With the increasing speed of placement machines and the continuous development of placement materials, stencil printing has become the main station to shorten the production cycle under the condition of ensuring 6σ quality. For reflow soldering, the vitality of stencil printing is that it is also the most suitable method to pre-set solder paste on PCB. For stencil printing machine manufacturers, their equipment must be able to meet the requirements of users to continuously print new materials in order to solve related process problems. Variations in solder paste technology, such as VOC-free (VOC-free) and lead-free solders, present challenges in maintaining constant printing parameters. Whether it is a production engineer looking to optimize an existing system, or a manager looking to purchase new equipment, it is important to understand the current mechanical condition of the stencil printer.

 

Efficiency is the decisive factor

 

Which aspect is more important for producers? Is it speed or precision? Of course, the complexity of specific production, testing and repair (or replacement) costs must also be taken into account. But the real goal should be to achieve the ideal combination of output and quality; in other words, efficiency. In some cases, the intersection of yield and quality determines the efficiency of production. The higher the efficiency, the faster the return on investment. The main parameters on the quality side of the efficiency equation are precision, reliability, and production flexibility.

 

Yield is another important factor

 

Improving the speed of a stencil printer is a challenging subject. During the printing process, the printing stroke and the separation speed of the PCB and the stencil are constant, while the printing speed of the squeegee depends on the size of the opening on the stencil and the composition of the solder paste. Therefore, improving the operating speed of printing becomes a key factor. The actual printing cycle of an automatic printing machine is usually 15 seconds to 20 seconds, while it requires less than 10 seconds for a high-speed production line. Although printing manipulation is not a decisive factor, in many actual productions, its importance is strengthened with the increase of output. However, to improve the accuracy and soft potential, the processing time must be increased. The faster the printing speed, the more time is left for visual correction, precise PCB positioning and stencil wiping, etc. By increasing the speed of these corresponding processes, more time can be left for 2D and/or 3D inspection after PCB printing, and the more thorough the difference from "slow speed" printing machines. The more time spent on 2D and 3D inspection, the higher the statistical accuracy of process samples, resulting in lower defect rates and improved yields.

 

Usually, the first thing that comes to mind when increasing the production speed of an automatic printing machine is the movement of each axis. From the perspective of machine development, the simplest and most commonly used method is to increase the speed of each axis of motion. The accuracy and reliability of this method are sufficient if there are no requirements for the accuracy and reliability of the machine.

 

A smarter approach is to employ real performance motion control engineering. Perform analysis, algorithm drills and experiments on each node of the movement, and analyze the fatigue of each axis. The actual performance of each drive system on the printing press is calculated to meet the best match for acceleration performance. Correspondingly, the printing speed is also determined by calculation, so that the printing press has the best potential. This efficiency and safety design concept has been written into the motion control board of the machine in the form of a curve, so that every movement of the machine can be quickly followed. In this way, the printing cycle can be shortened by 20%. Mobile vision system, the vision system of the automatic stencil printing machine is used to align the PCB with the stencil, which can improve the speed both mechanically and electronically. For the latter, first of all, a dedicated video processor can be combined with the CPU of the printing press system for collaborative processing, so that the resulting image can be processed quickly. Also, processing can be done while the system is moving to the next fiducial, hence the term "moving" vision system.

 

First, to improve the speed at which the vision system moves between reference points, the same motion analysis and design methods as for other motion axes must be used. The vision system must be able to be supported by advanced algorithms in the motion control board. It should be noted that true continuous motion is not possible since the position of the reference point usually requires motion in two mutually perpendicular axes. However, high video speeds can be achieved through performance motion control engineering. Optimized acceleration and deceleration can significantly improve the speed of video. This reduces dwell times and image capture times for fiducials, 2D, 3D inspections, etc. If the device uses air bearings, it can also ensure smooth and precise movement.

 

The system should use direct drive to avoid the obvious movement gap that belt drive has, intelligent software, intelligent software can automatically measure the efficiency of the stencil printing machine. High-efficiency printing machines can control the function and process of the machine, so as to realize the rapid changeover between various processes. The latest software features motion-controlled simultaneity, enabling parallel jobs in the printing press to be carried out simultaneously. Advanced printing systems have at least four axes that can move simultaneously, so that multiple actions can be performed simultaneously.

 

Another latest technology is "adaptive compensation", which can be used to continuously analyze the functions of the printing press to improve production efficiency. For example, in the process of template and PCB calibration, the original position of the template can be automatically re-established by analyzing the size of the calibration amount, thereby shortening the calibration time.

 

Today's smart software also enables analysis beyond the stencil printer. The operating system can monitor the equipment at the next station, verify the demand for the board, and can also propose strategies to maximize production efficiency for the entire production line. For example, a printing press is constantly "monitoring" the downstation and feeding it a PCB before it completes an operation such as stencil cleaning. If the operation is completed first, the printing press will change the routine order until it meets the needs of the entire production line.

 

Other Factors Affecting Yield

 

Now more and more users tend to clean the stencil during the printing interval. It is important to keep the stencil opening free of solder paste and the underside of the stencil clean. This is the basis for ensuring print quality with no missing prints and good print resolution.

 

Various forms of automatic wiping systems that can be assembled on printing presses have come out one after another, most of which have strong vacuum suction and improved wiping management procedures. A more advanced approach is to perform a high-efficiency wet wipe first, followed by a dry wipe to remove residual solvent. Residual solder paste can be removed with a cleaning solvent. Using fast and efficient stencil wiping not only shortens the printing cycle, but also reduces system maintenance. Of course, it is more meaningful to carry out multiple operations at the same time, such as filling the solder paste while wiping the stencil.

 

While one board is being printed, another board can be inserted directly into its queue position. Transferring the boards to the closest location to the printing station also saves axis movement time. Proper center positioning design is also conducive to accurate and repeatable printing.

 

The adjustment of equipment is also an aspect. Although it is before printing, it will still have an impact on the total product output. Changes in this area, such as the auto-support pinout system, both ensure accuracy and speed up adjustments. The automatic solder paste filling unit reduces the frequency of downtime and gains valuable time. In addition, the off-line programming function is also essential in automatic printing machines.

 

Flexibility, precision and reliability

 

Flexibility can be assessed in two ways. First of all, the printing machine must have a flexible mechanical structure to obtain the best fixture solution. Second, the printing press must have advanced software that enables the system to automatically adapt to different substrates. Tight and precise clamping of the PCB on the printing press is also one of the many fixture difficulties in the surface mount production line. A printing press must have a variety of positioning options that can be used individually or in conjunction with each other as appropriate. A vision system with low contrast is more adaptable to various substrate materials. If equipped with such a system, coupled with appropriate mechanical positioning devices, ceramic substrates and even flexible substrates can be easily handled.

 

For automatic printing machines, accuracy is affected by three aspects. The first is the positioning correction of the circuit board substrate on the printing machine. Some presses use mechanical shutters. With this method, its inherent accuracy is affected by substrate variation from board to board. Some high-end printers use non-contact motion sensors that, unlike mechanical positioning, can compensate for different substrates when changing boards without manual intervention. Thanks to these sensors, plate auto-calibration is faster and more repeatable. This is especially important when printing a second side of a board that already has components. The use of structural actuators, such as castings, can improve the repeatability and reliability of the system. Finite element analysis proves that in order to achieve high stability, a stable foundation and framework must first be established. Therefore, equipment usually prefers to use castings to achieve the required stability, without having to consider the additional stress caused by higher operating speeds.

 

Of course, any machine will eventually break. Performance engineering design only greatly improves system reliability and minimizes downtime. When a part or a subsystem is damaged, it should be easy to remove or replace. Modular design is important to reduce downtime. In addition, spare parts and engineering support from the manufacturer should be quick and easy, no matter where the user is in the world.

 

Product changeover time. Usually, the main considerations when choosing a printing machine are its flexibility and ease of use, and whether it can print many different types of plates. Today's printing presses often have a variety of options. If there is a pin arrangement function with automatic support, arrange the support pins to the board to be printed according to a certain pattern for support. The arrangement of pins can be programmed and controlled, and no special fixture is needed, which shortens the product change time, and is most suitable for production lines where product models are frequently changed.

 

Another option is online replacement of templates. That is, the template is automatically selected and loaded from an online template storage rack (usually located at the back of the printing press). Typical template change times are 20 seconds. In another system, the flexible clamps of the Y-axis and Z-axis of the printing table can clamp the substrate with appropriate strength, so that it can effectively adapt to substrates of different sizes. Finally, the printing machine can also be equipped with off-line programming software, so that the printing machine can download the set parameters and reference point data from the computer.

 

The requirement of "cheaper, faster and better quality" for product production has prompted manufacturers to continuously introduce more competitive stencil printing machines. With higher production speed requirements come higher precision requirements. All in all, the system must be faster, more stable, and last longer. Therefore, an ideal printing machine must be designed through performance engineering. It can meet the requirements of current smt production, have fast and efficient performance, and have certain flexibility and reliability. At the same time, it should also meet the ever-changing performance requirements of solder paste .