Research and Application of Aluminum Substrate PCB Light Bar in LCD TV

Author: Chao Yang

Source: "Science and Technology Communication" Issue 18, 2017

Abstract This article introduces the application of light bar using aluminum substrate PCB in LCD TV backlight module. The main precautions can be used as a reference for practical engineering applications. Keywords LCD TV; backlight module; aluminum substrate PCB; LED light bar

CLC number TN94 Document code A Article number 1674-6708 (2017) 195-0036-02

With the development of LED technology, the power of a single LED continues to increase, and its reliability is closely related to heat dissipation; in order to solve the heat dissipation problem of high-power LEDs, copper-clad aluminum substrate PCBs have been widely used in the field of LED light bars. Combined with the characteristics of the aluminum substrate PCB, the application of the aluminum substrate in the backlight strip is analyzed and summarized in the following aspects:

1) Analysis of copper-clad aluminum substrate characteristics and industry conditions.

2) Precautions for the application of aluminum substrate light strips.

3) Adjustment for the design scheme of the aluminum substrate light bar.

1 Copper-clad aluminum substrate characteristics and industry situation analysis 1.1 Basic product structure: conductive layer, insulating layer, metal layer Conductive layer: the main material is copper foil, the thickness of copper foil is 1 oz-6 oz and can be customized, and it is widely used in the light bar industry 1 oz;

Insulation layer: the main material is phenolic resin (glass fiber cloth), film, the thickness is generally 3mil ~ 6mil and can be customized; the light bar industry generally uses 4mil ~ 5mil products;

Metal layer: the main material is aluminum plate (1 series, 5 series, etc.), the thickness is 0.4mm ~ 2.0mm and the thickness can be customized;

1.2 Aluminum substrate PCB industry situation

Due to the good heat conduction and heat dissipation performance of the aluminum substrate, from 2009 to 2010, since the LED backlight started, the industry's interior type light bars have all used aluminum substrate light bars, and the use specifications include 0.5W/mk ~ 2W/mk thermal conductivity.

With the development of direct-lit backlights, the PCB market for LED light bars mainly includes 3 types of boards: FR4, CEM3, and aluminum substrates. The use of FR4/CEM3 boards in direct-lit models is gradually decreasing, and the proportion of aluminum substrates is gradually increasing. , By 2017, the aluminum substrate light bar has exceeded 90%.

The proportion of aluminum substrate usage is increasing year by year, the main reasons are as follows:

1) The power of a single LED continues to increase. When it reaches 1.5W to 2W, FR4/CEM3 cannot meet the heat dissipation performance requirements, and only the aluminum substrate can meet the heat dissipation requirements. At present, 2W-LEDs have become the mainstream of the backlight LED market, so the proportion of aluminum substrates has increased.

2) Rising commodity prices, especially copper prices have a greater impact on PCB prices. In early 2016, 1W/mk aluminum substrates were 30 yuan/m2 higher than FR4, but by Q4 2016, aluminum substrates were already cheaper than FR4 (FR4 is double-sided Copper clad, using a large amount of copper foil); more original use of FR4 is also switched to the state of aluminum substrate.

2 Precautions for the application of aluminum substrate light bar

The aluminum substrate PCB has good thermal conductivity and heat dissipation performance. In terms of insulation, the breakdown voltage of large plates can basically reach AC: 3kV, DC: 4kV, but the insulation performance is greatly attenuated after being processed into a finished PCB; at the same time, because the distance between the copper foil and the aluminum substrate is only 75um ~ 150um thick insulation thickness, its capacitive effect is also more obvious than FR4 sheet. The following is an analysis of the relevant precautions in the application from the aspects of insulation performance and distributed capacitance.

2.1 Poor insulation analysis The basic reason for poor insulation is insufficient spacing. Common PCB insulation failure points include: lead PAD flashover, edge line margin, discharge between lines, discharge at the tip of the line, and discharge between copper foil and aluminum base. For the possibility of poor insulation, the main measures at present are: 1) Enlarge each safety distance, which is 0.3mm ~ 0.5mm larger than that of the FR4 board; the distance between the line and the edge of the board is at least 1mm. 2) Increase the thickness of white oil. The thickness of FR-4 white oil is generally 10um ~ 15um. In order to improve the line spacing and board edge discharge, the thickness of aluminum substrate white oil is increased to 25um.

3) In the case of ensuring sufficient heat dissipation and current margin, minimize the copper foil area of the wiring to reduce the probability of poor insulation.

4) Strengthen the control of the production process of large plates, avoid the entry of impurities and ensure the uniformity of the thickness of the insulating layer; avoid the discharge of the insulating layer.

5) 100% inspection of the finished PCB; including open and short circuits between lines, copper foil and aluminum base insulation performance and other inspections.

6) The driving voltage value of the light bar should not be too high during the scheme design.

2.2 Analysis of distributed capacitance

Because there is an insulating layer between the copper foil and the aluminum substrate, a capacitor effect is formed; when the aluminum base of the aluminum substrate light bar is in contact with the backplane, the equivalent capacitance will be superimposed on the corresponding driving circuit, especially when the LED driver has a MOS modulation When the light circuit is used, the distributed capacitance will generate a corresponding pulse current. When the pulse current is large enough, it may cause the corresponding backlight drive protection and LED high current loss damage.

In order to reduce the impact of distributed capacitance on the circuit of the whole machine, the following points should be paid attention to in the design of aluminum substrate light strips: 1) Reduce the distributed capacitance of the light strip, select a suitable dielectric constant plate, and reduce the copper foil area.

2) The LED- in the light strip wiring should be as short as possible, and the LED+ can be long to reduce the influence of distributed capacitance on the drive.

3) When the driver meets the performance requirements, the non-dimming MOS solution is preferred.

4) If there is a dimming MOS solution, the light bar provides the equivalent capacitance of the light bar in the early stage of development, and the driver end carries out tolerance design to ensure that the driver end has enough design margin for the aluminum-based distributed capacitance to avoid pulse current Or the pulse current meets the specification.

3 Adjustment for the design scheme of the aluminum substrate light bar

3.1 Improve wiring design rules

1) The distance between the traces follows the following principles: the distance between the direct-type aluminum substrate traces and the edge of the board is greater than 1.5mm (more than 1mm in special cases).

2) LED- is required to be short, and the wiring width should be reduced outside the range of 10mm to 20mm from the +- terminal of the LED pin, so as to reduce the distributed capacitance and reduce the probability of bad insulation between copper foil and aluminum base.

3.2 Scheme Design Optimization

1) For the aluminum substrate light bar solution, the single-channel driving voltage should not be higher than 150V.

2) Evaluate the drive end of the power supply and seek new drive solutions, which can provide timely protection for poor insulation faults of aluminum substrates.

3) The heat dissipation design requires that the aluminum substrate light bar must also meet the temperature rise requirements when it does not touch the backplane (suspended state).

4) The insulating layer is thickened to 150um aluminum substrate.

4 Conclusion

This article introduces the basic principles and characteristics of aluminum substrates, and focuses on its application precautions and design suggestions in LCD TVs.

references

[1] Li Xiuzhen. Design of LED backlight module for ultra-thin LCD TV [J]. Modern Display, 2010, 21 (12): 40-42.

[2] Hao Yaru, Deng Zhaoqi, Deng Chunjian. Research on Acceleration Method of Memory Transfer Frequency in LED Display Control System [J]. Liquid Crystal and Display, 2016, 31 (5): 470-476.