**PCB Factory Basic Technical Requirements for Copper Plating**
1. **Must Have Good Mechanical Properties**
The mechanical properties of the electroplated layer in the PCB factory primarily refer to toughness, which is a concept in metallurgy. Toughness is determined by relative elongation and tensile strength. The toughness (Tou) specified by metal science is given by Tou = ε * σ, where ε is relative elongation and σ is tensile strength. Relative elongation (ε) is calculated as ε = (L – L₀) / L₀ * 100%, representing the metal’s deformability, while tensile strength is the tensile force per unit cross-section, indicating resistance to deformation. Toughness combines these two quantities, indicating the total energy required for material failure. Generally, relative elongation should be no less than 10%, and tensile strength should range from 20-50 kg/m² to ensure that, after leveling the PCB with hot air or during electrical assembly, wave soldering will not cause Z-direction fractures in the copper-plated layer due to the expansion coefficient differences between the epoxy base material and the copper plating.
2. **The Ratio of the Thickness of the Copper Plating Layer on the Board Surface (Ts) to the Thickness of the Copper Plating Layer on the Hole Wall (Th) Should Be Close to 1:1**
Practically, ensuring that both the plate surface and the plating layer in the holes have adequate strength and conductivity requires that the plating solution exhibits good dispersion ability. If the solution does not disperse well, meeting the national standard for the thickness of the hole wall plating layer will necessitate prolonged plating times. This approach not only wastes time and raw materials but also adversely affects subsequent imaging accuracy.
3. **The Plating Layer Should Be Firmly Bonded to the Substrate**
A strong bond between the plating layer and the substrate is crucial. If the bond is weak, the plating layer may blister and peel, potentially leading to PCB scrapping in severe cases.
4. **The Plating Layer Should Have Good Conductivity**
The circuit board relies primarily on the electroplated copper layer for electrical conduction. To ensure good conductivity, the purity of the plating layer must be high, and the inclusion of impurities should be minimized. Impurities mainly originate from additives in the plating solution and certain components in the anode.
5. The copper plating layer should be uniform, meticulous, and exhibit a good appearance.
The design of PCB A/D partition and ground division
When connecting the analog ground and digital ground pins of the A/D converter, most A/D converter manufacturers recommend connecting these grounds through the shortest low-impedance path. This is because many A/D converter chips do not internally connect the analog and digital grounds, necessitating external connections. Any impedance connected to the digital ground can lead to increased digital noise coupling into the analog circuits within the IC. Therefore, both the analog ground (AGND) and digital ground (DGND) pins of the A/D converter should be connected to the analog ground.
If the system contains only one A/D converter, these issues are easily managed by separating the grounds and connecting the analog and digital grounds together under the A/D converter. However, with multiple A/D converters, connecting the analog and digital grounds separately under each converter can lead to multiple connection points, which can render the isolation between the two grounds ineffective. In this case, the optimal approach is to start with a unified ground and then partition it into analog and digital sections. This method not only adheres to IC device manufacturers’ requirements for low-impedance connections but also avoids creating loop or dipole antennas.
Separate ground design Some suggest separating the digital and analog grounds on a mixed-signal circuit board to achieve isolation between them. While feasible, this method can introduce potential problems, especially in complex large-scale systems. The primary issue is routing across the division gap, which can significantly increase electromagnetic radiation and signal crosstalk. Common PCB design problems include signal lines crossing divided grounds or power supplies, leading to EMI issues. When designing mixed-signal PCBs, the following points are crucial:
1) Divide the PCB into independent analog and digital sections.
2) Place the A/D converter in the appropriate partitions.
3) Avoid dividing the ground; use a continuous ground plane under both the analog and digital sections.
4) Route digital signals exclusively within the digital section of the PCB.
5) Route analog signals exclusively within the analog section of the PCB.
6) Implement separate analog and digital power supplies.
7) Avoid routing across gaps between divided power planes.
8) Route any signal lines that must span power plane gaps on layers close to a large ground area.
9) Analyze the actual path and method of return ground current flow.
10) Adhere to proper wiring rules.
1. **Must Have Good Mechanical Properties**
The mechanical properties of the electroplated layer in the PCB factory primarily refer to toughness, which is a concept in metallurgy. Toughness is determined by relative elongation and tensile strength. The toughness (Tou) specified by metal science is given by Tou = ε * σ, where ε is relative elongation and σ is tensile strength. Relative elongation (ε) is calculated as ε = (L – L₀) / L₀ * 100%, representing the metal’s deformability, while tensile strength is the tensile force per unit cross-section, indicating resistance to deformation. Toughness combines these two quantities, indicating the total energy required for material failure. Generally, relative elongation should be no less than 10%, and tensile strength should range from 20-50 kg/m² to ensure that, after leveling the PCB with hot air or during electrical assembly, wave soldering will not cause Z-direction fractures in the copper-plated layer due to the expansion coefficient differences between the epoxy base material and the copper plating.
2. **The Ratio of the Thickness of the Copper Plating Layer on the Board Surface (Ts) to the Thickness of the Copper Plating Layer on the Hole Wall (Th) Should Be Close to 1:1**
Practically, ensuring that both the plate surface and the plating layer in the holes have adequate strength and conductivity requires that the plating solution exhibits good dispersion ability. If the solution does not disperse well, meeting the national standard for the thickness of the hole wall plating layer will necessitate prolonged plating times. This approach not only wastes time and raw materials but also adversely affects subsequent imaging accuracy.
3. **The Plating Layer Should Be Firmly Bonded to the Substrate**
A strong bond between the plating layer and the substrate is crucial. If the bond is weak, the plating layer may blister and peel, potentially leading to PCB scrapping in severe cases.
4. **The Plating Layer Should Have Good Conductivity**
The circuit board relies primarily on the electroplated copper layer for electrical conduction. To ensure good conductivity, the purity of the plating layer must be high, and the inclusion of impurities should be minimized. Impurities mainly originate from additives in the plating solution and certain components in the anode.
5. The copper plating layer should be uniform, meticulous, and exhibit a good appearance.
The design of PCB A/D partition and ground division
When connecting the analog ground and digital ground pins of the A/D converter, most A/D converter manufacturers recommend connecting these grounds through the shortest low-impedance path. This is because many A/D converter chips do not internally connect the analog and digital grounds, necessitating external connections. Any impedance connected to the digital ground can lead to increased digital noise coupling into the analog circuits within the IC. Therefore, both the analog ground (AGND) and digital ground (DGND) pins of the A/D converter should be connected to the analog ground.
If the system contains only one A/D converter, these issues are easily managed by separating the grounds and connecting the analog and digital grounds together under the A/D converter. However, with multiple A/D converters, connecting the analog and digital grounds separately under each converter can lead to multiple connection points, which can render the isolation between the two grounds ineffective. In this case, the optimal approach is to start with a unified ground and then partition it into analog and digital sections. This method not only adheres to IC device manufacturers’ requirements for low-impedance connections but also avoids creating loop or dipole antennas.
Separate ground design Some suggest separating the digital and analog grounds on a mixed-signal circuit board to achieve isolation between them. While feasible, this method can introduce potential problems, especially in complex large-scale systems. The primary issue is routing across the division gap, which can significantly increase electromagnetic radiation and signal crosstalk. Common PCB design problems include signal lines crossing divided grounds or power supplies, leading to EMI issues. When designing mixed-signal PCBs, the following points are crucial:
1) Divide the PCB into independent analog and digital sections.
2) Place the A/D converter in the appropriate partitions.
3) Avoid dividing the ground; use a continuous ground plane under both the analog and digital sections.
4) Route digital signals exclusively within the digital section of the PCB.
5) Route analog signals exclusively within the analog section of the PCB.
6) Implement separate analog and digital power supplies.
7) Avoid routing across gaps between divided power planes.
8) Route any signal lines that must span power plane gaps on layers close to a large ground area.
9) Analyze the actual path and method of return ground current flow.
10) Adhere to proper wiring rules.
