Introduction:
In SMT production process, the layout of PCB and the pattern of component solder pad are very important to production efficiency and product quality. Through long-term practice, it is found that good layout and welding panel graphics can greatly improve production efficiency and product quality.
Principles of Component Arrangement:
The component arrangement at design time should conform to and follow the following principles.
a) Principle of uniform heat distribution of welding. The arrangement of components on PCB should be as regular and uniform as possible (especially for large chips, where more heat is absorbed), so that the components can get uniform welding heat when backflow welding, thus reducing defects such as the tombstoning, the pseudo soldering and solder balls. In this way, the welding quality of components will be improved.
b) Uniform arrangement of components on PCB. The direction and polarity arrangement of components on PCB should be regular. In principle, they should change with the change of component type. For SMD, they should be arranged in the same direction, uniform polarity and uniform spacing as much as possible. This is conducive to assembly, welding, maintenance and testing.
The element spacing shall comply with relevant national standards or IPC series standards. Generally the spacing between two SMT elements should be greater than or equal to 1 mm. The recommended layout and unreasonable layout are shown below.
Pasting components should be placed on the same layer of PCB as far as possible, but when double-sided pasting is necessary, one side of PCB components should be of relatively small quality, and the process operation will be simpler. It is also required for PCB base material. The base material and thickness of PCB determine the warp degree, strength and degree of PCB. In product design, the designer should consider ensuring that PCB does not deform during SMT production.
The pad design should match the running away. In addition, the wire should be drawn as far as possible in the middle of the pad to ensure thermal balance; The weldability of welding pad should be good, the surface of welding pad should be smooth and anti-oxidation measures should be taken; In order to meet the above conditions, we should try our best to adapt to different parts of different manufacturers.
SMT improves the PCB routing requirements and density to a certain extent, and of course also increases the wiring difficulty. In order to ensure the reliability of the product, the connection should be designed as wide as possible under the design permission. No holes are allowed on the inside of the common plate soldering pads. If there are holes, the soldering tin of the plate printing will be lost, resulting in poor welding such as false soldering and false soldering. The spacing between pads is required to be greater than 1.27mm. The wire width is less than 0.2mm, and the resistance welding film should be added between the welding pads.
Nowadays, enterprises pay more and more attention to the traceability of products, and ask operators to make different PCB processing quality marks in the production process. Early SMT process quality identification method is used more marker on the edge of the PCB, and affix a seal, etc., due to the most in the production process now uses the ultrasonic cleaning process, the logo will be washed away, the subsequent process is untraceable quality responsibility, and identification of pigment wash off after will also contaminated cleaning fluid, circuit board cleaning effect.
Through practice, we put one or two rows of soldering pads without electrical connection in the area of no copper cover or the area of copper cover with the same potential on the surface of PCB boards where most of the PCB components are attached as quality marks, and increase the number of welding pads to at least 8. After such treatment, the PCB board can be used as the quality mark in production through the quality identification of each solder pad, which facilitates the traceability of product quality. The specific placement rules are shown in the figure below.
