The development of modern science and technology leads to the miniaturization of electronic components and the extensive application of SMT technology and equipment in electronic products. SMT manufacturing device has the characteristics of full automation, high precision and high speed. With the improvement of automation, higher requirements are put forward for PCB design. The PCB design in Nanjing, Suzhou and other areas must meet the requirements of SMT equipment, otherwise it will affect the production efficiency and quality, and may even fail to complete automatic SMT.
SMT and its attributes
SMT is the abbreviation of surface mounting technology, and it is an advanced electronic manufacturing technology, which can weld and install components in the specified position of PCB. Compared with the traditional THT (through hole technology), the most remarkable feature of SMT is the improvement of 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. From the PCB loader, it will be transported along the path and through the equipment until the production is completed. Then, the PCB will be subjected to high-temperature welding through the reflow oven, and will be sent to the unloading machine during the printing, installation and welding process. This process can be shown in Figure 1 below.
PCB design elements affecting SMT manufacturing
PCB design is the key link of SMT technology, and SMT technology is an important factor to determine the quality of SMT manufacturing. This paper will analyze the PCB design elements that affect the quality of SMT equipment from the perspective of manufacturing. The PCB design requirements of SMT manufacturing equipment 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 the production is completed after printing, chip mounting and welding. Finally, it will be generated by the unloader as a finished plate. In this process, the PCB is transmitted on the path of devices, which requires that the PCB pattern should be consistent with the path transmission between 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 right angles are designed as chamfers.
For the PCB design in Figure 3, the path clamping edge is not a straight line, so the position of PCB and the transmission in the device will be affected. The open space in fig. 3 can be supplemented so that its clamping edge becomes a straight line as shown in fig. 4. Another method is to add crack edges on PCB, as shown in Figure 5.
•PCB size
The design size of PCB must meet the maximum and minimum size requirements of mounter. Up to now, the size of most devices is in the range of 50mmx50mm to 330mmx250mm (or 410mmx360mm).
If the thickness of PCB is too thin, its design size should not be too large. Otherwise, the reflow temperature will cause PCB deformation. The ideal aspect ratio of is 3: 2 or 4: 3.
If the PCB size is smaller than the minimum size requirement of the equipment, it should be spliced. The number of panels depends on the size and thickness of 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 edge pressing
As the PCB is transmitted on the path of the device, the component should not be placed in the direction of the clamping edge, otherwise the component will be squeezed by the device, thus affecting 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 edge and the lower edge 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 fig. 6(b).
mark
PCB marking is the marking point of all automatic equipment identification and location, and is used to correct PCB manufacturing errors.
One. Shape: solid circle, square, triangle, diamond, cross, hollow circle, oval, etc. A solid circle is preferred.
1. Size: The size must be within the range of 0.5mm to 3mm. A solid circle with a diameter of 1mm is preferred.
2. Surface: Its surface is the same as the soldering plane of PCB pad, the soldering plane is even, neither thick nor thin, and the reflection effect is excellent.
A forbidden cloth area should be arranged around Mark points and other pads, which cannot contain screen printing and solder resist, as shown in Figure 7.
Fig. 8 shows an excellent MARK design method, while fig. 9 shows some unreasonable MARK designs.
Silk-printed characters and silk-printed lines are arranged around the MARK in Figure 9, which will affect the identification of MARK points by equipment, and will cause frequent alarms due to the identification of MARK, seriously affecting the manufacturing efficiency.
Splicing method
In order to improve the manufacturing efficiency, a plurality of small PCBs with the same or different shapes can be combined together to form a panel. For some PCB with two sides, the top side and the bottom side can be designed as a panel, so that a template can be produced and the cost can be reduced. This method also helps to reduce the shift time of the top side and the bottom side, thus improving the manufacturing efficiency and device utilization rate.
The joining method of the panels includes punching holes and V-shaped grooves, as shown in Figure 10.
One requirement of the V-groove connection method is to keep the rest of the plate (uncut) equal to one quarter to one third of the thickness of the plate. If too many circuit boards are cut off, the notch may be broken due to the high temperature of reflow soldering, resulting in the PCB falling off, and the PCB will burn in the reflow oven.
PCB design is a complex technology, which must consider device requirements and component layout, pad design and circuit design at the same time. Excellent PCB design is an important factor to ensure product quality. This paper brings some problems that should be considered in PCB design from the point of view of SMT manufacturing. As long as enough attention is paid to these problems, the full-automatic SMT manufacturing of SMT devices can be carried out.
