Flexible printing electronic inkjet technology

Printed electronic inkjet technology is a new technology that has emerged in recent years. In recent years, the author has done a lot of meaningful research and exploration on flexible printed electronic inkjet technology, and elaborates on this technology here.

Approaching flexible printed electronic inkjet technology

Flexible printed electronic inkjet technology is a technology for printing conductive lines on substrates such as phenolic resin and epoxy resin, and is also called full-printing electronic technology. This technology is suitable for making flexible shaped circuit boards, ordinary circuit boards, photovoltaic glass, PCB, LCD, etc.
"Flexible" has a double solution here, one refers to flexible media; the second refers to the need for mass production compared to conventional PCBs, and the use of inkjet technology features for small batch production on demand. Compared with the conventional process of printing conductive lines, the process for preparing conductive lines by using flexible printed electronic inkjet technology has the advantages of high manufacturing speed, environmental friendliness, simple process, low cost, and diversified functions. Flexible printed electronic inkjet technology can not only use conductive ink (such as nano silver) to prepare circuit boards, but also use special semiconductor, carbon, silicon and other media to prepare resistors, capacitors, inductors, batteries, transistors and even integrated circuits. All of this can be done by printing on a certain medium, which greatly improves production efficiency and reduces production costs.

The structure and working principle of flexible printed electronic inkjet equipment

The flexible printed electronic inkjet device is composed of a mechanical system, a drive system, a motion control system, and a visual inspection system (optional), as shown in FIG. Specifically, the flexible printed electronic inkjet system includes one or a group of nozzles, a nozzle control board, a data processing motion control card, a PC, a media placement platform, an ink path system, a nozzle holder, and a nozzle frame motion system. as shown in picture 2. The specific implementation manner is as follows: according to the circuit diagram that needs to be printed drawn in the PC, the nozzle control board controls one or one group of nozzles to eject a special ink, and prints a precision circuit on the medium through the movement of the nozzle holder.

Figure 1 Composition of flexible printed electronic inkjet equipment

Figure 2 Flexible printed electronic inkjet system

Master control system
For flexible printed electronic inkjet devices to achieve high-speed, high-resolution performance indicators, the main control system is the key. Since the inkjet device has many nozzles, the data transmission rate is fast, and the transmission amount is large. Therefore, the data transmission and data processing design of the main control system is the main point of the design of the main control system. The dashed box in Figure 3 is the main control system. The main control system is connected to the peripheral control module through the IIC bus. It is driven by the embedded processing system (MPC8548E), USB interface circuit, data buffer, CPLD nozzle and peripheral control circuit, and sprinkler data bus. composition.
   When the computer starts the printing command through the RIP program, the command is first transmitted to the controller MPC8548E through the command channel of the USB interface circuit, and the main control program of the MPC8548E analyzes the command, and sends the command to each control through the IIC bus. The module completes the monitoring of the thickness of the medium, the adjustment of the upper and lower positions of the nozzle, and the control of the X and Y direction movement of the nozzle car. When the inkjet device completes the initial setup, RIP transmits the data to the MPC8548E. The master program of the MPC8548E sets the travel distance of the nozzle car motor and its printable area according to the length of the required print. The USB interface circuit initiates data transmission and receives RIP. The image dot matrix data fills the data buffer. When the data buffer is filled, the USB interface circuit sends an instruction to the MPC8548E to start the nozzle control state system and the nozzle car motor in the CPLD nozzle and the peripheral control circuit. The inkjet pulse and the enable signal are emitted, and the head control state system reads data from the data buffer, writes it into each of the nozzles, and controls it to perform the inkjet operation.

Figure 3 main control system block diagram
   2. Ink supply system
(1) Ink supply system analysis Ink supply system is an indispensable part of inkjet equipment. Good ink supply system is an important guarantee for high quality and high reliability of inkjet equipment. The properties of the nozzles vary, requiring the use of specific properties of the ink and setting a reasonable ink flow negative pressure. At the same time, ink as a chemical substance, its characteristics will change with the change of the environment and working state of the inkjet device. This requires the design and development of an independent working mechatronics ink supply system, through the control of ink characteristics and ink flow, with the nozzle to complete the printing task with high quality and efficiency. The viscosity and surface tension of the ink have a great influence on the formation of the ink droplets. Among them, the surface tension of the ink is not affected by the change of the ambient temperature, but the viscosity of the ink has a large fluctuation with the temperature change, which has a great influence on the printing work. Therefore, controlling the ink viscosity is one of the main tasks for the ink supply system. The ink viscosity has a negative relationship with the temperature, and the specific temperature range determines the viscosity range of the ink. For print jobs, different nozzles correspond to different optimal ink viscosities, which in turn are temperature dependent. Therefore, by controlling the ink temperature, the ink viscosity can be effectively controlled. The ink temperature is mainly affected by the temperature difference between the seasons and the morning and evening. In particular, the high viscosity of the low temperature ink has a great influence on the nozzle, which is easy to cause the plug failure, which not only causes the printing to be interrupted, but also causes a lot of time and ink loss.
In addition, insufficient ink supply negative pressure will cause ink leakage, white line and other phenomena, and too large will reduce the ejection speed or even the ejection is not smooth, resulting in the ink dot is not full, the printed image edge is incomplete, the picture surface is broken. Therefore, controlling the negative pressure of the ink supply is also one of the main tasks of the ink supply system. The negative pressure of the ink supply generally adopts two methods of negative pressure pump supply and high differential vacuum supply. Among them, the former can provide stable and accurate negative pressure through the control of the negative pressure pump, but requires additional components such as a negative pressure sensor and a negative pressure pump, which will increase the equipment cost and structural complexity.
(2) Ink supply system structure design The ink supply system consists of main ink cartridge, secondary ink cartridge, ink pump, filter and detection feedback control system, as shown in Figure 4. The small-capacity secondary cartridge stores a small amount of filtered ink and provides ink flow to the nozzle at a specified pressure. As the ink in the secondary ink cartridge is consumed, the ink pump draws ink from the primary ink cartridge and replenishes the ink in the secondary ink cartridge through the filter. The detection feedback control system includes a hydraulic sensor and a temperature sensor in the secondary ink cartridge, a liquid level switch in the main ink cartridge, and a system control panel, etc., which detect information in the feedback control system, feedback ink temperature and liquid level in the ink cartridge. Give the system control panel. The system control panel analyzes and processes the collected data according to the set program, and generates a switch control signal for the heating chip and the ink pump, thereby controlling the ink temperature in the ink supply system, adjusting the negative pressure of the nozzle, and ensuring the ink capacity in the ink cartridge. Suitable for current print job requirements.

Figure 4 Ink system

3. Nozzle control and auxiliary circuit
The CPLD head and peripheral control circuit includes the following modules: a register set for storing commands sent by the controller and external I/O data and status; a decoder for completing address encoding and chip selection of the register; and a packet counter to complete Packet count and point count, when the data of each point of the nozzle is transmitted to the nozzle, the packet count is reset to zero, and the request is interrupted at the same time; the nozzle control state system completes the transmission of the nozzle data, controls the nozzle inkjet; I/O port, completes Some relay control, status detection, etc.
The chamber of each nozzle of the nozzle is wrapped by mutually independent piezoelectric ceramics, and the trigger pulse voltage is excited by the graphic data to control whether the nozzle is ink-jetted, and the amount of ink ejection from the nozzle is controlled by a pulse voltage applied to the piezoelectric ceramic. On the inkjet printing control software interface, you can select the image to be printed through the “picture path”, “sprinker/nozzle selection” to select the nozzle to be opened, and “waveform setting” to set the print waveform of the nozzle. The computer sends the selected data to the CPLD nozzle and the peripheral control circuit through the network port communication, and then is converted into a level signal and sent to the pulse power source. When the printing is needed, the pulse waveform is triggered, and the nozzle realizes the inkjet; When the pulse waveform is not triggered, the nozzle does not eject ink.

4. Media drying technology
As a media placement and transport platform, the media placement platform should be suitable for both flat and flexible media. In order to fully dry the ink and ensure its effective conductivity and other key indicators, the technique and method of heating the substrate at the bottom of the medium are usually adopted, and the heating temperature is controlled at 100 to 160 °C. In order to ensure constant heating of the substrate, a temperature control system should be used to ensure proper temperature.

Ink used in flexible printed electronic inkjet technology

The flexible printed electronic inkjet technology uses a special ink having conductive properties, which is usually composed of conductive silver or the like. For inkjet systems, when the nozzle is selected, the ink is adapted to the nozzle and typically requires consideration of parameters such as conductivity, viscosity, maximum particle size, tension, stability, and pH. Table 1 shows the different ink characteristics corresponding to the nozzles. It can be seen that the ink index must be adapted to the performance of the nozzle to make the entire system work well.

Table 1 Ink characteristics corresponding to different nozzles

For flexible printed electronic inkjet technology, nano-conductive silver ink is the most basic ink. If you can develop suitable inks made of semiconductors such as silicon, germanium, selenium, gallium arsenide, etc., you can print resistors, capacitors, inductors, semiconductor components, and integrated circuit components on any medium. I believe that with the development of materials science, this day will be getting closer and closer to us.

Print precision control

The application objects of flexible printing electronic inkjet technology are often some special-shaped flexible small circuits, so the printing precision is high, and the general inkjet printer cannot guarantee the printing precision. To achieve this, you can usually take the following approach.

1. Select high precision nozzle
When selecting the nozzle, first consider the width of the ink path to suit the conductive silver glue ink, semiconductor ink, etc. Secondly, according to the requirements of the printing medium and precision, select the appropriate nozzle to ensure the proper size of the ink droplet.

2. Choose the right fine ink
The ink is composed of some particles and liquid. The diameter and uniformity of the particles are very important. The basic production process is pulverization, grinding, stirring and the like. The development of ink and low-cost industrialization have always been an unavoidable problem.

3. Guarantee the motion accuracy in the X direction
The use of metal gratings and linear motors instead of conventional plastic scales and linear guides will greatly emphasize the motion accuracy in the X direction, increasing it from 50 to 150 μm to 1 to 5 μm.

4. Guarantee the accuracy of the mechanical platform
For higher precision requirements, a marble platform with a small deformation coefficient is required to increase the accuracy by 5 to 10 times.

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