Before starting the PCB design of a multilayer PCB, the designer must first determine the board structure based on several factors, including circuit size, board dimensions, and electromagnetic compatibility (EMC) requirements. The first step is to decide the number of layers the PCB will have. Once the number of layers is established, the designer must plan the placement of the inner electrical layers. This involves strategically distributing different signal types across these layers to optimize performance and signal integrity. The choice of laminate structure plays a critical role in achieving the desired EMC performance and is essential for minimizing electromagnetic interference (EMI). In this article, we will explore the key considerations and design principles for selecting the appropriate laminate structure in multilayer PCB design.
### Recommendations for PCB Stacking and SMT Process
#### 1. PCB Stacking Method
The **Foil Stacking Method** is highly recommended for PCB designs. This method provides better reliability and manufacturability due to its consistent layer alignment and lower risk of signal integrity issues. Foil stacking is preferred over other methods such as the conventional PP (Prepreg) stacking.
#### 2. Minimizing PP Sheets and CORE Models in Stacks
In a PCB stack, it is critical to minimize the number of **PP sheets** and **CORE models/types** used. It’s recommended that **no more than three PP stacks** should be used per layer. This limitation helps to maintain consistency in the overall PCB structure and reduces manufacturing complexity. The use of excessive PP layers can increase costs and processing time, so careful layer management is necessary.
#### 3. Thickness of PP Medium
The **thickness of PP (Prepreg) medium** between two layers should not exceed **21 MIL**. Thicker PP layers are challenging to process due to their rigidity and potential impact on signal integrity. Additionally, adding excessively thick prepreg can lead to a need for extra core boards, thus increasing the total PCB stack count, which raises both the manufacturing cost and time. Therefore, it is crucial to carefully choose the appropriate thickness to avoid unnecessary overhead.
#### 4. Copper Foil Thickness for PCB Layers
– **Outer Layers**: The top and bottom copper layers typically use **0.5 OZ** copper foil. This is standard for most PCBs as it strikes a balance between manufacturability and signal performance.
– **Inner Layers**: The inner layers usually employ **1 OZ** copper foil. This copper thickness is sufficient for most applications but may need adjustment based on current requirements and trace width. For example, **power boards** may require **2-3 OZ** copper for higher current handling, while **signal boards** typically use **1 OZ**. In cases where trace width is small, a thinner **1/3 OZ** copper foil may be selected to improve yield and reduce defects.
**Note**: Avoid using core boards with inconsistent copper thickness on both sides of the inner layer. This can lead to asymmetrical signal behavior and manufacturing complications.
#### 5. Symmetry in PCB Stack Design
The **distribution of wiring layers and plane layers** must be **symmetrical** about the centerline of the PCB stack. This symmetry ensures optimal signal integrity and minimizes issues such as skew and impedance mismatching. Key parameters that need to be symmetrical include:
– The number of layers
– Distance from the centerline
– Copper thickness of the wiring layers
Symmetry is crucial to maintaining consistent electrical characteristics across the PCB, particularly in multi-layer designs.
#### 6. Sufficient Margins for Line Width and Dielectric Thickness
When designing the **line width** and **dielectric thickness**, it is important to leave ample margins. Insufficient margins can lead to design issues, especially with **Signal Integrity (SI)** simulations, where trace widths and spacing may violate electrical performance standards. Always design with some safety margin to avoid potential simulation failures and performance degradation in the final product.
### Overview of SMT Process Components
**SMT (Surface-Mount Technology)** is a key assembly process for modern PCBs. The basic components of SMT include the following steps:
1. **Silk Screen Printing**:
– Function: Applies solder paste or adhesive to the PCB pads, preparing them for component placement.
– Equipment: **Screen printing machine**.
– Location: Typically at the beginning of the SMT production line.
2. **Dispensing**:
– Function: Drops glue at designated positions on the PCB, fixing components in place before soldering.
– Equipment: **Glue dispenser**.
– Location: Positioned at the start of the SMT line or behind inspection systems.
3. **Component Placement**:
– Function: Accurately places surface-mount components onto the PCB.
– Equipment: **Placement machine**.
– Location: Positioned after the silk screen in the SMT line.
4. **Curing**:
– Function: Melts the adhesive, securing the surface-mount components to the PCB.
– Equipment: **Curing oven**.
– Location: After the placement machine.
5. **Reflow Soldering**:
– Function: Melts solder paste to form permanent bonds between components and the PCB.
– Equipment: **Reflow oven**.
– Location: Positioned after the placement and curing stations.
6. **Cleaning**:
– Function: Removes residual flux and other contaminants from the assembled PCB, ensuring safety and reliability.
– Equipment: **Washing machine**.
– Location: May be online or offline, depending on the line configuration.
7. **Inspection**:
– Function: Checks the assembly quality, including soldering and placement integrity.
– Equipment: Tools such as magnifying glasses, microscopes, **ICT (In-Circuit Test)**, **AOI (Automatic Optical Inspection)**, **X-ray** systems, and functional testers.
– Location: Integrated into the production line, based on inspection needs.
8. **Rework**:
– Function: Corrects any faults detected during inspection, ensuring the final product meets quality standards.
– Equipment: **Soldering iron** and **rework stations**.
– Location: Rework stations can be located at various points in the production line.
### Conclusion
Effective PCB design and assembly require careful attention to both the stacking method and SMT processes. By following the guidelines for Foil stacking, minimizing PP layers, managing copper thickness, and ensuring symmetry in layer distribution, you can create a PCB that is both reliable and cost-effective. Additionally, proper SMT practices, from silk screening to rework, ensure that the final assembled product meets the required quality standards and performs well in its intended application. These principles, when adhered to, improve manufacturability, yield, and overall product performance.
### Recommendations for PCB Stacking and SMT Process
#### 1. PCB Stacking Method
The **Foil Stacking Method** is highly recommended for PCB designs. This method provides better reliability and manufacturability due to its consistent layer alignment and lower risk of signal integrity issues. Foil stacking is preferred over other methods such as the conventional PP (Prepreg) stacking.
#### 2. Minimizing PP Sheets and CORE Models in Stacks
In a PCB stack, it is critical to minimize the number of **PP sheets** and **CORE models/types** used. It’s recommended that **no more than three PP stacks** should be used per layer. This limitation helps to maintain consistency in the overall PCB structure and reduces manufacturing complexity. The use of excessive PP layers can increase costs and processing time, so careful layer management is necessary.
#### 3. Thickness of PP Medium
The **thickness of PP (Prepreg) medium** between two layers should not exceed **21 MIL**. Thicker PP layers are challenging to process due to their rigidity and potential impact on signal integrity. Additionally, adding excessively thick prepreg can lead to a need for extra core boards, thus increasing the total PCB stack count, which raises both the manufacturing cost and time. Therefore, it is crucial to carefully choose the appropriate thickness to avoid unnecessary overhead.
#### 4. Copper Foil Thickness for PCB Layers
– **Outer Layers**: The top and bottom copper layers typically use **0.5 OZ** copper foil. This is standard for most PCBs as it strikes a balance between manufacturability and signal performance.
– **Inner Layers**: The inner layers usually employ **1 OZ** copper foil. This copper thickness is sufficient for most applications but may need adjustment based on current requirements and trace width. For example, **power boards** may require **2-3 OZ** copper for higher current handling, while **signal boards** typically use **1 OZ**. In cases where trace width is small, a thinner **1/3 OZ** copper foil may be selected to improve yield and reduce defects.
**Note**: Avoid using core boards with inconsistent copper thickness on both sides of the inner layer. This can lead to asymmetrical signal behavior and manufacturing complications.
#### 5. Symmetry in PCB Stack Design
The **distribution of wiring layers and plane layers** must be **symmetrical** about the centerline of the PCB stack. This symmetry ensures optimal signal integrity and minimizes issues such as skew and impedance mismatching. Key parameters that need to be symmetrical include:
– The number of layers
– Distance from the centerline
– Copper thickness of the wiring layers
Symmetry is crucial to maintaining consistent electrical characteristics across the PCB, particularly in multi-layer designs.
#### 6. Sufficient Margins for Line Width and Dielectric Thickness
When designing the **line width** and **dielectric thickness**, it is important to leave ample margins. Insufficient margins can lead to design issues, especially with **Signal Integrity (SI)** simulations, where trace widths and spacing may violate electrical performance standards. Always design with some safety margin to avoid potential simulation failures and performance degradation in the final product.
### Overview of SMT Process Components
**SMT (Surface-Mount Technology)** is a key assembly process for modern PCBs. The basic components of SMT include the following steps:
1. **Silk Screen Printing**:
– Function: Applies solder paste or adhesive to the PCB pads, preparing them for component placement.
– Equipment: **Screen printing machine**.
– Location: Typically at the beginning of the SMT production line.
2. **Dispensing**:
– Function: Drops glue at designated positions on the PCB, fixing components in place before soldering.
– Equipment: **Glue dispenser**.
– Location: Positioned at the start of the SMT line or behind inspection systems.
3. **Component Placement**:
– Function: Accurately places surface-mount components onto the PCB.
– Equipment: **Placement machine**.
– Location: Positioned after the silk screen in the SMT line.
4. **Curing**:
– Function: Melts the adhesive, securing the surface-mount components to the PCB.
– Equipment: **Curing oven**.
– Location: After the placement machine.
5. **Reflow Soldering**:
– Function: Melts solder paste to form permanent bonds between components and the PCB.
– Equipment: **Reflow oven**.
– Location: Positioned after the placement and curing stations.
6. **Cleaning**:
– Function: Removes residual flux and other contaminants from the assembled PCB, ensuring safety and reliability.
– Equipment: **Washing machine**.
– Location: May be online or offline, depending on the line configuration.
7. **Inspection**:
– Function: Checks the assembly quality, including soldering and placement integrity.
– Equipment: Tools such as magnifying glasses, microscopes, **ICT (In-Circuit Test)**, **AOI (Automatic Optical Inspection)**, **X-ray** systems, and functional testers.
– Location: Integrated into the production line, based on inspection needs.
8. **Rework**:
– Function: Corrects any faults detected during inspection, ensuring the final product meets quality standards.
– Equipment: **Soldering iron** and **rework stations**.
– Location: Rework stations can be located at various points in the production line.
### Conclusion
Effective PCB design and assembly require careful attention to both the stacking method and SMT processes. By following the guidelines for Foil stacking, minimizing PP layers, managing copper thickness, and ensuring symmetry in layer distribution, you can create a PCB that is both reliable and cost-effective. Additionally, proper SMT practices, from silk screening to rework, ensure that the final assembled product meets the required quality standards and performs well in its intended application. These principles, when adhered to, improve manufacturability, yield, and overall product performance.
