Good SMT processing quality is inseparable from good PCB design. If the characteristics and requirements of SMT production equipment and process can be fully considered in PCB design, then SMT processing can achieve twice the result with half the effort. So what are the requirements for SMT processing technology for PCB design? Next, Shenzhen Viasion Electronics introduced to everyone.
The basic requirements for 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-quality components during reflow soldering is large. If the concentration is too concentrated, the local temperature will be low and the solder joint will be caused. At the same time, the uniform layout is also beneficial to the balance of gravity. In the vibration shock test. It is not easy to cause damage to components, metallized holes and pads.
2. The arrangement direction of components on the PCB, the same components are arranged in the same direction as much as possible, the characteristic direction should be consistent, which is convenient for component placement, soldering and inspection. For example, the positive electrode of the electrolytic capacitor, the positive pole of the diode, the single-pin terminal of the transistor, and the first pin arrangement direction of the integrated circuit are as uniform as possible. All component numbers are printed in the same orientation.
3. The size of the SMD rework equipment heating head can be operated around the large components.
4. The heating components should be kept away from other components as far as possible, usually placed in the corners and in the ventilation position inside the chassis. The heating components are supported by other leads or other supports (such as heat sinks), so that the heating components are kept at a certain distance from the surface of the PCB, and the minimum distance is 2 mm. The heating component connects the heating component body to the PCB in the multi-layer board, and the metal pad is designed during the design, and the solder is connected during processing to dissipate heat through the PCB.
5. Keep temperature sensitive components away from hot components. Such as triodes, integrated circuits, electrolytic capacitors and some molded case components, should be as far as possible from the bridge stack, high-power components, heat sinks and high-power resistors.
6. Components and components that need to be adjusted or frequently replaced, such as potentiometers, adjustable inductors, variable capacitor microswitches, fuses, buttons, plug-ins and other components, should consider the structure of the machine It is required to be placed in a position that is easy to adjust and replace. If it is adjusted inside the machine, it should be placed on the PCB for easy adjustment; if it is adjusted outside the machine, its position should be adapted to the position of the adjustment knob on the chassis panel to prevent conflict between the three-dimensional space and the two-dimensional space. For example, the panel opening of the toggle switch and the position of the switch on the PCB should match.
7. Fixing holes should be provided at the terminals, near the plugging and inserting parts, at the center of the long string terminals and at the parts where the force is often applied, and there should be a corresponding space around the fixing holes to prevent deformation due to thermal expansion. For example, if the long string terminal is thermally expanded, it is more serious than the PCB, and the wave soldering is prone to tilting.
8. Some body (face) product tolerances, low precision, components and components that need secondary processing (such as transformers, electrolytic capacitors, varistors, bridge stacks, heat sinks, etc.), and other components The interval is increased by a certain margin based on the original setting.
9. It is recommended that the increase margin of electrolytic capacitors, varistors, bridge stacks, polyester capacitors, etc. should be no less than 1mm, and transformers, heat sinks and resistors exceeding 5W (including 5W) should be no less than 3mm.
10. Electrolytic capacitors must not touch heating components, such as high-power resistance thermistors, transformers, heat sinks, etc. The distance between the electrolytic capacitor and the heat sink is at least 10 mm, and the interval between other components and the heat sink is at least 20 mm.
11. Stress-sensitive components should not be placed on the corners, edges or close-up of PCBs, connectors, mounting holes, slots, cuttings, cutouts, corners, etc. These locations are high-stress areas of the PCB, which can easily cause solder joints. Cracks or cracks in components.
12. PCB design should meet the process requirements and spacing requirements of reflow soldering and wave soldering. Reduce the shadowing effect produced by wave soldering.
13. The position of the PCB positioning hole and the fixing bracket should be reserved.
14. In large-area PCB design with an area of more than 500cm2, in order to prevent PCB bending during the tin furnace, a 5~10mm wide gap should be left in the middle of the PCB, and no components (wireable) should be used. A tin bar that prevents bending of the PCB is added to the tin furnace.
15. The component layout direction of the reflow soldering process.
▪ The direction in which components are placed should be considered in the direction of the PCB entering the reflow oven.
▪ In order to synchronize the soldering ends on both sides of the two end chip components and the pins on both sides of the SMD component, the tombstoning, displacement, and solder joint disengagement are reduced due to the simultaneous soldering of the soldering ends on both sides of the component. For soldering defects such as discs, the long axis of the two end chip components on the PCB should be perpendicular to the conveyor belt of the reflow oven.
▪ The long axis of the SMD component should be parallel to the direction of reflow soldering. The long axis of the Chip component at both ends should be perpendicular to the long axis of the SMD component.
▪ A good PCB design, in addition to considering the uniformity of heat capacity, also consider the direction and sequence of components.
▪ For large PCBs, in order to keep the temperature on both sides of the PCB as uniform 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 greater than 200mm, the requirements are as follows:
a) The long axis of the Chip component at both ends is perpendicular to the long side of the PCB.
b) The long axis of the SMD component is parallel to the long side of the PCB.
c) Double-sided assembled PCB with the same orientation of the components on both sides.
d) The arrangement direction of components on the PCB, the same components are arranged in the same direction as much as possible, and the characteristic direction should be consistent to facilitate the mounting, soldering and testing of components. For example, the positive electrode of the electrolytic capacitor, the positive electrode of the diode, the single-pin terminal of the transistor, and the arrangement direction of the first pin of the integrated circuit are as uniform as possible.
16. In order to prevent interlayer short circuit caused by touching the printed wiring during PCB processing, the conductive pattern of the inner and outer edges should be greater than 1.25 mm from the edge of the PCB. When the ground wire is already laid on the edge of the outer layer of the PCB, the ground wire can occupy the edge position. For PCB board positions that have been occupied by structural requirements, components and printed conductors cannot be placed. There should be no through holes in the bottom pad area of SMD/SMC to avoid solder reheating after wave soldering after reflow soldering. Diversion.
17. Component mounting pitch: The minimum mounting pitch of components must meet the requirements for manufacturability, testability and maintainability of SMT assembly.