1. PCB surface treatment problem
Although the LED display can realize its value under working conditions, from a professional point of view, we still pay attention to the effect of the black screen. The black screen in this article refers to the visual effect of the LED display in the inoperative state.
In the design of the array driver board for high-density products, we encountered two problems that affected the black screen effect. First, the color difference of the PCB surface. In the industry, the color difference of the mass-produced PCB has not been reflected yet. Most of them are observed by the human eye, but the consistency of the PCB surface is better, and it is difficult to be consistent with the mask. Beautiful. If no mask is added, secondary processing is required on the PCB. However, this secondary machining accuracy control is very demanding and requires a special process design. The second is the problem of scan line segmentation. The dividing line refers to the gap between the two sets of LED connecting lines of an I/O port of the driving chip in the design scanning circuit. This gap is like dividing the printed board into two parts, so it is called dividing. Line (Figure 1).
The generation of the dividing line is an inevitable result of the design of the scanning circuit, which has always existed and is only highlighted by the high density. There are three ways to remove this problem. One is to add a mask, the other is to use a higher-level light-emitting tube to shield, and the third is to add a false-line design. The fake line means that there is no design in the PCB layout. The line defined by the signal.
In the split line area, draw a data line with no signal to avoid a fault. The left picture of Fig. 1 is the design without the false line. The figure has the obvious dividing line. The right picture of Fig. 1 is the design of adding the false line. There is no dividing line.
In the face of these two major problems, the best solution at present is still to install a mask. Just the strength of the mask and the difficulty of processing will be difficult to design.
2. PCB size accuracy problem
Most high-density displays have problems with bright and dark lines when lit. The appearance of bright and dark lines is caused by the unevenness of the entire line or the entire column of pixels. An important reason is the assembly flaw caused by the inaccurate PCB size. According to experience, when the pitch error between pixel edges of the module exceeds 10% of the pixel pitch, the problem of bright and dark lines will appear. The precision of the high-precision PCB cutting machine on the market is ±0.1mm, and the precision of the laser cutting machine is ≤±0.01mm. From the data point of view, the use of high-precision PCB cutting machine does not lead to the appearance of bright and dark lines, but this is not the case. The edge of the PCB that we cut with a high-precision panel is compared to the edge of the PCB cut with a laser cutter for several times magnification. It can be found that the high-precision PCB cutting machine can achieve an accuracy of ±0.1mm, but the trimming is not smooth, and it is easy to cause burrs, and the laser cutting can not only ensure the cutting precision, but also ensure that the edge of the PCB is relatively flat. Therefore, in order to avoid the assembly problem caused by the inaccurate PCB edge size, high-density PCB cutting should use laser cutting technology.
3. Mounting problems
The placement of high-density LED displays is one of the difficulties. The design of the pad, the design of the stencil, and the placement process are all control points.
The pad size of the 1010SMD LED tube is only 0.3mm. If the hole is smaller than the pad size according to the conventional thickness of the stencil, then when the solder paste itself has poor fluidity, it is easy to block the stencil hole. Affects production efficiency. If the solder paste does not land on the pad, the solder joint may cause poor soldering. At present, the method adopted is to reduce the thickness of the stencil and increase the size of the stencil opening.
In Fig. 2, h represents the thickness of the stencil, and a represents the opening size of the stencil. In the case of ensuring that the total amount of solder paste is constant, a2>a1, h1>h2, the contact surface of the solder paste and the pad in the right figure Large, because the adhesion of the solder paste to the pad is proportional to the size of the contact surface, the opening of the mesh plate in the right figure facilitates the adhesion of the solder paste to the pad, thereby avoiding the blockage of the mesh plate hole. However, it should be noted that the stencil opening cannot be larger than the pad size, otherwise the solder paste is scattered during the melting process. After the reflow, the excess tin climbs up the LED pin, causing the display pin to glisten and black. The effect is not good.
In the past, when the SMD light-emitting diode was mounted, the naked eye was more difficult to shift, and unless the mask was used as a reference, the light-emitting tube with a relatively large offset would appear in the original shape. Even if the arc tube is offset, it does not affect the soldering reliability. The high density is different. The deviation of the LED tube not only affects the viewing effect, but also causes a false soldering. If there is a slight external force, the LED will fall off. Therefore, it is necessary to set the center of the arc tube to be aligned with the center of the pad before the patch, and pay attention to the control of the wind speed in the reflow oven.
When debugging a high-density module, if there is a blind spot, the first thing to do is to observe under the magnifying glass, because the blind spot is not necessarily the LED dead point or the virtual soldering, and sometimes the LED is turned over, which is not high. Through research and analysis, the main reasons are as follows:
1) When the nozzle of the placement machine sucks the light-emitting tube, the control precision is not good or the nozzle is not perpendicular to the PCB. In addition, when the light-emitting tube is fed, there is jitter on the conveyor belt, and the light-emitting tube is in the carrier tape, some of which are to the left and some of which are to the right, and the nozzles are not necessarily able to be sucked in the center position of the light-emitting tube.
2) During the process of throwing and repairing, the artificial touch causes the arc tube to shift or flip.
Therefore, in the production process of high-density products, we place special emphasis on human influence. At the same time, the addition of automatic optical inspection (AOI) before reflow can avoid similar problems.
4. Picture quality problem
In the past two years, high-density LED displays have become the main competitors of display products such as DLP, PDP, LCD, etc. The biggest advantages are seamless, high-brightness, high refresh and rich colors. However, it should be emphasized here that the highlighting advantage of the LED display is only visible when the ambient brightness is relatively high. The main applications of DLP, PDP, LCD and other display products are indoor environments, and most of the ambient illumination will not exceed 300lx. For this reason, we have obtained the relationship between ambient illumination and display brightness through experiments and subjective evaluations ( See Table 1), we believe that when the ambient illumination is less than 300lx, the brightness of the high-density LED display only needs 120-600nit to meet the requirements. However, some customers do not accept low brightness, which is related to the early display promotion. Therefore, we need to actively guide the customer. At the same time, pay attention to the configuration of the switching power supply under high brightness and low brightness, and use the brightness adaptive function.
At present, in the industry, the driver chip for solving the high-density display with low brightness, high gray level and high refresh rate is relatively mature, and there are typically MBI515* and SUM203*. As an LED display application manufacturer, we focus on the following parameters in the design of high-density products:
1) Minimum output current
At present, the minimum output current of the constant current driving chip in the industry is as low as 0.5 mA, which can meet the low brightness requirement of high density LED display, but the stability and reliability under small current is the focus of our attention.
2) Current output accuracy
The accuracy of the current output greatly affects the uniformity of the LED display. The current output accuracy of the driver chip is mainly divided into two types, one is the current error between the channels of the same chip, and the other is the current between the chip and the chip. error. The color error caused by the current error between the channel and the channel is not obvious to the naked eye. The current error between the chips is ±3%-±6%. The error between the chip and the chip can clearly show the excellent block. This is the current stage. The focus of attention.
3) Data shift clock
This data determines the transmission rate of the display data, which is a key indicator that affects the display refresh rate. At present, the clock frequency of some manufacturers has achieved 30MHz, and the display refresh rate can reach 4000-16000Hz.
4) Grayscale
The larger the grayscale value, the more the level of the picture, the finer the picture quality. In the design of high-density LED display screens, we pay more attention to the picture layer effect below 100 levels. Of course, this is the result of the joint adjustment between the driver chip and the control system.
5. Blanking problem
The number of scans of high-density displays is getting higher and higher, and the problem of ghosting becomes more and more serious. When the system is driving the next line, the upper line appears dark. We call it "upper light". The reason is that when the previous line is turned off, the residual power in the circuit is nowhere to be released, and is only released by the LED illumination. At this stage, the brightening is mainly solved by adding a blanking circuit (for example, using the CD4051 and 595 chips), and the parasitic charge on the line is discharged by staggering or serial discharge, but at the same time, the breakdown/short circuit/open circuit is avoided. There are problems, and this will be more difficult. In particular, the reverse voltage causes a reverse breakdown of the LED, and in terms of circuit design, the possible reverse voltage is reduced.
When the system drives the previous line, the next line appears dark. We call it "lower light". The LED display panel has LED lamp beads and circuit board parasitic capacitance, which forms a downward ghost. The lower band is mainly solved by a driver chip with a blanking function.
High-density LED display products are in a period of rapid maturity, with high prices and high profits, and soaring demand. It is a blue ocean in the current LED display market. However, it is not easy to do well. A slight inadvertent design and production process will affect the overall situation. Therefore, manufacturers should develop their respective advantages in the early stage, and use patent protection methods appropriately, so that in the near future, high-density products will become another victim of price wars like other LED display conventional products.
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