Good SMT processing quality is inherently linked to effective PCB design. By thoroughly considering the characteristics and requirements of SMT production equipment and technology during PCB design, SMT processing can yield significantly better results with reduced effort.
The fundamental 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. Large-mass components have a higher heat capacity during reflow soldering. Over-concentration can lead to localized low temperatures and potential soldering failures. Additionally, a balanced layout aids in stabilizing the center of gravity, reducing the risk of damage to components, metallized holes, and pads during vibration and shock tests.
2. The orientation of components on the PCB should ensure that similar components are aligned in the same direction. This alignment facilitates the mounting, soldering, and testing processes. For instance, the anode of the electrolytic capacitor, the anode of the diode, the single-pin end of the transistor, and the first pin of the integrated circuit should be arranged in a consistent direction. Moreover, the printing orientation of all component designators should be uniform.
3. Adequate space should be allocated around larger components to accommodate the size of the heating head of the SMD rework equipment.
4. Heat-generating components should be positioned as far away from other components as possible, typically located in corners or well-ventilated areas of the chassis. These components should be supported by additional leads or structures (such as heat sinks) to maintain a minimum distance of 2mm from the PCB surface.
1. The heating components are integrated into the multi-layer PCB design using metal pads, with solder connections facilitating heat dissipation through the board.
2. Maintain a significant distance between temperature-sensitive components, such as transistors, integrated circuits, electrolytic capacitors, and certain plastic-encased parts, and heating elements like bridge stacks, high-power components, radiators, and high-power resistors.
3. When arranging components that require frequent adjustments or replacements—such as potentiometers, adjustable inductors, variable capacitors, micro switches, fuses, buttons, and plugs—consider the overall machine structure. Ensure easy access for adjustment; internal adjustments should be placed on easily reachable PCB areas, while external adjustments should align with chassis panel knobs to avoid spatial conflicts. For instance, ensure the toggle switch panel opening matches the PCB switch position.
4. Fixing holes should be positioned near wiring terminals, plug-in parts, centers of long terminal arrays, and frequently stressed areas, with adequate surrounding space to accommodate thermal expansion. If thermal expansion in long terminal arrays exceeds that of the PCB, warping during wave soldering may occur.
5. For components requiring secondary processing due to large tolerances and low precision—such as transformers, electrolytic capacitors, varistors, bridge stacks, and radiators—increase the spacing from adjacent components based on initial settings.
6. It is advisable to increase the clearance for electrolytic capacitors, varistors, and bridge stacks to no less than 1mm, and for transformers, radiators, and resistors above 5W (including 5W) to at least 3mm.
7. Ensure that electrolytic capacitors do not touch heating components like high-power resistors, thermistors, transformers, or radiators. Maintain a minimum distance of 10mm between electrolytic capacitors and radiators, and at least 20mm between other components and radiators.
8. Avoid placing stress-sensitive components at PCB corners, edges, or near connectors, mounting holes, slots, cutouts, or gaps, as these areas are prone to high stress, which can lead to solder joint failures or component cracking.
9. PCB design must comply with the process and spacing requirements for reflow and wave soldering, minimizing shadow effects during wave soldering.
10. Reserve space for PCB positioning holes and the areas occupied by fixing brackets.
11. In large-area PCB designs exceeding 500cm², to prevent bending during soldering, leave a 5–10mm gap in the middle of the PCB without components (though routing is allowed) to accommodate beads that prevent bending during tin furnace processes.
12. The orientation of components for reflow soldering should consider the PCB’s entry direction into the oven.
13. To ensure synchronized heating of both ends of chip components and SMD pins, orient the long axis of both end chip components perpendicularly to the conveyor belt of the reflow oven. The long axis of SMD components should run parallel to the conveyor, with the long axes of the two ends’ chip components and SMD components perpendicular to each other.
14. A well-designed PCB should consider both heat capacity uniformity and the arrangement of components. For PCBs larger than 200mm, adhere to these guidelines:
a) The long axes of end chip components should be perpendicular to the PCB’s long sides.
b) The long axis of SMD components should align with the PCB’s long side.
c) For double-sided assemblies, components on both sides should have identical orientations.
d) Arrange similar components in the same direction as much as possible to ensure consistency, facilitating mounting, soldering, and testing. For example, align the anode of electrolytic capacitors, anodes of diodes, single-pin ends of transistors, and first pins of integrated circuits in the same direction whenever feasible.
15. To prevent interlayer shorts during processing, maintain a minimum distance of 1.25mm between conductive patterns on the PCB’s inner and outer edges. If a ground wire occupies the PCB edge, components and printed wires should not be placed in structural areas, and no through holes should be located in the bottom pad area of SMD/SMC to prevent remelting of solder during wave soldering.
16. Component installation spacing must satisfy the manufacturability, testability, and maintainability requirements for SMT assembly.
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