emits light overall.
Because the LED module of the COB technology package has a simple structure, a simple manufacturing process and soft light, it is widely used in the market, but there are also some problems.
First, although the amount of the glue is controlled to slightly bulge the middle of the fluorescent glue, forming a convex lens improves the light extraction efficiency. However, there is still a reflection problem with light at the colloidal and air interfaces. Especially for chips far from the center, the role of the convex lens is weaker.
Second, the potting compound is made up of organic packaging materials (such as silica gel) and phosphors mixed in a certain proportion, and the heat dissipation effect is poor. In the experiment, under the premise of having a heat sink, the temperature of the surface of the normally illuminated COB module colloid is as high as tens of degrees Celsius, and even some devices with higher power are as high as one hundred degrees Celsius.
Third, during normal operation, due to the higher temperature of the colloid, it has a serious impact on silica gel and phosphor. As the temperature increases, the thermal stress inside the silica gel increases, and the refractive index of the silica gel decreases, which affects the optical distribution of the device and reduces the light extraction efficiency.
In response to the above mentioned problems, we propose a new COB package structure scheme, the structure of which is shown in Figure 2.
The structure is formed on the substrate of the previous process such as chip placement and gold wire bonding, and the surface of the chip is covered with a phosphor (fluorescent glue) by conformal coating technology, and then externally sealed on each chip. A specially designed free-form lens can achieve a specific optical distribution (such as near Lambertian distribution, uniform illumination, etc.), and avoid the total reflection of light at the interface between the silica gel and the air, improving the light extraction of the device. At the same time, since each free-form surface lens is close to each other and the optical distribution is uniform, the light field superimposition of many free-form surface lenses does not cause a spot light effect when viewed in the far field.
Under the premise that the color temperature of the device is constant, the concentration of the fluorescent glue using conformal coating technology is much higher than that of the ordinary COB package. Therefore, the thermal conductivity of the new structure of the fluorescent glue is greatly improved. At the same time, since the fluorescent glue is only coated on the surface of the chip, the coverage is small, and the heat generated by the phosphor operation can be quickly transmitted to the substrate through the chip for heat dissipation. Thus, the colloidal temperature rise is prevented from affecting the physical properties of the phosphor and the silica gel.
Freeform lens optical design
Ideally, the output of the chip is a Lambertian source and is a point source. Ignore the absorption of silica gel material and the reflection, diffuse transmission, diffuse reflection and other factors at the interface between silica gel and air. Only the influence of key factors such as the surface of the silicone lens and the relative position of the chip and the lens on the light distribution of the LED is considered.
This type of outer sealing primary optical lens is usually designed using the structure shown in FIG. It is assumed that the light source (chip) is located at the coordinate origin O, the chip light emitting surface is the XY plane, and the normal direction of the chip plane is the Z axis. After the light is refracted through the free-form surface P(r, θ), it conforms to the optical distribution we need. The free-form surface P(r, θ) is calculated by solving the differential equation according to the law of conservation of energy and Snell's law of refraction.
Since the entire lens is rotationally symmetrical, only the two-dimensional case is considered, taking the XZ plane as an example. The vector form of the catadioptric theorem can be expressed as:
Where n and n' are respectively the refractive index of the silicone lens and air; N is the unit normal vector of the free-form surface at the point of incidence of the light, and Q and Q' are the unit vectors of the incident and outgoing rays, respectively.
The normal differential form of the freeform surface P(r, θ) is:
According to the law of conservation of energy, the radiant flux of the light source is equal to the flux of the exiting light after passing through the free-form surface, ie:
Where I(θ) and I'(θ) are the light intensity distribution of the light source and the light intensity distribution of the light after the refracting.
The general light source is set to the Lambertian source, ie:
I(θ)=I0cosθ (4)
The light intensity distribution of the emitted light has different expression forms according to different actual needs. For example, a class Lambertian source, then:
I'(θ)=I'0cosmθ (5)
Where m is determined by the divergence half angle θ1/2:
m=-ln2/ln(cosθ1/2) (6)
If the outgoing light is to be illuminated at a large angle on the target surface, according to the cosine cube theorem:
I'(θ)=E0·L/cos3θ (7)
Where E0 is the average illuminance of the target surface, and L is the distance between the light source and the target surface.
As can be seen from Fig. 3, the initial condition is P = P0 (R0, 0), and R0 is the initial lens height. According to the initial conditions, the equations (2)~(5), or simultaneous (2)~(4) and (7), can be solved by using the Runge-Kutta method or other mathematical algorithms to solve a series of data points P0 (R0). , 0), P(R1, θ1), ..., P(R n', θn). The 3D lens model was built by fitting the point set to a curve using 3D modeling software. Finally, we can import the Tracepro software to get the optical model we want. After setting the parameters and performing simulation and optimization, we can get the light distribution effect we need.
Experiment and result analysis
(1) Simulation experiment
Here, for example, uniform illumination on the target surface is taken as an example. Assume that each chip is a point source with a power of 1W. The 9 chips are in a 3×3 array with a row to pitch of 1.25 mm. The initial height of the lens is R0 = 0.4 mm, the refractive index is 1.5, and uniform illumination is achieved outside the distance of 15 mm, and the illumination range is a circle having a diameter of 60 mm. The calculated shape of a single free-form surface lens is shown in Fig. 4(a).
We fit the calculated data point set into the curve in the modeling software SolidWorks and build the COB lens array model as set by the previous parameters. The three-dimensional model of the COB lens was introduced into Tracepro software for simulation, and the refractive index of the lens and the light-emitting characteristics of the chip were set according to the above assumptions, and a receiving surface of 100 mm × 100 mm was established 15 mm away from the lens. In order to ensure the accuracy of the illumination distribution on the receiving surface, we use Tracepro software to trace 1.5 million rays of light to the lens, and the target light field distribution of the COB lens array is shown in Fig. 4(b).
It can be seen from Fig. 4(b) that the trace simulation result of the light has reached the expected result, and most of the light is irradiated in a circular area of ​​60 mm in diameter. The structural energy utilization rate of a single free-form lens is 96%, and the energy efficiency of the COB free-form lens array is higher than 90%. It can be seen that the energy utilization rate of the free-form lens structure is much higher than that of the existing planar structure and the micro-convex structure.
The uniformity of general illumination is defined as the ratio between the minimum illuminance value and the maximum illuminance value in the target plane:
In the simulation results in Fig. 4(b), the uniformity was 98.3%.
(2) Actual test
In order to verify the feasibility of the above structure, based on the above simulation results, we conducted experimental demonstration. We use a 0.889mm chip, 9 chips into a 3 × 3 array, the row and column distance are 2.8mm, the outer cover of the optimized free-form lens structure, the actual object is shown in Figure 5 (a). The device is illuminated, and the spot of the receiving screen 15 mm away from the light source is as shown in Fig. 5(b).
As can be seen from Fig. 5(b), most of the light falls in a predetermined circular area of ​​60 mm in diameter, and the spot is uniform. The actual results are in good agreement with the previous simulation results.
in conclusion
The article introduces the structural characteristics of the new LED module COB package and the design method of the free-form lens.
Phosphor coating is achieved by the conformal coating technology, which is beneficial to the overall heat dissipation of the module, and avoids the influence of the temperature rise of the colloid on the physical properties of the phosphor and the silica gel. Under the premise of known light source luminescence characteristics and required illumination requirements, the equations of energy are established according to the law of conservation of energy and the Snell equation, and the freeform surface is directly obtained by numerical solution. According to the parameters such as the spacing of the chip, the calculated free-form lens array is externally sealed, thereby effectively preventing the occurrence of total reflection of light at the interface between the silica gel and the air, and achieving a specific optical distribution.
The optical simulation results show that the proposed scheme can achieve better design results, and can achieve light-emitting effects such as near-Lambertian light emission and uniform illumination, and the structure light-emitting efficiency is higher than 90%, which is higher than the commonly used planar structure. The experimental results of the physical light field distribution are in good agreement with the simulation results. In theory, this structural design is applicable to all types of LEDs. The desired optical distribution of the light is obtained by changing the corresponding parameters in the program during design, so the structure has a wide range of applications.
The power density increases with the number of chips in the module. If the heat dissipation method is improper, the heat accumulated rapidly not only affects the electronic performance of the LED, but also affects the brightness and color of the LED. As the temperature of the chip increases, the spectrum appears red. Shift, the luminous efficiency is reduced. Studies have shown that the thermal conductivity of a mixture of polymer composites increases as the concentration of doped particles increases. Increasing the concentration of the fluorescent glue can effectively improve the thermal conductivity of the colloid.
In addition, different package structures have a significant impact on the light extraction efficiency of the module. In particular, since the refractive index of the encapsulant colloids is too different from the refractive index of air, the full emission occurring at the interface will seriously affect the light extraction of the device as a whole. Therefore, compared with the commonly used planar structure, the outer sealed free-form lens structure can significantly improve the light extraction of the module and improve the light extraction efficiency. At the same time, the COB package can also achieve a specific optical distribution.
This paper presents a new COB package structure based on conformal coating technology and free-form lens array. A layer of phosphor is coated on the surface of the chip by conformal coating technology, and a properly designed free-form lens is encapsulated for each chip mold to improve the light extraction of the module and achieve a specific optical distribution.
New COB package structure
Nowadays, the structure of COB on the market is complete, mainly filling the fluorescent glue on the substrate for completing the core placement and the gold wire bonding, and forming mechanical protection for the die and the gold wire. At the same time, by adjusting the amount of fluorescent glue, an optical structure of a plane or a circular arc surface is formed. Figure 1 is a schematic diagram of the mainstream COB package structure on the market. As can be seen from Fig. 1, this type of COB package structure covers the fluorescent glue over a large area to form a surface light source and emits light overall.
Because the LED module of the COB technology package has a simple structure, a simple manufacturing process and soft light, it is widely used in the market, but there are also some problems.
First, although the amount of the glue is controlled to slightly bulge the middle of the fluorescent glue, forming a convex lens improves the light extraction efficiency. However, there is still a reflection problem with light at the colloidal and air interfaces. Especially for chips far from the center, the role of the convex lens is weaker.
Second, the potting compound is made up of organic packaging materials (such as silica gel) and phosphors mixed in a certain proportion, and the heat dissipation effect is poor. In the experiment, under the premise of having a heat sink, the temperature of the surface of the normally illuminated COB module colloid is as high as tens of degrees Celsius, and even some devices with higher power are as high as one hundred degrees Celsius.
Third, during normal operation, due to the higher temperature of the colloid, it has a serious impact on silica gel and phosphor. As the temperature increases, the thermal stress inside the silica gel increases, and the refractive index of the silica gel decreases, which affects the optical distribution of the device and reduces the light extraction efficiency.
In response to the above mentioned problems, we propose a new COB package structure scheme, the structure of which is shown in Figure 2.
The structure is formed on the substrate of the previous process such as chip placement and gold wire bonding, and the surface of the chip is covered with a phosphor (fluorescent glue) by conformal coating technology, and then externally sealed on each chip. A specially designed free-form lens can achieve a specific optical distribution (such as near Lambertian distribution, uniform illumination, etc.), and avoid the total reflection of light at the interface between the silica gel and the air, improving the light extraction of the device. At the same time, since each free-form surface lens is close to each other and the optical distribution is uniform, the light field superimposition of many free-form surface lenses does not cause a spot light effect when viewed in the far field.
Under the premise that the color temperature of the device is constant, the concentration of the fluorescent glue using conformal coating technology is much higher than that of the ordinary COB package. Therefore, the thermal conductivity of the new structure of the fluorescent glue is greatly improved. At the same time, since the fluorescent glue is only coated on the surface of the chip, the coverage is small, and the heat generated by the phosphor operation can be quickly transmitted to the substrate through the chip for heat dissipation. Thus, the colloidal temperature rise is prevented from affecting the physical properties of the phosphor and the silica gel.
Freeform lens optical design
Ideally, the output of the chip is a Lambertian source and is a point source. Ignore the absorption of silica gel material and the reflection, diffuse transmission, diffuse reflection and other factors at the interface between silica gel and air. Only the influence of key factors such as the surface of the silicone lens and the relative position of the chip and the lens on the light distribution of the LED is considered.
This type of outer sealing primary optical lens is usually designed using the structure shown in FIG. It is assumed that the light source (chip) is located at the coordinate origin O, the chip light emitting surface is the XY plane, and the normal direction of the chip plane is the Z axis. After the light is refracted through the free-form surface P(r, θ), it conforms to the optical distribution we need. The free-form surface P(r, θ) is calculated by solving the differential equation according to the law of conservation of energy and Snell's law of refraction.
Since the entire lens is rotationally symmetrical, only the two-dimensional case is considered, taking the XZ plane as an example. The vector form of the catadioptric theorem can be expressed as:
Where n and n' are respectively the refractive index of the silicone lens and air; N is the unit normal vector of the free-form surface at the point of incidence of the light, and Q and Q' are the unit vectors of the incident and outgoing rays, respectively.
The normal differential form of the freeform surface P(r, θ) is:
According to the law of conservation of energy, the radiant flux of the light source is equal to the flux of the exiting light after passing through the free-form surface, ie:
Where I(θ) and I'(θ) are the light intensity distribution of the light source and the light intensity distribution of the light after the refracting.
The general light source is set to the Lambertian source, ie:
I(θ)=I0cosθ (4)
The light intensity distribution of the emitted light has different expression forms according to different actual needs. For example, a class Lambertian source, then:
I'(θ)=I'0cosmθ (5)
Where m is determined by the divergence half angle θ1/2:
m=-ln2/ln(cosθ1/2) (6)
If the outgoing light is to be illuminated at a large angle on the target surface, according to the cosine cube theorem:
I'(θ)=E0·L/cos3θ (7)
Where E0 is the average illuminance of the target surface, and L is the distance between the light source and the target surface.
As can be seen from Fig. 3, the initial condition is P = P0 (R0, 0), and R0 is the initial lens height. According to the initial conditions, the equations (2)~(5), or simultaneous (2)~(4) and (7), can be solved by using the Runge-Kutta method or other mathematical algorithms to solve a series of data points P0 (R0). , 0), P(R1, θ1), ..., P(R n', θn). The 3D lens model was built by fitting the point set to a curve using 3D modeling software. Finally, we can import the Tracepro software to get the optical model we want. After setting the parameters and performing simulation and optimization, we can get the light distribution effect we need.
Experiment and result analysis
(1) Simulation experiment
Here, for example, uniform illumination on the target surface is taken as an example. Assume that each chip is a point source with a power of 1W. The 9 chips are in a 3×3 array with a row to pitch of 1.25 mm. The initial height of the lens is R0 = 0.4 mm, the refractive index is 1.5, and uniform illumination is achieved outside the distance of 15 mm, and the illumination range is a circle having a diameter of 60 mm. The calculated shape of a single free-form surface lens is shown in Fig. 4(a).
We fit the calculated data point set into the curve in the modeling software SolidWorks and build the COB lens array model as set by the previous parameters. The three-dimensional model of the COB lens was introduced into Tracepro software for simulation, and the refractive index of the lens and the light-emitting characteristics of the chip were set according to the above assumptions, and a receiving surface of 100 mm × 100 mm was established 15 mm away from the lens. In order to ensure the accuracy of the illumination distribution on the receiving surface, we use Tracepro software to trace 1.5 million rays of light to the lens, and the target light field distribution of the COB lens array is shown in Fig. 4(b).
It can be seen from Fig. 4(b) that the trace simulation result of the light has reached the expected result, and most of the light is irradiated in a circular area of ​​60 mm in diameter. The structural energy utilization rate of a single free-form lens is 96%, and the energy efficiency of the COB free-form lens array is higher than 90%. It can be seen that the energy utilization rate of the free-form lens structure is much higher than that of the existing planar structure and the micro-convex structure.
The uniformity of general illumination is defined as the ratio between the minimum illuminance value and the maximum illuminance value in the target plane:
In the simulation results in Fig. 4(b), the uniformity was 98.3%.
(2) Actual test
In order to verify the feasibility of the above structure, based on the above simulation results, we conducted experimental demonstration. We use a 0.889mm chip, 9 chips into a 3 × 3 array, the row and column distance are 2.8mm, the outer cover of the optimized free-form lens structure, the actual object is shown in Figure 5 (a). The device is illuminated, and the spot of the receiving screen 15 mm away from the light source is as shown in Fig. 5(b).
As can be seen from Fig. 5(b), most of the light falls in a predetermined circular area of ​​60 mm in diameter, and the spot is uniform. The actual results are in good agreement with the previous simulation results.
in conclusion
The article introduces the structural characteristics of the new LED module COB package and the design method of the free-form lens.
Phosphor coating is achieved by the conformal coating technology, which is beneficial to the overall heat dissipation of the module, and avoids the influence of the temperature rise of the colloid on the physical properties of the phosphor and the silica gel. Under the premise of known light source luminescence characteristics and required illumination requirements, the equations of energy are established according to the law of conservation of energy and the Snell equation, and the freeform surface is directly obtained by numerical solution. According to the parameters such as the spacing of the chip, the calculated free-form lens array is externally sealed, thereby effectively preventing the occurrence of total reflection of light at the interface between the silica gel and the air, and achieving a specific optical distribution.
The optical simulation results show that the proposed scheme can achieve better design results, and can achieve light-emitting effects such as near-Lambertian light emission and uniform illumination, and the structure light-emitting efficiency is higher than 90%, which is higher than the commonly used planar structure. The experimental results of the physical light field distribution are in good agreement with the simulation results. In theory, this structural design is applicable to all types of LEDs. The desired optical distribution of the light is obtained by changing the corresponding parameters in the program during design, so the structure has a wide range of applications.
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Shenzhen Mingxue Optoelectronics CO.,Ltd , https://www.led-lamp-china.com