As we all know, semiconductor light-emitting devices include semiconductor light-emitting diodes (referred to as LEDs), digital tubes, symbol tubes, meter tubes, and dot matrix display screens (referred to as matrix tubes). In fact, each light-emitting unit in the digital tube, symbol tube, meter tube and matrix tube is a light-emitting diode. Let's take a look at the simple principles, features, applications and packaging one by one.

The main component of the lens light bar is LED. The following mainly introduces the light-emitting principle of the lens light bar LED, the characteristics of the lens light bar LED, the classification of the lens light bar LED, the application of the lens light bar LED, the detection of the lens light bar LED, the lens light bar LED packaging technology, lens light bar LED structure.

1  Lens light bar LED light-emitting principle

Light-emitting diodes are made of III-IV compounds, such as GaAs (gallium arsenide), GaP (gallium phosphide), GaAsP (gallium arsenide phosphide) and other semiconductors, and their core is a PN junction. Therefore, it has the I-N characteristics of the general P-N junction, that is, forward conduction, reverse cut-off, and breakdown characteristics. In addition, under certain conditions, it also has luminescent properties. Under forward voltage, electrons are injected into P region from N region, and holes are injected into N region from P region. Part of the minority carriers (minor carriers) entering the opposing region recombine with the majority carriers (multiple carriers) to emit light, as shown in Figure 1.

Assuming that the luminescence occurs in the P region, the injected electrons recombine directly with the valence band holes to emit light, or are first captured by the luminescent center and then recombine with the holes to emit light. In addition to this luminescent recombination, some electrons are captured by the non-luminescent center (this center is near the middle of the conduction band and the dielectric band), and then recombine with the holes. The energy released each time is not large and cannot form visible light. The larger the ratio of the luminescent recombination amount to the non-luminescent recombination amount, the higher the photon quantum efficiency. Since recombination emits light in the minority carrier diffusion region, light is only generated within a few μm near the PN junction.

Theory and practice have proved that the peak wavelength λ of light is related to the forbidden band width Eg of the semiconductor material in the light-emitting region, that is, λ≈1240/Eg (mm) where the unit of Eg is electron volts (eV). If visible light can be generated (violet light at wavelengths from 380nm to red light at 780nm), the Eg of the semiconductor material should be between 3.26 and 1.63 eV. Light with a wavelength longer than red light is infrared light. Now there are infrared, red, yellow, green and blue light emitting diodes, but the blue light diodes are very expensive and are not widely used.

2  Characteristics of lens strip LED

2.1 The meaning of limit parameters

1) Allowable power consumption Pm: the maximum value that is allowed to be applied to the product of the forward DC voltage across the LED and the current flowing through it. If this value is exceeded, the LED will heat up and be damaged.

2) Maximum forward DC current IFm: the maximum allowable forward DC current. Exceeding this value can damage the diode.

3) Maximum reverse voltage VRm: the maximum allowed reverse voltage. Exceeding this value, the light-emitting diode may be damaged by breakdown.

4) Working environment topm: The ambient temperature range in which the light-emitting diode can work normally. Below or above this temperature range, the light-emitting diode will not work properly, and the efficiency will be greatly reduced.

2.2 The meaning of electrical parameters

1) Spectral distribution and peak wavelength: The light emitted by a certain LED is not a single wavelength, and its wavelength is roughly as shown in Figure 2. It can be seen from the figure that the light intensity of a certain wavelength λ0 in the light emitted by the light-emitting tube is the largest, and this wavelength is the peak wavelength.

2) Luminous intensity IV: The luminous intensity of a light-emitting diode usually refers to the luminous intensity in the direction of the normal line (or its axis for a cylindrical light-emitting tube). If the radiation intensity in this direction is (1/683) W/sr, it will emit 1 candela (symbol cd). Since the luminous intensity of the general LED is small, the luminous intensity is often used as a unit of candela (mcd).

3) Spectral half-width Δλ: It represents the spectral purity of the light-emitting tube, which refers to the interval between the two wavelengths corresponding to the 1/2 peak light intensity in Figure 3.

4) Half-value angle θ1/2 and viewing angle: θ1/2 refers to the angle between the direction where the luminous intensity value is half of the axial intensity value and the luminous axis (normal direction). 2 times the half value angle is the viewing angle (or half power angle).

Figure 3 shows the angular distribution of the luminous intensity of two different types of LEDs. The coordinates of the mid-perpendicular (normal) AO are the relative luminous intensity (ie the ratio of the luminous intensity to the maximum luminous intensity). Obviously, the relative luminous intensity in the normal direction is 1, and the greater the angle away from the normal direction, the smaller the relative luminous intensity. From this figure, the half-value angle or viewing angle value can be obtained.

5) Forward working current If: It refers to the forward current value of the light-emitting diode when it emits normally. In actual use, the IF should be selected below 0.6·IFm according to the needs.

6) Forward working voltage VF: The working voltage given in the parameter table is obtained at a given forward current. It is generally measured when IF=20mA. The forward working voltage VF of the light-emitting diode is 1.4~3V. When the external temperature increases, VF will drop.

7) V-I characteristics: The relationship between the voltage and current of the light-emitting diode can be shown in Figure 4. When the forward voltage is less than a certain value (called the threshold), the current is extremely small and no light is emitted. When the voltage exceeds a certain value, the forward current increases rapidly with the voltage and emits light. From the V-I curve, parameters such as forward voltage, reverse current and reverse voltage of the LED can be obtained. The reverse leakage current IR of the forward light-emitting tube is less than 10μA.

3  Classification of lens light bar LED

3.1 According to the light-emitting color of the light-emitting tube

According to the light-emitting color of the light-emitting tube, it can be divided into red, orange, green (subdivided into yellow-green, standard green and pure green), blue light, etc. In addition, some light-emitting diodes contain chips of two or three colors. According to whether the light-emitting diode is doped with or not doped with a scattering agent, colored or colorless, the above-mentioned light-emitting diodes of various colors can also be divided into four types: colored and transparent, colorless and transparent, colored scattering and colorless scattering. Scattered light-emitting diodes and Dayu are used as indicator lights.

3.2 According to the characteristics of the light-emitting surface of the light-emitting tube

According to the characteristics of the light-emitting surface of the light-emitting tube, it can be divided into round light, square light, rectangular light, surface light-emitting tube, lateral tube, micro-tube for surface installation, etc. Circular lights are divided into φ2mm, φ4.4mm, φ5mm, φ8mm, φ10mm and φ20mm according to their diameters. In foreign countries, the light-emitting diode of φ3mm is usually recorded as T-1; the light-emitting diode of φ5mm is recorded as T-1 (3/4); the light-emitting diode of φ4.4mm is recorded as T-1 (1/4). The angular distribution of circular luminous intensity can be estimated from the half-value angle. From the luminous intensity angle distribution map, there are three categories:

(1) High directivity. Generally, it is a pointed epoxy package, or a package with a metal reflective cavity, and no scattering agent is added. The half-value angle is 5°~20° or less, and has high directivity. It can be used as a local illumination light source, or combined with a light detector to form an automatic detection system.

(2) Standard type. It is usually used as an indicator light, and its half-value angle is 20°~45°.

(3) Scattering type. This is an indicator light with a larger viewing angle, the half-value angle is 45°~90° or more, and the amount of scattering agent is larger.

3.3 According to the structure of light-emitting diodes

According to the structure of light emitting diode, it can be divided into full epoxy encapsulation, metal base epoxy encapsulation, ceramic base epoxy encapsulation and glass encapsulation.

3.4 According to luminous intensity and working current

According to the luminous intensity and working current, there are ordinary brightness LEDs (luminous intensity < 10mcd); ultra-high brightness LEDs (luminous intensity > 100mcd); high-brightness light-emitting diodes with luminous intensity between 10 and 100mcd. Generally, the working current of LED is in the range of tens of mA to tens of mA, while the working current of low-current LED is below 2 mA (the brightness is the same as that of ordinary light-emitting tubes).

In addition to the above classification methods, there are methods of classification by chip material and classification by function.

4  The application of lens light bar LED

Due to the difference in color, size, shape, luminous intensity and transparency of light-emitting diodes, appropriate selection should be made according to actual needs when using light-emitting diodes. Due to the limitation of the maximum forward current IFm and the maximum reverse voltage VRm of the light-emitting diode, it should be guaranteed not to exceed this value when using it. For the sake of safety, the actual current IF should be below 0.6IFm; the possible reverse voltage VR<0.6VRm should be allowed. LEDs are widely used in various electronic instruments and electronic equipment, and can be used as power indicator, level indicator or For micro light source. Infrared light-emitting tubes are often used in remote controls for TVs, VCRs, etc.

(1) The circuit for making a miniature flashlight using high-brightness or ultra-high-brightness light-emitting diodes is shown in Figure 5. The value of the resistor R in the figure is a current limiting resistor, and its value should ensure that the current of the LED should be less than the maximum allowable current IFm when the power supply voltage is the highest.

(2) Figure 6 (a), (b), (c) are the DC power supply, rectified power supply and AC power supply indicating circuits.

(3) Single LED level indicating circuit. At the output end of the amplifier, oscillator or pulse digital circuit, LEDs can be used to indicate whether the output signal is normal, as shown in Figure 7. R is the current limiting resistor. The LED may emit light only when the output voltage is greater than the threshold voltage of the LED.

(4) A single LED can be used as a low voltage regulator. After the LED is turned on in the forward direction, the current changes very quickly with the voltage, which has the characteristics of ordinary voltage regulators. The stabilized voltage of the light-emitting diode is between 1.4~3V, and VF should be selected according to the needs.

(5) Level meter. Currently, LED level meters are widely used in audio equipment. It uses multiple light-emitting tubes to indicate the output signal level, that is, the number of light-emitting LEDs is different, which means the change of the output level. When the input signal level is very low, it does not emit light at all. When the input signal level increases, first LED1 lights up, and then increases LED2 lights up.

5  Detection of lens light bar LED

5.1 Detection of ordinary light-emitting diodes

(1) Check with a multimeter. Using a pointer-type multimeter with a ×10kΩ block can roughly judge the quality of the light-emitting diode. In normal conditions, the forward resistance of the diode is tens to 200kΩ, and the reverse resistance is ∝. If the forward resistance value is 0 or ∞, and the reverse resistance value is small or 0, it is easy to damage. This detection method cannot see the light-emitting condition of the LED on the spot, because the ×10kΩ block cannot provide a large forward current to the LED.

If there are two pointer multimeters (preferably the same model), you can better check the light-emitting diodes. Use a wire to connect the "+" terminal of one of the multimeters to the "-" terminal of the other meter. The remaining "-" pens are connected to the positive electrode (P area) of the LED under test, and the remaining "+" pens are connected to the negative electrode (N area) of the LED under test. Both multimeters are set to ×10Ω. Under normal circumstances, it can light up normally after it is turned on. If the brightness is very low or even does not emit light, you can turn both multimeters to ×1Ω. If it is still very dark or even does not emit light, it means that the performance of the LED is poor or damaged. It should be noted that the two multimeters cannot be placed at × 1Ω at the beginning of the measurement, so as to avoid excessive current and damage to the light-emitting diode.

(2) External power measurement. The optical and electrical characteristics of light-emitting diodes can be measured more accurately with a 3V voltage regulator or two dry batteries connected in series and a multimeter (either pointer type or digital type). For this purpose, the circuit can be connected as shown in Figure 10. If the measured VF is between 1.4~3V, and the luminous brightness is normal, it can indicate that the luminescence is normal. If VF=0 or VF≈3V is measured, and no light is emitted, it means that the LED is broken

5.2 Detection of infrared light-emitting diodes

Thanks to infrared light-emitting diodes, itIt emits infrared light of 1~3μm, which is invisible to the human eye. Usually, the emission power of a single infrared LED is only a few mW, and the angular distribution of the luminous intensity of different types of infrared LEDs is also different. The forward voltage drop of the infrared LED is generally 1.3~2.5V. It is precisely because the infrared light emitted by the infrared light cannot be seen by the human eye, so the detection method of the above visible light LED can only determine whether the forward and reverse electrical characteristics of the PN junction are normal. It is impossible to determine whether its light emission is normal or not. To this end, it is best to prepare a photosensitive device (such as 2CR, 2DR silicon photovoltaic cells) as a receiver. Use a multimeter to measure the change in voltage across the battery. To judge whether the infrared LED emits infrared light after adding an appropriate forward current. Its measurement circuit is shown in Figure 8.

6  Encapsulation technology of lens strip LED

6.1 Particularities of LED Packages

LED packaging technology is mostly developed and evolved on the basis of discrete device packaging technology, but it has great particularity. Typically, the die of a discrete device is sealed in a package that protects the die and completes electrical interconnections. The LED package is to complete the output of electrical signals, protect the normal operation of the die, and output: the function of visible light, which has both electrical parameters and optical parameters. Design and technical requirements, it is impossible to simply use the packaging of discrete devices for LEDs.

The core light-emitting part of the LED is a pn junction die composed of p-type and n-type semiconductors. When the minority carriers injected into the pn junction recombine with the majority carriers, visible light, ultraviolet light or near-infrared light will be emitted. However, the photons emitted by the pn junction area are non-directional, that is, they have the same probability of being emitted in all directions. Therefore, not all the light generated by the die can be released, which mainly depends on the quality of the semiconductor material, die structure and geometry. , Encapsulation internal structure and encapsulation materials, application requirements to improve the internal and external quantum efficiency of LED. The conventional Φ5mm LED package is to bond or sinter a square die with a side length of 0.25mm on the lead frame. The positive pole of the die is connected to the gold wire through the spherical contact point, and the inner lead is connected to a pin, and the negative pole is reflected by reflection. The cup is connected to the other pin of the lead frame, and the top of it is then encapsulated with epoxy. The function of the reflector cup is to collect the light emitted from the side and interface of the die and emit it in the desired direction angle. The epoxy resin encapsulated at the top is made into a certain shape, which has several functions: protect the die from external erosion; use different shapes and material properties (with or without dispersing agent) to act as a lens or diffuser lens Function, control the divergence angle of light; the refractive index of the die is too related to the refractive index of the air, so that the critical angle of total reflection inside the die is very small, and only a small part of the light generated by the active layer is taken out, and most of it is easy to be in the tube. The inside of the core is absorbed by multiple reflections, which is prone to total reflection and leads to excessive light loss. The epoxy resin with the corresponding refractive index is selected as the transition to improve the light output efficiency of the die. The epoxy resin used to form the tube shell must have moisture resistance, insulation, mechanical strength, and high refractive index and transmittance to the light emitted by the tube core. When packaging materials with different refractive indices are selected, the effect of packaging geometry on photon escape efficiency is different, and the angular distribution of luminous intensity is also related to the die structure, light output method, and the material and shape of the packaging lens. If a pointed resin lens is used, the light can be concentrated in the axial direction of the LED, and the corresponding viewing angle is smaller; if the top resin lens is circular or flat, the corresponding viewing angle will be increased.

Under normal circumstances, the luminous wavelength of LED changes with temperature to 0.2-0.3nm/℃, and the spectral width increases accordingly, which affects the color vividness. In addition, when the forward current flows through the pn junction, the thermal loss causes the temperature rise in the junction area. Near room temperature, for every 1°C increase in temperature, the luminous intensity of the LED will correspondingly decrease by about 1%, and the package will dissipate heat; Purity and luminous intensity are very important. In the past, the method of reducing the driving current was used to reduce the junction temperature. The driving current of most LEDs was limited to about 20mA. However, the light output of LED will increase with the increase of current. At present, the driving current of many power LEDs can reach 70mA, 100mA or even 1A. It is necessary to improve the packaging structure, new LED packaging design concept and low thermal resistance packaging structure and technology to improve thermal characteristics. For example, a large-area chip flip-chip structure is used, silver paste with good thermal conductivity is used, the surface area of the metal bracket is increased, and the silicon carrier of the solder bump is directly mounted on the heat sink. In addition, in the application design, the thermal design and thermal conductivity of the PCB circuit board are also very important.

6.2 Package structure type

Since the 1990s, a number of breakthroughs have been made in the research and development of LED chip and material manufacturing technology, such as the trapezoidal structure of transparent substrates, textured surface structures, flip-chip structures, and commercialized ultra-high brightness (above 1cd) red, orange, Yellow, green and blue LED products have been on the market one after another. As shown in Table 1, they have been used in special lighting with low and medium luminous flux since 2000. The upstream and midstream industries of LEDs have received unprecedented attention, further promoting the downstream packaging technology and industrial development. Using different packaging structures and sizes, dies with different luminous colors and their two-color or three-color combinations, a variety of products can be produced. series, varieties and specifications of products.

The type of LED product package structure is shown in Table 2, and it is also classified according to the characteristics of light emission color, chip material, light emission brightness, size and so on. A single die generally constitutes a point light source, and multiple die assemblies can generally constitute a surface light source and a line light source for information, status indication and display. In parallel) combined with a suitable optical structure, it constitutes the light-emitting segment and light-emitting point of the light-emitting display. Surface mount LEDs can gradually replace pin-type LEDs, and the application design is more flexible. It has occupied a certain share in the LED display market and has an accelerated development trend. Some solid-state lighting source products are listed, which will become the medium and long-term development direction of LED in the future.

6.3 Lead Package

The LED pin-type package uses lead frames as the pins of various package shapes. It is the first package structure that has been successfully developed and put on the market. There are many varieties and high technology maturity. The internal structure of the package and the reflective layer are still being continuously improved. Standard LEDs are considered by most customers to be the most convenient and economical solution in the display industry today. A typical traditional LED is housed in a package that can withstand 0.1W input power, and 90% of its heat is generated by the negative lead frame. Dissipate to the PCB board and then to the air. How to reduce the temperature rise of the pn junction during operation must be considered in packaging and applications. The encapsulation material is mostly high temperature curing epoxy resin, which has excellent optical properties, good process adaptability, and high product reliability. It can be made into colored transparent or colorless transparent and colored scattering or colorless scattering. The shape constitutes a variety of shapes and sizes. For example, the circle is divided into Φ2mm, Φ3mm, Φ4.4mm, Φ5mm, Φ7mm and so on according to the diameter. Different components of epoxy resin can produce different luminous effects. There are many different package structures for the color point light source: the ceramic base epoxy resin package has better operating temperature performance, the pins can be bent into the required shape, and the volume is small; the metal base plastic reflector package is an energy-saving indicator light , suitable for power indication; the flickering type combines the CMOS oscillator circuit chip with the LED die, which can produce flickering light with strong visual impact; the two-color type is composed of two different luminous colors of the die, packaged in the same epoxy In the resin lens, in addition to the two-color, a third mixed color can be obtained, which is widely used in large-screen display systems and can be packaged to form a two-color display device; the voltage type combines the constant current source chip and the LED die to package, It can directly replace various voltage indicators of 5-24V. The surface light source is formed by using a plastic reflective frame cover and encapsulating epoxy resin on the specified position of the CB board with multiple LED dies. The different designs of the PCB board determine the arrangement and connection of the outer leads. There are double columns In-line and single-in-line and other structural forms. Point and area light sources have now developed hundreds of package shapes and sizes for the market and customers.

6.4 Surface Mount Packages

In 2002, surface mount packaged LEDs (SMD LEDs) were gradually accepted by the market and gained a certain market share. The shift from lead package to SMD conformed to the general development trend of the entire electronics industry, and many manufacturers launched such products.

Most of the early SMD LEDs used the improved SOT-23 with transparent plastic body, the overall size was 3.04×1.11mm, and the reel container was packaged with tape. On the basis of SOT-23, the SLM-125 series and SLM-245 series of high-brightness SMD with lenses are developed. The former is single-color emitting, and the latter is two-color or three-color emitting. In recent years, SMD LED has become a development hotspot, which has well solved the problems of brightness, viewing angle, flatness, reliability, consistency, etc. It adopts lighter PCB board and reflective layer materials, and needs to be filled in the display reflective layer. Less epoxy resin and removal of heavier carbon steel pins, by reducing the size and weight, the product weight can easily be reduced in half, and ultimately make the application more perfect, especially suitable for indoor, semi-outdoor full color display screen application.

6.5 Power Package

LED chips and packages are developing in the direction of high power. Under high current, the luminous flux is 10-20 times larger than that of Φ5mm LED. Effective heat dissipation and non-deteriorating packaging materials must be used to solve the problem of light decay. Therefore, the shell and packaging are also the key. technology, LED packages that can withstand several watts of power have emerged. 5W series of white, green, blue-green, blue power LEDs have been available since early 2003. The white LED light output reached 1871m, and the luminous efficiency was 44.31m/W. Tube; The size of the dagger is 2.5×2.5mm, it can work under the current of 5A, and the light output reaches 2001m. It has a lot of room for development as a solid-state lighting source.

Luxeon series power LEDs are made by flip-chip welding A1GalnN power type flip-chip die on a silicon carrier with solder bumps, and then placing the flip-chip soldered silicon carrier into a heat sink and a tube case, and bonding wires for packaging. This kind of package is the best for light extraction efficiency, heat dissipation performance, and design to increase the working current density. Its main features: low thermal resistance, generally only 14°C/W, only 1/10 of conventional LEDs; high reliability, filled with stable flexible gel inside the package, in the range of -40-120°C, it will not The internal stress caused by the sudden temperature change breaks the gold wire from the lead frame and prevents the epoxy lens from yellowing, and the lead frame will not be stained by oxidation; the optimal design of the reflector cup and lens makes the radiation pattern controllable and Highest optical efficiency. In addition, its output optical power, external quantum efficiency and other excellent performance, the LED solid-state light source has been developed to a new level.

As LEDs are widely used in large-area graphic display full-color screens, status indicators, sign lighting, signal displays, LCD backlights, automotive combination taillights and interior lighting, etc., their development prospects have attracted global lighting manufacturers. Join LED light source and market development. The most promising development and application is white LED, which is economical and environmentally friendly as a solid-state lighting device. It is gradually replacing traditional incandescent lamps. The annual growth rate of the world is more than 20%. The United States, Japan, Europe and China Taiwan Province has launched a semiconductor lighting program. The excellent heat dissipation characteristics and optical characteristics of power LEDs are more suitable for the field of general lighting, and are considered by academia and industry as the only way for LEDs to enter the lighting market. Therefore, LED is known as a new light source in the 21st century, and is expected to become the fourth-generation light source after incandescent lamps, fluorescent lamps, and high-intensity gas discharge lamps.