Introduction
The SMT process utilizes solder or solder paste to establish a mechanical and electrical connection between components and the PCB board. Its key advantages include small size, light weight, and excellent interconnectivity. High-frequency circuits benefit from improved performance and reduced parasitic impedance, while also exhibiting good shock and vibration resistance. By eliminating the need for leads to pass through the circuit board, the SMT process helps prevent signal interference, ultimately enhancing the circuit’s signal-to-noise ratio.
To assess the efficacy of the SMT process, the correct formation of solder joints is paramount. This requires adequately designed pad sizes for components on the PCB board. Furthermore, the density of components must be carefully considered during PCB layout to fulfill test point requirements. Circuit board design is executed through DFM (Design for Manufacturability), a critical aspect of concurrent engineering (CE). DFM ensures manufacturability and detectability considerations are integrated from the outset of product design, making it an indispensable tool for successful circuit board development from design to manufacturing.
PCB Board Material Selection
There are two main types of printed circuit board substrates: organic and inorganic materials, with organic materials being the most commonly used. Different substrates are used for different layers of PCB boards. For instance, prefabricated composite materials are used for 3 to 4-layer boards, while glass-epoxy materials are predominantly used for double-sided boards. During lead-free electronic assembly, the temperature increase can cause the PCB to bend more when heated. Therefore, in surface mount technology (SMT), it is important to use boards with minimal curvature, such as FR-4 and other substrates. The selection of materials should consider the expansion coefficient, especially for components larger than 3.2×1.6mm. PCB boards used in surface assembly require high thermal conductivity, heat resistance (150℃ for 60 minutes), solderability (260℃ for 10 seconds), high copper foil adhesion strength (above 1.5×104Pa), bending strength (25×104Pa), high electrical conductivity, low dielectric constant, good punch ability (accuracy ±0.02mm), compatibility with cleaning agents, and a smooth, flat appearance without warping, cracks, scars, or rust spots. The thickness of PCB boards can vary from 0.5mm to 6.4mm. Thicknesses like 0.7mm and 1.5mm are used for double-sided boards with gold fingers, while 1.8mm and 3.0mm are non-standard sizes. The size of the printed circuit board should not be less than 250 × 200mm, with an ideal size generally ranging from 250 × 200mm to 350 × 250mm.
PCB Board Via Holes and Component Layout
Via Layout
1) Avoid placing vias within or less than 0.6mm of the surface mount pad.
2) Component pads without external pins should not have through-holes between the pads to ensure cleaning quality.
3) Consider the spacing of probes during automatic online testing when using vias for test support.
4) Ensure appropriate gap between via hole diameter and component lead diameter to facilitate soldering.
5) Avoid connecting vias and pads to prevent solder loss or thermal isolation. If connection is needed, use thin wires and keep a distance between via holes and pad edges.
Component Layout
1) Components should be evenly distributed on the board for uniform heat and space distribution.
2) Align components in the same direction to reduce welding issues.
3) Maintain a spacing of at least 0.5mm between components to avoid temperature compensation issues.
4) Provide maintenance and testing space around large devices like PLCC, SOIC, QFP, etc.
5) Distribute power components separately to enhance ventilation and heat dissipation.
6) Avoid placing valuable components in high-stress areas to reduce the risk of cracks or breakages.
Component Orientation
1) Arrange passive components parallel to each other.
2) Align passive components and SOIC axes perpendicular to each other.
3) Align passive components with reflow soldering direction.
4) Set tin stealing pads for multi-pin components to prevent bridging.
5) Arrange components of similar types in the same direction for easier placement, inspection, and soldering.
6) Consider the adaptability of component pins and weight during different assembly processes to prevent part damage or poor soldering.
PCB Circuit and Pad Design
Circuit Process Design
1) Maintain a clamping edge of 5mm in the PCB process.
2) Avoid connecting wires to pads at angles and connect wires perpendicular to component pads.
3) Reduce width of wire connections, with the width around 0.4mm or half the pad width, to control heat dissipation and solder flow.
4) Vary trace width and spacing based on etching technology and assembly method.
5) Avoid interconnection lines passing through adjacent pads.
6) Consider different lead widths for insertion, mount, and fine pitch assembly methods.
7) Avoid short connections between pin pads of multi-pin components.
8) Ground square-shaped pads of bare chips and evenly plate with gold for reliable bonding.
Circuit Electrical Design
1) Adhere to wire-passing principle within pin spacing for low, medium, and high density requirements.
2) Ensure consistent line width for impedance matching.
3) Use short lines, especially for small signal circuits, to reduce resistance and interference.
4) Separate power, ground, and signal layers to reduce interference.
5) Design large-area power and ground layers adjacent to each other to form a capacitor for filtering.
Pad Design
– Follow specific component specifications when designing pads to ensure reliability and prevent process defects.
– Design pad size based on component package shape, pins, and special requirements.
– Follow industry standards like IPC-SM-782, IPC-7095, etc., for common pad designs.
– Consider pad symmetry, size calculations, and solder joint reliability during pad design.
– Design pads appropriately to prevent non-wetting joints and ensure good soldering.
– Ensure thermal isolation for pads connected to large conductive areas and provide dummy pads for wave soldering.
Fiducial Mark Production Requirements
– Use square, circle, triangle, or cross shapes for datum marks with a diameter of 0.5mm to 3mm.
– Maintain consistent mark size on the same board, at least 5mm away from the board edge.
– Use bare copper or plated marks with specified thickness and surface flatness.
– Ensure symmetry and space around reference point marks for accurate alignment.
– Use stamp plate or V-groove separation technology for jigsaw PCBs to improve equipment utilization.
The SMT process utilizes solder or solder paste to establish a mechanical and electrical connection between components and the PCB board. Its key advantages include small size, light weight, and excellent interconnectivity. High-frequency circuits benefit from improved performance and reduced parasitic impedance, while also exhibiting good shock and vibration resistance. By eliminating the need for leads to pass through the circuit board, the SMT process helps prevent signal interference, ultimately enhancing the circuit’s signal-to-noise ratio.
To assess the efficacy of the SMT process, the correct formation of solder joints is paramount. This requires adequately designed pad sizes for components on the PCB board. Furthermore, the density of components must be carefully considered during PCB layout to fulfill test point requirements. Circuit board design is executed through DFM (Design for Manufacturability), a critical aspect of concurrent engineering (CE). DFM ensures manufacturability and detectability considerations are integrated from the outset of product design, making it an indispensable tool for successful circuit board development from design to manufacturing.
PCB Board Material Selection
There are two main types of printed circuit board substrates: organic and inorganic materials, with organic materials being the most commonly used. Different substrates are used for different layers of PCB boards. For instance, prefabricated composite materials are used for 3 to 4-layer boards, while glass-epoxy materials are predominantly used for double-sided boards. During lead-free electronic assembly, the temperature increase can cause the PCB to bend more when heated. Therefore, in surface mount technology (SMT), it is important to use boards with minimal curvature, such as FR-4 and other substrates. The selection of materials should consider the expansion coefficient, especially for components larger than 3.2×1.6mm. PCB boards used in surface assembly require high thermal conductivity, heat resistance (150℃ for 60 minutes), solderability (260℃ for 10 seconds), high copper foil adhesion strength (above 1.5×104Pa), bending strength (25×104Pa), high electrical conductivity, low dielectric constant, good punch ability (accuracy ±0.02mm), compatibility with cleaning agents, and a smooth, flat appearance without warping, cracks, scars, or rust spots. The thickness of PCB boards can vary from 0.5mm to 6.4mm. Thicknesses like 0.7mm and 1.5mm are used for double-sided boards with gold fingers, while 1.8mm and 3.0mm are non-standard sizes. The size of the printed circuit board should not be less than 250 × 200mm, with an ideal size generally ranging from 250 × 200mm to 350 × 250mm.
PCB Board Via Holes and Component Layout
Via Layout
1) Avoid placing vias within or less than 0.6mm of the surface mount pad.
2) Component pads without external pins should not have through-holes between the pads to ensure cleaning quality.
3) Consider the spacing of probes during automatic online testing when using vias for test support.
4) Ensure appropriate gap between via hole diameter and component lead diameter to facilitate soldering.
5) Avoid connecting vias and pads to prevent solder loss or thermal isolation. If connection is needed, use thin wires and keep a distance between via holes and pad edges.
Component Layout
1) Components should be evenly distributed on the board for uniform heat and space distribution.
2) Align components in the same direction to reduce welding issues.
3) Maintain a spacing of at least 0.5mm between components to avoid temperature compensation issues.
4) Provide maintenance and testing space around large devices like PLCC, SOIC, QFP, etc.
5) Distribute power components separately to enhance ventilation and heat dissipation.
6) Avoid placing valuable components in high-stress areas to reduce the risk of cracks or breakages.
Component Orientation
1) Arrange passive components parallel to each other.
2) Align passive components and SOIC axes perpendicular to each other.
3) Align passive components with reflow soldering direction.
4) Set tin stealing pads for multi-pin components to prevent bridging.
5) Arrange components of similar types in the same direction for easier placement, inspection, and soldering.
6) Consider the adaptability of component pins and weight during different assembly processes to prevent part damage or poor soldering.
PCB Circuit and Pad Design
Circuit Process Design
1) Maintain a clamping edge of 5mm in the PCB process.
2) Avoid connecting wires to pads at angles and connect wires perpendicular to component pads.
3) Reduce width of wire connections, with the width around 0.4mm or half the pad width, to control heat dissipation and solder flow.
4) Vary trace width and spacing based on etching technology and assembly method.
5) Avoid interconnection lines passing through adjacent pads.
6) Consider different lead widths for insertion, mount, and fine pitch assembly methods.
7) Avoid short connections between pin pads of multi-pin components.
8) Ground square-shaped pads of bare chips and evenly plate with gold for reliable bonding.
Circuit Electrical Design
1) Adhere to wire-passing principle within pin spacing for low, medium, and high density requirements.
2) Ensure consistent line width for impedance matching.
3) Use short lines, especially for small signal circuits, to reduce resistance and interference.
4) Separate power, ground, and signal layers to reduce interference.
5) Design large-area power and ground layers adjacent to each other to form a capacitor for filtering.
Pad Design
– Follow specific component specifications when designing pads to ensure reliability and prevent process defects.
– Design pad size based on component package shape, pins, and special requirements.
– Follow industry standards like IPC-SM-782, IPC-7095, etc., for common pad designs.
– Consider pad symmetry, size calculations, and solder joint reliability during pad design.
– Design pads appropriately to prevent non-wetting joints and ensure good soldering.
– Ensure thermal isolation for pads connected to large conductive areas and provide dummy pads for wave soldering.
Fiducial Mark Production Requirements
– Use square, circle, triangle, or cross shapes for datum marks with a diameter of 0.5mm to 3mm.
– Maintain consistent mark size on the same board, at least 5mm away from the board edge.
– Use bare copper or plated marks with specified thickness and surface flatness.
– Ensure symmetry and space around reference point marks for accurate alignment.
– Use stamp plate or V-groove separation technology for jigsaw PCBs to improve equipment utilization.