Electronic Assembly for Refrigeration Dryer
SMT assembly process requirements for PCB design:
1. Overall layout of components;
2. Spacing design;
3. Components layout direction of reflow welding process
4. Wave soldering process requirements
5. Wiring design requirements
6. Setting up the connection between the welding pads and printed wires
7. Hole design
8. soldering resistance, screen setting
9. printed board thermal design
The overall layout of components and devices setting process requirements. Layout is an important link.Layout results will directly affect the effectiveness of wiring, so a reasonable layout is the first step of PCB design success.
1. The distribution of components should be as uniform and orderly as possible. During reflow welding of large quality devices, the heat capacity is large, and too much concentration can easily lead to low local temperature and virtual welding. At the same time uniform distribution is also conducive to the center balance of gravity, in the shock impact test, it is not easy to appear that the components, metallized hole and welding pad is damaged phenomenon.
2. Components direction arrangement in PCB. Similar components should be arranged in the same direction as far as possible.For example, the polarity of electrolytic capacitor, the positive pole of diode, the single pin end of transistor, the first pin of integrated circuit, etc.
3. There should be a certain maintenance gap around the large device (the size of SMD repair equipment heating head that can be operated);
4. The heating element should be kept away from other components as far as possible, and generally placed in the corner and ventilated position in the cabinet.The heating element shall be kept at a minimum distance of 2mm from the circuit board surface.It is generally supported by its lead or other supports, such as heat sink, etc.The heating element connects the heating element body with the PCB in the multilayer pcb board. The metal welding pad is made at the time of design, and the soldering tin is used at the time of processing to dissipate heat through the PCB.
5. Temperature-sensitive components should be kept away from heating elements.For example, triode tubes, integrated circuits, electrolytic capacitors and some plastic shell elements should be kept as far away from bridge stacks, high-power devices, radiators and high-power resistors (plug-ins) as possible.
6. For the components and parts that need to be adjusted or changed frequently, such as potentiometer, adjustable inductor coil, variable capacitor, micro switch, safety pipe, button, plug and unplug, etc., components layout and parts should consider the structural requirements of the whole machine and be placed in a position that is convenient for adjustment and replacement.If it's machine internal adjustment, should be placed on the printed board above the place for easy adjustment;If it's outside machine adjustment, its position should be adjusted with the adjustment knob on the chassis panel position, prevent three space and two space conflict. For example, the panel opening of the button switch should match the position of the switch hole on the PCB.
7. Fixed holes shall be set near wiring terminals, plug and unplug parts, the center of long series of terminals and the parts often subject to force, and corresponding space shall be left around the fixed holes. Prevent deformation due to thermal expansion.If the thermal expansion of long series terminals is more serious than PCB, warping occurs during wave soldering.
8. For some big volume (area) tolerance, low accuracy and instrumentation to be secondary processing components, parts (such as transformer, electrolytic capacitors, varistors, bridge pile, radiator, etc.) and the gap between the other components on the basis of the original set to add a certain amount of rich volume, we suggest rich content not less than 1 mm for electrolytic capacitor, pressure sensitive resistance, bridge pile and polyester capacitor, for pressure machine, radiator, and more than 5w(include 5w) resistance the rich content not less than 3 mm.
9. All component Numbers are printed in the same direction.
10. Valuable components should not be placed in the PCB angle, edge, or close to connector, mounting hole, groove, splitter cutting, gap and corner, etc., these positions are high stress areas for printed circuit board, easy to cause solder joints and component cracks.
11. Component layout shall meet the technical requirements of reflow welding and wave soldering as well as the spacing requirements.
12. The position occupied by the position hole and fixed bracket should be set aside.
13. If the PCB area is too large, a gap of 5-10mm wide should be left in the middle of PCB board to prevent PCB from bending when passing through the tin furnace.