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Technical Article


PCB Design Requirements for SMT Assembly

Time:2020-12-29 14:05:37 Click:

1. PCB design should be designed according to the characteristics and requirements of SMT production equipment and technology;

 

Different processes, such as reflow soldering and wave soldering, have different component layouts. When double-sided reflow soldering, there are different requirements for the layout of the A side and the B side; Selective wave soldering and traditional wave soldering also have different requirements.

 

The basic requirement of SMT Assembly for PCB design is that 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. In the meanwhile, too much concentration can easily cause local low temperatures and lead to false soldering; Furthermore, uniform layout is also beneficial The center of gravity is balanced, and it is not easy to damage the components, metallized holes and pads in the vibration and shock experiment.

 

 

2. The arrangement direction of the components on the PCB board. 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 instance, 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. All component numbers are printed in the same orientation. The size of the heating head of the SMD rework equipment that can be operated should be reserved around the large components. Heating components should be as far away from other components as possible, generally placed in the corners, ventilated positions in the chassis. The heating components should be supported by other leads or other supports (such as heat sinks can be added) to keep the heating components and the PCB surface at a certain distance, the minimum distance is 2mm. The heating components are connected to the PCB board in the multi-layer board, the metal pads are used in the design, and the solder connection is used during the processing to dissipate the heat through the PCB board. Keep temperature sensitive components away from heating components. For example, triodes, integrated circuits, electrolytic capacitors and some plastic shell components should be kept away from bridge stacks, high-power components, radiators and high-power resistors as much as possible.

 

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 structural requirements of the whole machine. Place it in a position that is easy to adjust and replace. If it is adjusted inside the machine, it should be placed on the PCB board where it is easy to adjust; 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 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. Fixing holes should be provided near the connection terminals, plug-in parts, the center of the 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 board, it is prone to warping during wave soldering.

 

  

 

3. The electrolytic capacitor cannot touch the heating components, such as high-power resistance thermistor, transformer, radiator, etc.

 

The minimum distance between the electrolytic capacitor and the heat sink is 10mm, and the minimum distance between other components and the heat sink is 20mm. Do not place stress-sensitive components on the corners, edges, or near connectors, mounting holes, slots, cutouts, gaps, and corners of the PCB board. These locations are high stress areas of the PCB board, which are likely to cause solder joints. And the cracks or cracks of the components. PCB design should meet the process requirements and spacing requirements of reflow soldering and wave soldering. Reduce the shadow effect produced during wave soldering. The positioning holes of the PCB board and the position occupied by the fixing bracket should be reserved.

 

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

 

 

 

 

4. The arrangement direction of the components of the reflow soldering process: The layout direction of the components should consider the direction of the PCB board entering the reflow oven;

 

In order to make the welding ends of the two end SMD components and the pins on both sides of the SMD component be heated synchronously, to reduce the tombstone, displacement, and separation of the welding end from the pad due to the simultaneous heating of the welding ends on both sides of the component For soldering defects, the long axis of the two end SMD 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. In addition to the uniformity of heat capacity, a good PCB design must also consider the arrangement and order of components.

 

 

 

 

 

5. For large-size PCB boards, in order to keep the temperature on both sides of the PCB board as consistent as possible, the long side of the PCB board should be parallel to the conveyor belt direction of the reflow oven;

 

When the PCB board size is greater than 200mm, the long axis of SMD components at the two ends is required to be perpendicular to the long side of the PCB board, and the long axis of the SMD component is parallel to the long side of the PCB board. The PCB board assembled on both sides has two sides. The orientation of the components should be the same, the arrangement direction of the components on the PCB board, 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.

 

 

 

 

6. In order to prevent short circuit between layers 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 a ground wire has been laid on the edge of the PCB outer layer, the ground wire can occupy the edge position. For the position on the PCB board that has been occupied due to structural requirements, components and printed wires cannot be laid out. 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. Installation spacing of components: The minimum installation spacing of components must meet the manufacturability, testability, and maintainability requirements of SMT Assembly.

 


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