The development of modern science and technology has led to the miniaturization of electronic components, and the SMT manufacturing device has the characteristics of full automatic, high precision and high speed. Due to the increased degree of automation, PCB design needs higher requirements. The PCB design must meet the requirements of the SMT device, otherwise it will affect the manufacturing efficiency and quality, and may even fail to complete the automatic SMT process. For example, when MARK is not fully satisfied, the machine may often fail, and the unreasonable design of the PCB shape, clamping edges and panels can affect manufacturing efficiency, and the pad defects can even cause mechanical manufacturing to fail.
PCB design is a key part of SMT technology and an important factor in determining the quality of SMT manufacturing. We will analyze PCB design from the perspective of SMT device manufacturing. PCB design requirements mainly include: PCB pattern, size, positioning hole, clamping edge, MARK, panel mode, etc.
In the automatic SMT production line, PCB production starts from the loader, printing, chip mounting, and soldering. Finally, it will be generated from the uninstaller as a finished board. During this process, PCB transfer over the device’s path, which requires the PCB pattern to be consistent with the path transfer between the devices.
Figure 1 shows a standard rectangular PCB with a via clamping edge that is as flat as a line, so this type of PCB is suitable for via transmission. Sometimes the right angle change chamfered.
For the PCB design in Figure 2, the path clamped edges are not straight, so the location of the PCB and the transmission in the device are affected. The open space in Figure 3 can be supplemented so that its clamping edge becomes a straight line, as shown in Figure 3. Another method is to add crack edges to the PCB, as shown in Figure 4.
The PCB design dimensions must match the maximum and minimum size requirements of the printer and chip mounter. To date, most devices range in size from 50mm x 50mm to 330mm x 250mm (or 410mm x 360mm). If the PCB is too thin, its design size should not be too large. Otherwise, the reflow temperature will cause the PCB to deform. A 3:2 or 4:3 aspect ratio is the best choice. If the PCB size is smaller than the minimum size of the device, it should be panelized. The number of panels is determined by the PCB size and thickness.
PCB positioning hole
There are two types of SMT positioning methods: positioning holes and edge positions and edge positions.
PCB Clamping edge
Since transporting the PCB board over the path of the device, the component must not be placed in the clamping direction, otherwise device will press the component, affecting the chip mounting. If the component is placed near the lower edge of the PCB, the upper and lower edges cannot act as clamping edges. However, there are no components near the two sides, so the two short sides can be used as clamping edges.
PCB markings are all automatic device identification and location identification points used to modify PCB manufacturing tolerances.
Shape: solid circle, square, triangle, diamond, cross, hollow circle, oval, etc. Solid circles are preferred.
Size: The size must be in the range of 0.5mm to 3mm. A solid circle with a diameter of 1 mm is preferred.
Surface: The surface is the same as the PCB pad plane, the soldering plane is uniform, and the reflection effect is excellent.
The background area should be placed around the Mark and other spacers, and the screen and solder mask should not be included in the background area.
In order to improve manufacturing efficiency, a plurality of small PCBs having the same or different shapes will combine in a panel. For some PCBs with double sided, the top and bottom sides can be designed as one panel, which will produce a template that can reduce costs. The method also helps to reduce shift times on the top and bottom sides, improving manufacturing efficiency and equipment utilization. The connection method of the panel includes a stamp hole and a V-shaped groove. One requirement of the V-groove joint method is to maintain the remainder of the panel (not cut) equal to one-quarter to one-third of the thickness of the panel. If too many boards are cut, the high temperature of the reflow will damage the cutting slots, causing the PCB to fall and burn out in the reflow oven.
PCB design is a complex technology that must consider device requirements and component layout, pad design and circuit design. Excellent PCB design is an important factor in ensuring product quality.