Introduction to LED Light Emitting Diode

LED lighting has quietly emerged in recent years, but do you know LED? Let's learn about LED today.

LED composition

LED (Light Emitting Diode) is mainly composed of five materials: bracket, silver glue, chip, gold wire and epoxy resin.

LED is a semiconductor that can convert electrical energy into light energy. It changes the principle of tungsten filament light emission of incandescent lamps and three-primary color powder light emission of energy-saving lamps, and adopts electric field light emission. According to analysis, the characteristics of LED are very obvious.

LED bulbs and lamps pictures (18 pictures) show long life, high light efficiency, low radiation and low power consumption. The spectrum of white light LED is almost entirely concentrated in the visible light band, and its luminous efficiency can exceed 150lm/W (2010). Comparing LED with ordinary incandescent lamps, spiral energy-saving lamps and T5 tri-color fluorescent lamps, the results show that the light efficiency of ordinary incandescent lamps is 12lm/W, and the life is less than 2000 hours, the light efficiency of spiral energy-saving lamps is 60lm/W, and the life is less than 8000 hours, and the light efficiency of T5 fluorescent lamps is 96lm/W, and the life is about 10000 hours. The light efficiency of white light LED with a diameter of 5 mm can theoretically exceed 150lm/W, and the life can be more than 100000 hours. Some people also predict that the upper limit of LED life will be infinite in the future. With the improvement of LED heat dissipation technology, high-power LED lighting fixtures such as outdoor lighting LED lamps and floodlights have been industrialized and have begun to be widely used. Stage lights and studio lights for indoor lighting that have high requirements for color temperature and color rendering have also been mass-produced and put into use. T8, T5, and T4 tubes for general lighting with the largest application range and the largest usage, and screw-type bulbs that replace incandescent lamps and energy-saving lamps have formed a series, with a service life of up to 50,000 hours. LED lighting has entered a period of rapid development.

LED packaging

According to different application occasions, different appearance dimensions, heat dissipation plans and luminous effects, there are various LED packaging methods. At present, LEDs are mainly classified by packaging methods into Lamp-LED, TOP-LED, Side-LED, SMD-LED, High-Power-LED, Flip Chip-LED, etc.

Lamp-led packaging process flow chart

Lamp-LED (vertical LED)

In the early days of Lamp-LED, direct-insert LEDs were packaged using potting. The potting process involves first injecting liquid epoxy resin into the LED molding cavity, then inserting the welded LED bracket, and then putting it into an oven to allow the epoxy resin to solidify. Then, the LED is removed from the mold cavity to form the LED. Due to its relatively simple manufacturing process and low cost, it has a high market share.

SMD-LED (SMD LED)

SMD LEDs are attached to the surface of the circuit board, suitable for SMT processing and reflow soldering. They have well solved the problems of brightness, viewing angle, flatness, reliability, consistency, etc., and have selected lighter PCB boards and reflective layer materials. After improvement, the heavier carbon steel pins of the direct-inserted LEDs have been removed, so that less epoxy resin needs to be filled in the reflective layer, with the intention of reducing the size and weight. In this way, SMD LEDs can easily reduce the weight of the product by half, and ultimately make the application more perfect.

Side-LED

At present, another type of LED package is the side-emitting package. If you want to use LED as the backlight source of LCD (liquid crystal display), the side emission of LED must be the same as the surface emission, so that the LCD backlight can emit light evenly. Although the design of the lead frame can also achieve the purpose of side emission, the heat dissipation effect is not good. However, Lumileds invented the design of the reflector, which uses the reflector principle to emit side light from the surface-emitting LED, and successfully applied high-power LED to large-scale LCD backlight modules.

TOP-LED (Top-emitting LED)

Top-emitting LEDs are relatively common SMD light-emitting diodes, and are primarily used as backlights and status indicators in multi-functional ultra-thin mobile phones and PDAs.

High-Power-LED

In order to obtain high-power and high-brightness LED light sources, manufacturers are developing in the direction of high power in terms of LED chip and package design. At present, LED packages that can withstand several W of power have appeared. For example, the package layout of the Norlux series of high-power LEDs is a multi-chip combination with a hexagonal aluminum plate as the base (making it non-conductive). The base diameter is 31.75mm, and the light-emitting area is located in the center. The diameter is about (0.375×25.4)mm, which can accommodate 40 LED tube cores. The aluminum plate also serves as a heat sink. This package uses a conventional tube core high-density combination package, with high luminous power, low thermal resistance, and high light output power under high current. It is also a promising LED solid light source.

It can be seen that the thermal characteristics of power LEDs directly affect the operating temperature, luminous power, luminous wavelength, service life, etc. of the LEDs. Therefore, the packaging description and manufacturing skills of power LED chips are becoming more and more important.

Flip Chip-LED

The LED flip chip packaging layout is that a plurality of through-holes are made on the PCB. Each through-hole on one side of the substrate is provided with two different areas of conductive materials that are open to each other, and the conductive materials are laid flat on the surface of the substrate. A plurality of unpackaged LED chips are placed at each through-hole on the side with the conductive materials. The positive and negative contacts of a single LED chip are connected to the conductive materials on the surface of the substrate using solder balls, and a transparent material sealant is applied to the surface of the plurality of LED chips facing the through-holes. The sealant is in the shape of a hemisphere and is located at each through-hole. It belongs to the flip chip layout light emitting diode.

LED Lighting Principle

LED can directly convert electrical energy into light energy. The heart of LED is a semiconductor chip with one end attached to it.

The LED lamp is on a bracket, which is the negative pole, and the other end is connected to the positive pole of the power supply. The whole chip is encapsulated by epoxy resin. The semiconductor chip consists of two parts, one is a P-type semiconductor, in which holes dominate, and the other end is an N-type semiconductor, which is mainly electrons. But when these two semiconductors are connected, a "PN junction" is formed between them. When the current acts on the chip through the wire, the electrons will be pushed to the P area, where the electrons and holes recombine, and then they will emit energy in the form of photons. This is the principle of LED luminescence. The wavelength of light determines the color of the light, which is determined by the material that forms the PN junction.

SMD LED

LED Classification

1. According to the color of the light emitting tube

It can be divided into red, orange, green (further 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 scattering agent or not, and whether it is colored or colorless, the above-mentioned light emitting diodes of various colors can also be divided into four types: colored transparent, colorless transparent, colored scattering and colorless scattering. Scattering type light emitting diodes are suitable for use as indicator lights.

2. Classification by the characteristics of the light-emitting surface of the light-emitting tube

There are round lamps, square lamps, rectangles, surface-emitting tubes, lateral tubes, and micro tubes for surface mounting. Round lamps are classified by diameter into φ2mm, φ4.4mm, φ5mm, φ8mm, φ10mm, and φ20mm. In foreign countries, φ3mm LEDs are usually recorded as T-1; φ5mm as T-1(3/4); and φ4.4mm as T-1(1/4).

The angular distribution of circular luminous intensity can be estimated from the size of the half-value angle.

There are three categories based on the luminous intensity angular distribution diagram:

(1) High directivity. It is usually a pointed epoxy package or a package with a metal reflective cavity without adding a scattering agent. The half-value angle is 5° to 20° or less, with high directivity. It can be used as a local lighting source, or used in conjunction with a light detector to form an automatic detection system.

(2) Standard type. Usually used as an indicator light, with a half-value angle of 20° to 45°.

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

3. According to the structure of light-emitting diodes

There are structures such as full epoxy encapsulation, metal base epoxy encapsulation, ceramic base epoxy encapsulation and glass encapsulation.

4. According to luminous intensity and working current

There are normal brightness LEDs (luminous intensity 10mcd); those with luminous intensity between 10 and 100mcd are called high brightness LEDs. The working current of general LEDs is between tens of mA and several tens of mA, while the working current of low current LEDs is below 2mA (the brightness is the same as that of ordinary light-emitting tubes).

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

Related knowledge

Ceramic LED Ceiling Light

The brightness of LED is inevitably related to the light-emitting angle of LED. The smaller the angle of LED, the higher its brightness. The production process is different and the service life is slightly different. The luminous intensity of white light of 5MM LED at 180 degrees is only a few hundred MCD, and the light intensity at 15 degrees is more than 10,000 or 20,000 MCD. When using high-power LED outdoors, the brightness is even higher. The single power is 1W, 3W, 5W, and some are combined into a high-power LED with multiple high-power LEDs, and the power can reach hundreds of W.

Color temperature has nothing to do with brightness, but brightness has something to do with lumen value

Let’s look at a few related concepts:

(1) Luminous flux (lm)

Since the human eye has different sensitivities to electromagnetic waves of different wavelengths, we cannot directly use the radiation power or radiation flux of the light source to measure the light energy. We must use the unit based on the human eye's perception of light - luminous flux. Luminous flux is represented by the symbol Φ and the unit is lumen (lm).

(2) Luminous intensity (cd)

Luminous flux is the total light energy emitted by a light source to the surrounding space. The distribution of luminous flux in space is different for different light sources. The unit of luminous intensity is candela, symbolized as cd, which represents the luminous flux emitted by a light source within a certain unit steradian solid angle (the angle formed by the surface of the object to the point light source). 1 cd = 1 lm/1 sr (sr: steradian unit of solid angle).

ceramics

(3) Brightness (cd/m2)

Brightness is the intensity of light emitted by an object seen by the eye from a certain direction. The unit is candela/square meter [cd/m2], symbolized by L, indicating the luminous flux per unit solid angle per unit area of ​​a luminous body in a specific direction, which is equal to the luminous intensity of 1 candela emitted on a surface of 1 square meter.

(4) Color Temperature

When the color of light emitted by a light source is the same as the color radiated by a black body at a certain temperature, the temperature of the black body is called the color temperature of the light source, expressed in absolute temperature K (Kelvin, Kelvin = Celsius + 273.15).

Color rendering property

In principle, artificial light should be the same as natural light so that the human eye can correctly distinguish the color of things. Of course, this depends on the location and purpose of the lighting.

The degree to which a light source renders the color of an object is called color rendering. It is usually called the "color rendering index" (Ra). Color rendering refers to the relationship between the true color of an object (its own color) and the color displayed under a certain standard light source. The determination of the Ra value is to compare the 8 test colors defined in the DIN6169 standard under the standard light source and the tested light source. The smaller the color difference, the better the color rendering of the color of the tested light source.

A light source with an Ra value of 100 means that the color of objects displayed under its light is the same as under a standard light source.