Surface Mount and In-line LED Outdoor Full Color Display Performance Analysis

- Mar 22, 2018 -

In the past two years, full-color surface mount (SMD) outdoor LED displays have been widely used, and the market scale has rapidly increased. However, according to the author's understanding of SMD outdoor display products, there are still many such products in practical applications. problem. Here we discuss the performance of SMD outdoor display and in-line outdoor display from four aspects, in order to provide a reference for the design makers and users.

1 Unit module internal basic structure analysis

We analyze the internal structure of common outdoor LED display unit modules from four aspects.

1.1 Single light

Figure 1 shows the outline of two typical full-color SMD LED lamps and in-line LED lamps.

Fig. 1 (a) is SMD LED lamp, the external dimension is 5.0mm × 5.0mm × 2.5mm, it is built-in red, green, blue three chips, namely three-in-one, can use as a picture element of the display; Fig. 1 b) It is a line-in LED lamp with an external dimension of 3.0mm x 4.1mm x 6.3mm. The built-in single-color chip requires at least three different colors of red, green and blue lamps to be assembled together to form a complete One pixel of the color display. So from the pixel point of view, the SMD lamp is much smaller than a plug-in lamp. However, because the plug-in lamp is a single lamp package, the volume is large, the forward height is high, and the length of the lamp leg is long, so its sealing and solidity degree and heat dissipation degree are higher than that of the SMD lamp.

1.2 Welding method

Figure 2 shows the welding of the two lamps.

As can be seen from the figure, the two lamps have different welding methods. The pads of the SMD lamp are soldered to the surface of the PCB board, and the pins of the in-line lamp are soldered after passing through the pad via holes on the PCB. The soldering firmness of the latter is better than former. In fact, SMD lamps are prone to false soldering, or bad contact occurs during transportation and use, which causes more trouble for use and maintenance. This is especially important when using outdoor environments where the environment is harsh. Therefore, ensuring welding quality is one of the key factors to ensure the quality of SMD outdoor displays.

1.3 Sealing method

Figure 3 shows the sealing methods within the two lamp panel unit modules.

       It can be seen from the figure that the sealing methods of the two types of lamp plates are the same, and most of the sealed materials currently use silicone. However, we can see from the figure that the thickness of the silica gel of the two types of lamp plates is quite different. The thickness of Figure 3(a) is about 1~1.5mm, and the thickness of Figure 3(b) is about 3.5~4.5mm. So, The latter is better protected against lamp and PCB board sealing. Of course, the specific advantages and disadvantages are also related to the glue filling process. It can be seen that SMD LED display screens can be selected for display screens with a high density requirement and a good environment. For the low density requirements, the use of harsh environment display, the author believes that choose to insert LED display better.

 2 Unit Module Production Process Analysis

 From the production process point of view, the SMD display unit module and the in-line display unit module are mainly different in the two points of welding and gluing, and there are no major differences.

 2.1 Welding

 The welding process of the SMD display unit module mainly includes glue application (conductive adhesive), patch, and welding. These processes are suitable for large-scale mechanized production. The position of the post-welding lamp is very regular.

The welding process of the plug-in unit module mainly includes inserting a lamp, welding, and cutting a leg. Although dedicated automation machinery is available, its production efficiency is low. In addition, because the lamp leg is high, it is easy to bend in the production process, thereby changing the direction of the light, affecting the quality of the subsequent processes and the display screen.

2.2 Filling glue

SMD display unit module gluing process requirements should be more stringent, because the glue layer is thinner, so the level of operation and the amount of glue should be strictly controlled, each module should be strictly consistent.

The filling process of the DIP unit module is required to be slightly lower, because the glue layer is thicker, so the degree of placement of the module and the amount of the glue when operating are relatively loose. However, the lights should be collated before filling, ensuring that the axis of the lamp is aligned with the normal direction of the PCB.

 In any case, the flatness and consistency of the plastic surface should be kept high, otherwise it will affect the quality of the display screen.

3. Reliability Analysis

For reliability, there are three main differences between the two.

3.1 Sealing issues

As mentioned earlier, the sealing performance of SMD lamps and modules is slightly inferior to that of in-line lamps and modules, and therefore they are less reliable in terms of waterproof, moisture, UV, and other harsh environmental factors.

3.2 Heat Dissipation

The full-color SMD lamp is a three-in-one package of red, green and blue, that is, the three wicks are packaged in a narrow space, resulting in heat dissipation. For line-in LED lamps, only one wick is enclosed in each lamp, and the lamp legs are long, so heat dissipation is not a problem.

In addition, due to the low brightness of the SMD lamp, the drive current is generally larger than that of the in-line lamp in order to meet the requirements of outdoor ultra-high brightness, which further aggravates the heat dissipation problem.

3.3 Failure problem

Due to packaging, heat dissipation, and environmental issues, SMD lamps have higher failure rates than in-line lamps. At present, the failure rate of full-color outdoor display SMD lamps within 3,000 hours should be less than 100,000.

4 Optical Performance Analysis

 For optical performance, we mainly discuss three aspects: brightness, viewing angle, and consistency.

 4.1 Brightness

At present, the brightness of the SMD outdoor display of the same specification is lower than the brightness of the display of the plug-in lamp. In other words, to achieve the brightness of the in-line lamp display, the quality of the SMD lamp should be better and the driving current may be greater. Taking a display with a dot pitch of 16 mm (P16) as an example, at present, the brightness of an SMD display screen can reach 5,000 cd/m2 or more, and the brightness of a full-screen display can reach 7,000 cd/m2 or more. Not difficult.

 4.2 Perspective

Fig. 4 and Fig. 5 are the horizontal relative light intensity and viewing angle diagrams of two typical SMD lamps and in-line lamps, respectively, which is the distribution of light energy according to the angle.

 Comparing Fig. 4 and Fig. 5, it can be seen that the horizontal viewing angle of the SMD lamp is obviously better than that of the in-line lamp (similar to the vertical viewing angle). Of course, the different types of lamps have different figures, but the viewing angle of the SMD lamp is better than Inline lighting is an indisputable fact. In general, the viewing angle of the SMD display can reach about 110 degrees, while the viewing angle of the inline lamp display can reach about 90 degrees.

 4.3 Light distribution and color mixing

 The full-color SMD lamp is a three-in-one structure of red, green and blue. These three chips are placed together in the same bowl. The spacing between the chips is small and uniform, so the light distribution and color mixing are very good.

 Directly inserting red, green, and blue three elliptical lights to compose one pixel. It is very difficult to match their composite light from different directions, and due to the large spacing between wicks, color mixing is also problematic. Especially when watching at close quarters, the problem of poor color mixing is more prominent.

 In addition, the direction of light emitted by the SMD lamp on the display screen can maintain a good consistency. In-line light is difficult to maintain the same direction of the light in the production process, so the directionality of the emitted light is difficult to maintain the same. Therefore, the SMD display screen image is more delicate than the line-in lamp.

 4.4 Attenuation problems

 Due to the large driving current, small chip spacing, and difficult heat dissipation, the SMD lamp's brightness decay speed is faster than that of the line-in lamp, and the color consistency is more likely to be degraded.

The in-line lamp is individually packaged in each chip and has high brightness, small drive current, good heat dissipation, and relatively small attenuation.

Therefore, the attenuation characteristics of SMD lamps are worse than those of in-line lamps. To overcome these disadvantages, better chip and packaging materials are needed.

5 Conclusion

Through the above analysis and comparison, we believe that full-color displays made with SMD lamps have a large viewing angle, good light distribution and color mixing, delicate images, and a medium brightness level, and are suitable for large-scale automated production. However, the failure rate and attenuation speed are high and the adaptability to harsh environments is poor. It is suitable for displays with good environment, high density and low brightness requirements.

The full-color display made with a line-in lamp has good sealing performance, high brightness, and strong environmental adaptability and is suitable for long-distance viewing. However, the viewing angle is slightly smaller, the color uniformity is slightly inferior, and the production process is slightly complicated. The performance of all aspects of the full-color display screen made by the in-line lamps is not a problem for the general needs.

Xiao Bian has a saying: outdoor surface-mounted LED display is the future development trend

Because the surface brightness of the surface-mount LED is low, and the waterproof, moisture-proof, and ultraviolet-resistant function indexes do not meet the requirement of fighting the harsh outdoor environment, the outdoor full-color display has been the world of in-line LED devices. The full-color surface LED display can only be used in a large number of indoor. However, it is undeniable that this product has the features and advantages that can not be surpassed by the conventional LED screen in-line light in many aspects such as color reproduction, color consistency, uniformity, viewing angle, and overall picture quality.

With the continuous improvement and development of the upstream LED chip technology and LED packaging technology, the brightness and protection level of SMD have been able to meet the needs of outdoor applications, and the full-color surface-mount LED display will obtain faster application. The prospects for the application market of future outdoor surface-mount products will, I think, increase as people's visual needs increase, outdoor surface-mount LED displays will gradually become popular.