SMT is the process of placing SMT components on PCB and reflow soldering. As the saying goes, a skillful woman cannot cook without rice. Without a standardized PCB substrate, no matter how advanced SMT technology is, it is difficult to produce high-quality and efficient PCBA. Generally speaking, PCB requirements for achieving high-quality and efficient SMT are easy to achieve:
1、 The size of PCB assembly is suitable (excluding LED light boards): Generally, the length and width range after assembly is 10-15 centimeters, which is more suitable.
It should not be too small, usually at least 5cm * 5cm, otherwise the SMT machine will be difficult to handle;
It should not be too large. According to the requirements of different brands of SMT machines, the processing limit of the SMT machine should generally not exceed 420 * 500mm. Otherwise, the SMT machine will be difficult to handle.
In non essential situations, do not be too thin or too soft. For thin and soft boards, printing positioning and SMT support are both challenges, and usually require the production of specialized fixtures to handle them, which is costly.
2、 There must be mark points and process edges that meet the specifications:
The mark points and process edges of a PCB are essential conditions for high-quality SMT. For specific explanations, please refer to the top SMT college paper "PCB MARK point and process edge design" (please check!).
3、 Reasonable design and placement of SMD components: Efficient SMT requires a board with many SMD components, so that the additional waste of time in and out of the board and the empty running of the chip head will be reduced.
Try to avoid situations where a PCB has only a few SMD components on one side, as the efficiency of attaching these components will be very low and the cost will be high. In this case, it is best to try to move the components to the other side and become a single sided SMD board. When there are only a few SMD components on a PCB, efforts should be made to reduce the area of a single PCB and assemble the SMD.
When designing the double-sided patch board, try to place the components that are sensitive to the reflow temperature, such as the lens and BGA, and the components with excessive self weight, such as the inductance card socket connector, on the same side. This side only needs reflow soldering once, and the other side needs reflow soldering twice. Try to only place the resistance capacitance diode triode and ordinary IC.
4、 Preferred PCB surface treatment: Currently, the most common and optimal surface treatment is gold deposition (gold plating).
When there are 0402 and smaller components, precision BGA/QFN/QFP, do not use the tin spraying process because the flatness of the tin spraying plate is not good, which affects the printing quality of the solder paste.
When double-sided mounting, it is better not to use the OSP process, because when the first side is mounted, the second side OSP pad is prone to oxidation at a high temperature of more than 200 degrees in the reflow soldering furnace.
When PCBs need to be stored for several months before being mounted, it is best not to use the OSP process, as OSP boards are prone to oxidation when stored unsealed at room temperature.
When there are test points on the PCB with OSP surface treatment, it is necessary to use a test stand to test the function. Please remember to open windows and tin the test points on the steel mesh, because the OSP protective film before welding itself is poorly conductive! After welding with the solder, the OSP protective film will evaporate and disappear.
5、 The shape, size, and positioning holes of component pads on PCBs should be designed carefully:
It should be known that the paste like solid solder paste will become liquid tin for welding during reflow soldering, and then be cooled and solidified. When tin becomes liquid, the surface tension is high, and the components float on the tin liquid. If the pad design is too large, the resistance capacitance components are easily skewed (there is hot air in the reflow soldering furnace), resulting in the tip being upright and skewed after reflow. The solder pad is too small, and the amount of tin added is difficult to guarantee.
The IC and connector pads with tight legs should not be too thick or too thin.
The positioning column at the bottom of the card holder and connector must match the positioning hole of the PCB, and if it is too small, it cannot be inserted; If it is too large, it will not have a positioning effect.
