LED modularization - a new trend in LED development

Lighting is a basic human need, and it also represents the level of economic activities. In recent years, due to factors such as global economic growth, continuous urbanization, and the expansion of non-urban power grids, the construction of general buildings and manufacturing plants has increased rapidly, which has led to a rapid increase in lighting demand and a continuous increase in energy consumption. As a new light source, LED is increasingly being integrated into people's lives, and its development is inseparable from people's demand for lighting. Compared with traditional light sources (such as fluorescent lamps and incandescent lamps), LEDs are 5-20 times more efficient than traditional light sources when they are close to the theoretical conversion efficiency. Even the current mass production light efficiency is between 2-15 times, and combined with its directional advantages, the gap will be even greater. Due to the change in the principle of light emission, its life span will also be much longer than that of traditional light sources. In addition, since LED also has a series of advantages such as no harm to health and the environment, it has been recognized as the most suitable light source for the next generation at this stage. However, LED lighting fixtures have not been widely promoted and applied so far, and cannot enter thousands of households. The main reason is that the price of LED lighting fixtures remains high. How to reduce the price of LED lighting fixtures? The market and experience tell us---LED modularization.

LED modular development model: (1) modularization of light source, (2) integration of device + PCB board, (3) integration of light source and power supply, (4) integration of light source, power supply, heat dissipation and optics, (5) standardization of connection port and base.

As an LED vertical integration enterprise, Zhenmingli Group has created the LMDM lighting integration service plan to provide customers with customized lighting solutions and assist customers in solving technical difficulties in four key aspects: light, heat, electricity and machinery at each product development stage.

This article mainly discusses the LED light source module, standard interface and heat dissipation design.

1. Light source module
This item includes two modes: COB and light source device + PCB. See the figure below.

1. Light source module
This item includes two modes: COB and light source device + PCB. See the figure below.

1. COB packaging
COB packaging, or chip on board, is to adhere the bare chip to the interconnect substrate with conductive or non-conductive glue, and then perform wire bonding to achieve its electrical connection (flip chip does not require wire bonding), that is, LED chip and substrate integration technology.
COB packaging can currently be divided into two categories: (1) low thermal resistance packaging process; (2) high reliability packaging process.
(1) Low thermal resistance packaging process
Low thermal resistance COB packaging is currently divided into aluminum substrate COB, copper substrate COB, and ceramic substrate COB.

Aluminum-based COB has a low cost of substrate, so the COB light source packaged has a very high cost-effectiveness. In addition, the highest luminous efficiency can reach 130LM/W. Based on the above advantages, it is widely used in LED bulbs, LED downlights and other lamps. However, due to the limitation of the thermal conductivity of the aluminum substrate (the thermal conductivity of the conventional substrate is currently 1-2W/mK), it is suitable for 5-10W COB light sources.

Copper-based COB, because the chip is directly fixed on copper, the thermal conductivity of copper is 380W/mK, the thermal conductivity effect is good, and 20-50W COB can be packaged. In addition, the luminous efficiency can reach 130LM/W. It is currently widely used in LED projection lamps, LED street lamps and other lamps, but in order to prevent local overheating, generally encapsulate 20-50W COB light sources.

Ceramic substrate COB, ceramic is currently recognized as the most suitable material for LED packaging substrates. It is widely used in high-end, high-reliability LED lamps with its excellent thermal conductivity, excellent insulation performance, and small thermal deformation. It can currently package 10-50W COB light sources, but due to its expensive substrate price, it is generally used in high-end LED lighting and lighting fields with high reliability requirements.

(2) High reliability packaging process
At present, the reliability requirements for LED lamps in some special occasions are particularly high, such as LED street lights, LED tunnel lights, LED explosion-proof lights, LED mining lamps, etc., and the reliability requirements for LED light sources are also very high. As

one of the top three factors affecting the reliability of LED packaging (heat effect, static electricity effect, and moisture effect), thermal effect is an important cause of LED attenuation. According to the Arrhenius model applicable to LED chips, for every 10°C increase in the junction temperature of the LED, the life of the LED itself will be reduced by half.

P=P0exp（-βt）
P0-initial light power, β-attenuation coefficient, t-LED aging time
β=β0Ifexp（-Ea/kTj）
β0-constant, k-Boltzmann constant, Tj-chip junction temperature, Ea-activation energy
Since the ceramic substrate has a high thermal conductivity and insulation performance, it can solve the thermal and static effects. In addition, the ceramic substrate and silica gel have good bonding properties, which can solve the moisture effect. In addition, the flip chip process is used to remove the gold wire to further greatly improve the reliability of the entire light source component. The use of COB packaging improves the cost performance of the light source component. Therefore, the use of ceramic substrate COB plus flip chip process can meet the high requirements of LED application fields.

2.
The light source module formed by light source device + PCB is aimed at promoting the standardization of light source modules and facilitating the manufacturing process of LED lamp manufacturers.

The above only explains 3014 and 5630. There are also imitation lumen, ceramic 3535, 3528 and other light source devices + PCB modes, which are used in LED spotlights, LED street lights, LED fluorescent tubes, LED bulbs, LED downlights, etc., which greatly facilitate LED lamp manufacturers.

From the above, it can be seen that the use of COB light sources has many advantages, such as low thermal resistance, high reliability, and high cost performance. It is the only way for LED lighting to be popularized. In addition, the LED device + PCB model greatly facilitates LED lamp manufacturers, simplifies the operation process, and further standardizes.

2. Standard interface
At present, LED lighting fixtures are booming, especially indoor LED lamps. New companies join and new lamps are born almost every day. Therefore, the LED indoor lamp market is diverse and each has its own way. This has brought great inconvenience to the standardized production and promotion of indoor LED lamps. For this reason, LED interface standardization is imperative. Here we mainly introduce the provision of standard interfaces for light source products.
The standard interfaces of light source components include: (1) COB standard interface; (2) light source device + PCB standard interface.
(1) COB standard interface
Here we take ceramic COB as an example. In the ceramic COB products currently in use, since the ceramic substrate has good thermal conductivity, it is more difficult to weld wires in the actual process; on the other hand, since ceramics are fragile, it is the first choice to develop a lamp holder for ceramic COB that combines compression and lead-out wires.

Lamp holder pictures

COB+lamp holder+radiator picture

By adding a lamp holder, it can fix the ceramic COB on the one hand and make electrical connections on the other.

(2) Light source device + PCB standard interface
Adding a connector to the PCB board saves manual soldering, shortens installation time, and makes maintenance easier.

3. Heat dissipation design
Currently, about 70%-80% of LED power is converted into heat. Therefore, heat dissipation design for LED applications, how to minimize the temperature of LED components, has always been a key project that the application side needs to consider.
For LEDs, choosing a suitable operating environment and quickly dissipating the heat generated after the LED is lit can maintain the original life and performance of the LED. The heat conduction pathways are mainly the following four:
(1) From the heat source (LED component) to the heat sink;
(2) Heat conduction from the heat sink;
(3) Heat dissipation into the air through convection;
(4) Heat removal through surface thermal radiation;

Path (1): The contact surface between the LED component and the heat sink is not perfectly flat and smooth, and cannot fit completely. If there is no complete contact, the high thermal resistance of the air in the gap will reduce the heat transfer effect. Therefore, in order to quickly conduct heat from the LED component, a key structure is whether the LED and the heat sink can be tightly combined. A simple and effective approach is to evenly apply an appropriate amount of thermal paste on the contact surface of the LED component and the heat sink to fill the gap between the contact surfaces, and use screws to strengthen the adhesion between the two to enhance the thermal conductivity.

Path (2): The heat sink itself will cause temperature differences at different locations over time. Its total heat conduction is (Q), which is the total heat that can be removed by the heat dissipation mechanism over a period of time. It is determined by
(1) the temperature difference between the heating element and the two ends of the heat sink (△T = Tj-Th);
(2) the thermal conductivity of the heat sink (K);
(3) the total heat transfer area (A);
(4) the straight-line distance of heat transfer (L);

The heat dissipation formula is:

When designing LED heat dissipation, Tj has a maximum limit. Improving the heat dissipation coefficient, increasing the heat dissipation area (number of fins), or shortening the straight-line distance of heat transfer can all increase the heat conduction per unit time of the heat dissipation mechanism. Among them, metal materials with high thermal conductivity and high cost performance are the primary choice.

Path (3): The heat dissipation mechanism includes convection and radiation. Regardless of convection or radiation, its effectiveness is proportional to the surface area of ​​the heat sink. The larger the total surface area of ​​the heat sink, the better the heat dissipation effect. The more heat sink fins there are, the larger the total surface area can be; but within a limited volume, designing too many fins will suppress the convection effect. There are many thermal designs that use external fans to force convection to achieve the effect of forced heat removal, but this design is subject to limitations such as noise and circuits, which will not be discussed here.

Path (4): Compared with an untreated heat sink, a layer of high emissivity material (emissivity equal to 1), such as ceramic or dark film, is covered on the surface of the heat sink to enhance the surface thermal radiation effect. Common surface anodizing or surface etching are methods of improving thermal radiation capabilities.

Key points of heat sink design:
(1) Tightness between LED components and heat sink and flatness of contact surface;
(2) Total surface area of ​​heat sink;
(3) Material selection of heat sink;
(4) Optimization of number of fins (gas flow design)

IV. Conclusion
LED packaging is a research topic involving multiple disciplines (such as optics, thermal, mechanical, electrical, mechanical, material, semiconductor, etc.). From a certain perspective, LED packaging is not only a manufacturing technology, but also a basic science. Good packaging requires the understanding and application of physical essences such as thermal, optics, material and process mechanics. LED packaging design should be carried out simultaneously with chip design, and it is necessary to uniformly consider the performance of light, heat, electricity, structure, etc., integrate many devices to form LED modules, and promote the better and faster development of LED lighting.