PCB Technical - The requirements of SMT processing technology for PCB design

Good SMT processing quality is inseparable from good PCB design. If the characteristics and requirements of SMT production equipment and technology can be fully considered in PCB design, then SMT processing can achieve twice the result with half the effort.

The basic requirements of SMT processing technology for PCB design are as follows:

1. The distribution of components on the PCB should be as uniform as possible. The heat capacity of large-mass components during reflow soldering is relatively large. Too much concentration can easily lead to low local temperature and lead to false soldering; at the same time, a uniform layout is also conducive to the balance of the center of gravity. In the vibration and shock experiment, It is not easy to damage the components, metallized holes and pads.

2. The arrangement direction of the components on the PCB, similar components should be arranged in the same direction as much as possible, and the characteristic directions should be consistent to facilitate the mounting, welding and testing of the components. For example, the anode of the electrolytic capacitor, the anode of the diode, the single-pin end of the triode, and the first pin of the integrated circuit are arranged in the same direction as possible. The printing orientation of all component numbers is the same.

3. The size of the heating head of the SMD rework equipment that can be operated should be reserved around the large components.

4. Heat-generating components should be as far away as possible from other components, generally placed in corners and in a ventilated position in the chassis. Heating components should be supported by other leads or other supports (for example, heat sinks can be added) to keep a certain distance between the heating components and the PCB surface, the minimum distance is 2mm.

The heating components are connected to the PCB in the multi-layer board, the metal pads are used in the design, and the solder connection is used during the processing, so that the heat is dissipated through the PCB.

5. Keep temperature sensitive components away from heating components. For example, triodes, integrated circuits, electrolytic capacitors and some plastic shell components should be kept as far away as possible from bridge stacks, high-power components, radiators and high-power resistors.

6. The layout of components and parts that need to be adjusted or frequently replaced, such as potentiometers, adjustable inductance coils, variable capacitor micro switches, fuses, buttons, plugs and other components, should consider the structure of the whole machine It is required to place it in a position that is convenient for adjustment and replacement. If it is adjusted inside the machine, it should be placed on the PCB where it is easy to adjust; if it is adjusted outside the machine, its position should be compatible with the position of the adjustment knob on the chassis panel to prevent conflicts between the three-dimensional space and the two-dimensional space. For example, the panel opening of the toggle switch and the empty position of the switch on the PCB should match.

7. Fixing holes should be provided in the vicinity of wiring terminals, plug-in parts, the center of long series of terminals and the parts that are often subjected to force, and there should be corresponding space around the fixing holes to prevent deformation due to thermal expansion. If the thermal expansion of the long series of terminals is more serious than that of the PCB, it is prone to warping during wave soldering.

8. Some components and parts (such as transformers, electrolytic capacitors, varistors, bridge stacks, radiators, etc.) that require secondary processing due to large volume (area) tolerances and low precision, and other components The interval is increased by a certain margin on the basis of the original setting.

9. It is recommended to increase the margin of electrolytic capacitors, varistors, bridge stacks, polyester capacitors, etc., not less than 1mm, and transformers, radiators, and resistances exceeding 5W (including 5W) not less than 3mm.

10. The electrolytic capacitor cannot touch heating components, such as high-power resistance thermistors, transformers, radiators, etc. The minimum distance between the electrolytic capacitor and the radiator is 10mm, and the minimum distance between other components and the radiator is 20mm.

11. Do not place stress-sensitive components on the corners, edges of the PCB or close to connectors, mounting holes, slots, cutouts, gaps and corners of the puzzle, these locations are high stress areas of the PCB, which are likely to cause solder joints. And the cracks or cracks of the components.

12. PCB design should meet the process requirements and spacing requirements of reflow soldering and wave soldering. Reduce the shadow effect produced during wave soldering.

13. The PCB positioning hole and the position occupied by the fixing bracket should be reserved.

14. In the large-area PCB design with an area of more than 500cm2, in order to prevent the PCB from bending when passing through the soldering furnace, a 5~10mm wide gap should be left in the middle of the PCB without any components (wires can be routed) to be used in the process. Add bead to prevent PCB from bending when tin furnace.

15. The component arrangement direction of the reflow soldering process.

▪ The placement direction of the components should consider the direction the PCB enters the reflow oven.

▪ In order to make the welding ends of the two end chip components and the pins on both sides of the SMD component be heated synchronously, to reduce the tombstone, displacement, and welding ends caused by the simultaneous heating of the welding ends on both sides of the components. For soldering defects such as disks, the long axis of the two end chip components on the PCB should be perpendicular to the conveyor belt direction of the reflow oven.

▪ The long axis of the SMD component should be parallel to the conveying direction of the reflow oven, and the long axis of the Chip component at the two ends and the long axis of the SMD component should be perpendicular to each other.

▪ A good PCB design should not only consider the uniformity of heat capacity, but also consider the arrangement direction and order of the components.

▪ For large-size PCBs, in order to keep the temperature on both sides of the PCB as consistent as possible, the long side of the PCB should be parallel to the direction of the conveyor belt of the reflow oven. Therefore, when the PCB size is larger than 200mm, the requirements are as follows:

a) The long axes of the Chip components at the two ends are perpendicular to the long sides of the PCB.

b) The long axis of the SMD component is parallel to the long side of the PCB.

c) For PCBs assembled on both sides, the orientation of the components on both sides is the same.

d) The arrangement direction of the components on the PCB. Similar components should be arranged in the same direction as much as possible, and the characteristic directions should be consistent to facilitate the mounting, welding and testing of the components. For example, the anode of the electrolytic capacitor, the anode of the diode, the single-pin end of the triode, and the first pin of the integrated circuit are arranged in the same direction as possible.

16. In order to prevent interlayer shorts caused by touching the printed wires during PCB processing, the distance between the conductive patterns on the inner and outer edges of the PCB should be greater than 1.25mm. When the edge of the PCB outer layer has been laid with a ground wire, the ground wire can occupy the edge position. For the position of the PCB board that has been occupied due to structural requirements, components and printed wires cannot be placed. There should be no through holes in the bottom pad area of SMD/SMC to avoid the solder being heated and remelted in the wave soldering after reflow. Diversion.

17. Installation spacing of components: The minimum installation spacing of components must meet the manufacturability, testability, and maintainability requirements of SMT assembly.