The development of modern science and technology has led to the miniaturization of electronic components and the large-scale application of SMT technology and equipment in electronic products. SMT manufacturing equipment has the characteristics of fully automatic, high precision and high speed. Due to the improvement of the degree of automation, higher requirements are put forward for PCB design. PCB design must meet the requirements of SMT equipment, otherwise it will affect production efficiency and quality, and may even fail to complete computer automatic SMT.
SMT and its attributes
SMT is the abbreviation of surface mount technology, which is an advanced electronic manufacturing technology that can solder and install components on the specified position of the PCB. Compared with the traditional THT (Through Hole Technology), the most significant feature of SMT is the increase in the degree of automated manufacturing, which is suitable for large-scale automated manufacturing.
SMT production line introduction
The basic integrated SMT production line should include loader, printer, chip mounter, reflow oven and unloader. The PCB starts from the loader, travels along the path and passes through the equipment until the production is completed. Then, the PCB will be subjected to high temperature soldering through the reflow oven and transferred to the unloader during the process of printing, mounting and soldering. This process can be shown in Figure 1 below.
PCB design elements that affect SMT manufacturing
PCB design is a key link in SMT technology, and SMT technology is an important factor that determines the quality of SMT manufacturing. This article will analyze the PCB design elements that affect its quality from the perspective of SMT equipment manufacturing. The design requirements of SMT manufacturing equipment for PCB mainly include: PCB pattern, size, positioning hole, clamping edge, MARK, panel wiring, etc.
•PCB pattern
In the automatic SMT production line, PCB production starts from the loader, and completes the production after printing, chip mounting, and soldering. Finally, it will be generated as a finished board by the unloader. In this process, the PCB is transmitted on the path of the device, which requires that the PCB pattern should be consistent with the path transmission between the devices.
Figure 2 shows a standard rectangular PCB whose channel clamping edge is as flat as a line, so this type of PCB is suitable for channel transmission. Sometimes the right angle is designed as a chamfer.
For the PCB design in Figure 3, the path clamping edge is not a straight line, so the position of the PCB and the transmission in the device will be affected. The open space in Figure 3 can be supplemented so that the clamping edge becomes a straight line as shown in Figure 4. Another method is to add crack edges on the PCB, as shown in Figure 5.
•PCB size
The PCB design size must meet the maximum and minimum size requirements of the placement machine. So far, the size of most equipment is in the range of 50mmx50mm to 330mmx250mm (or 410mmx360mm).
If the thickness of the PCB is too thin, its design size should not be too large. Otherwise, the reflow temperature will cause PCB deformation. The ideal aspect ratio is 3:2 or 4:3.
If the PCB size is smaller than the minimum size requirement of the device, it should be joined together. The number of panels depends on the size and thickness of the PCB.
•PCB positioning hole
SMT positioning methods are divided into two types: positioning holes and edge positions and edge positions. However, the most commonly used positioning method is Mark point alignment.
•PCB blanking
Since the PCB is transmitted on the path of the device, the component must not be placed in the direction of the clamping edge, otherwise the component will be squeezed by the device, which will affect the installation of the chip. Take the PCB in Figure 6(a) as an example. Some components are placed near the lower edge of the PCB. Therefore, the upper and lower edges cannot be regarded as clamping edges. However, there are no components near the two side edges, so the two short edges can be used as clamping edges, as shown in Figure 6(b).
•mark
The PCB mark is an identification point for the identification and location of all fully automatic equipment, which is used to correct PCB manufacturing errors.
one. Shape: solid circle, square, triangle, rhombus, cross, hollow circle, oval, etc. A filled circle is preferred.
1. Size: The size must be in the range of 0.5mm to 3mm. A solid circle with a diameter of 1mm is preferred.
2. Surface: The surface is the same as the soldering plane of the PCB pad, the soldering plane is uniform, neither thick nor thin, and the reflection effect is excellent.
A forbidden area should be arranged around the Mark point and other pads, and the screen printing and solder mask should not be included in this area, as shown in Figure 7.
Figure 8 is an excellent MARK design method, and Figure 9 is some unreasonable MARK design.
The silk-screened characters and silk-screened lines are arranged around the MARK in Figure 9, which will affect the device's recognition of the MARK point, and will cause frequent alarms due to the MARK recognition, which seriously affects the manufacturing efficiency.
• Jigsaw
In order to improve manufacturing efficiency, multiple small PCBs with the same or different shapes can be combined to form a panel. For some double-sided PCBs, the top and bottom sides can be designed as one panel, so that a template can be produced, which can reduce costs. This method also helps to reduce the displacement time on the top and bottom sides, thereby improving manufacturing efficiency and device utilization.
The connection method of the jigsaw panel includes punching holes and V-shaped grooves, as shown in Figure 10.
A requirement of the V-groove connection method is to keep the rest of the board (uncut) equal to one-fourth to one-third of the thickness of the board. If too many circuit boards are cut off, the cut groove may be broken due to the high temperature of the reflow soldering, which may cause the PCB to fall, and the PCB will burn in the reflow oven.
PCB design is a complex technology, and device requirements and component layout, pad design and circuit design must be considered at the same time. Excellent PCB design is an important factor to ensure product quality. This article brings up some issues that should be considered in PCB design from the perspective of SMT manufacturing. As long as sufficient attention is paid to these issues, fully automatic SMT manufacturing of SMT devices can be carried out.
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