In modern electronics assembly, Surface Mount Technology has become the mainstream due to its advantages in automation and miniaturization. The entire SMT process involves a series of automated steps, including pick-and-place, printing, soldering, etc. While these processes enhance production efficiency, they also bring new challenges. Surprisingly, about 70% of failures in the SMT process are attributed to the solder paste printing.
To ensure the success of your project, TechSparks presents a comprehensive guide to the solder paste printing process in this article. By delving into the process flow and relevant considerations, we aim to help you improve the accuracy of surface mount soldering and reduce the likelihood of potential failures.
Automated Solder Paste Printing Process
Solder paste is a paste-like mixture of solder powder and flux, forming a viscous substance that, when melted at high temperatures and cooled at low temperatures, creates robust solder joints on the surface of a PCB. Solder paste printing, in simple terms, refers to the process of applying solder paste to predetermined soldering points on a PCB. While individuals unfamiliar with the electronics industry may dismiss it, thinking that even syringes or toothpicks can complete this process, it may be feasible for small or DIY projects but not for commercial activities. In commercial operations, we typically use automated equipment to complete the entire production line. Below is a detailed overview of the automated solder paste printing process:
- Place the PCB to be assembled on the loading machine, and these boards will go through the entire assembly process along the conveyor belt, including solder paste printing.
- The vision system equipped on the solder paste printer recognizes and locates the main outline of the PCB.
- The Z-axis moves up to the position of the vacuum plate, and a vacuum is formed to fix the PCB in place, preventing displacement during the printing process.
- The vision axis moves to the first target or mark point on the PCB. After confirming the mark point, the vision system uses image processing techniques to locate the reference point beneath the PCB stencil.
- Move the stencil horizontally to align the opening of the stencil with the PCB pad. Then, the Z-axis moves up to bring the stencil closer to the PCB.
- Solder paste is applied over the stencil, and, with the push of a squeegee, it rolls on the stencil. Finally, under pressure, the solder paste is forced through the holes in the stencil onto the pads on the PCB.
- After printing, the Z-axis moves down, separating the PCB from the stencil.
- Perform SPI Inspection to verify the quality of the solder paste printing.
Types of Solder Paste Printing
In fact, the aforementioned solder paste printing process is stencil printing. Its advantages lie in high precision, and the stencil is easy to replace, making it widely used in the SMT process. For the process of using a stencil for solder paste printing, the key factors are the manufacturing quality of the stencil and the setting of printing parameters.
It is an effective method for small-scale projects that require quick turnaround. It uses a nozzle to spray solder paste in a mist onto the pad positions, eliminating the need for stencil manufacturing. Jet printing is suitable for projects with lower precision requirements, such as consumer electronics.
Manual printing is typically used for small-scale, rework, or hobbyist projects. This method involves aligning solder paste or solder wire with the soldering areas and then using a soldering iron to complete the soldering.
Key Parameters for Solder Paste Printing
Pressure and Angle
These are two critical parameters associated with the squeegee. To ensure uniform solder paste application and avoid residue issues, it is recommended to use as much pressure as possible, ensuring that the squeegee maintains a tight contact with the stencil surface throughout its movement. The angle between the squeegee and the stencil should be kept around 60°; too small an angle may result in insufficient pressure, while too large an angle could cause solder paste smearing and sticking problems.
Simply put, it indicates the height of the solder paste on the stencil, usually denoted by the symbol ∮h. In general, it is recommended to set this value between 13-23mm to achieve an ideal coating effect. If the value is too low, it may lead to insufficient solder paste, resulting in printing defects; conversely, if the value is too high, it may fail to create the desired rolling effect.
To maintain the solder paste condition, TechSparks suggests checking the solder paste on the stencil every half an hour during the operation. This serves two main purposes: on one hand, it helps prevent prolonged exposure of the solder paste to the air, affecting its quality; on the other hand, it aids in promptly addressing solder paste beyond the squeegee length range to ensure coating accuracy and consistency.
This encompasses two aspects: printing speed and separation speed.
Printing speed is inversely proportional to the viscosity of the solder paste. When facing narrow spacing and high-density layouts, it is advisable to appropriately reduce the printing speed; otherwise, it may result in incomplete penetration of the solder paste into apertures, leading to defects such as insufficient formation or misprinting.
Separation speed refers to the instantaneous speed at which the stencil detaches from the PCB after printing. Excessive speed may cause solder paste to pull and string, while too slow a speed may make it challenging for the solder paste to detach from the walls of the stencil apertures. Modern solder paste printing machines often address the issue of separation speed by employing multi-stage separation, involving brief pauses during the separation process to balance the speed effectively.
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