### 1. **Pad Overlap**
**Problem Description:**
– Pad overlap, particularly in non-surface-mount pads, occurs when two pads share the same area. This leads to the overlapping of drilling holes during the fabrication process.
– The most significant risk is that the drill bit may break when attempting to drill multiple holes in a single location, leading to damaged vias or pads.
– On multilayer PCBs, hole overlap can also occur when one hole is designed as an isolation pad, and another serves as a connection via (flower pad). In this case, the resultant film will incorrectly display an isolation pad, which may cause the board to be scrapped during manufacturing.
**Solution:**
– Always verify the spatial layout of pads, especially in multi-layer designs, to ensure that holes do not overlap.
– Use design rule checks (DRC) to confirm that there is no overlap, especially in the drilling area. Modern PCB design software can automatically detect and flag such issues.
### 2. **Misuse of Graphics Layers**
**Problem Description:**
– Designers sometimes add unnecessary connections to the graphics layers, which can create confusion. For example, a four-layer PCB may appear to have more than five layers because extraneous connections are visible on the graphics layers.
– Some designers, in an attempt to simplify the design process, might use the “Board” layer in Protel software to draw lines, marking connections without a clear distinction between electrical and graphical information. As a result, the “Board” layer lines may not be selected during the light drawing process, causing missing connections. In other cases, the marking lines could inadvertently cause short circuits if they overlap critical traces.
**Solution:**
– Ensure that the graphics layer is used strictly for non-electrical elements such as board outlines or layer definitions. Keep the design data for electrical traces and vias on the appropriate signal layers.
– During the design process, carefully manage layer visibility to avoid unnecessary connections being drawn on the graphics layers.
– Always double-check that the correct layers are selected before exporting the design for fabrication to avoid broken or short-circuited connections.
### Conclusion:
By addressing common design issues such as pad overlap and misuse of graphics layers, PCB designers can significantly reduce errors in the manufacturing process. As PCB design continues to evolve, experienced designers know the value of running design rule checks and maintaining clear, accurate layer management. By following these best practices, you can minimize the risk of costly mistakes and ensure a smooth transition from design to production.
**Common PCB Design Issues and Best Practices**
In PCB design, certain mistakes and improper practices can significantly affect both manufacturing and functionality. Below are key issues that frequently occur during the design process, along with recommendations to avoid these problems.
### 1. Violations of Conventional Design Practices
When components are placed incorrectly—for instance, surface-mount devices (SMDs) on the bottom layer and welding pads on the top—it can create substantial inconvenience. This not only complicates the manufacturing process but also disrupts the overall functionality of the board.
### 2. Random Placement of Text and Symbols
The placement of text and symbols on the PCB should follow systematic guidelines to avoid issues during both manufacturing and inspection:
1. **SMD Pads Overlapping Text**: SMD pads placed over text can make continuity testing and component soldering difficult. It’s essential to place these pads without interfering with readable text areas.
2. **Size of Characters**: Text that is too small can lead to difficulties in screen printing, while overly large text can overlap, making it hard to distinguish individual characters. Maintaining an optimal size is crucial for clarity and ease of production.
### 3. Single-Sided Pad Aperture Design
Single-sided pads typically do not require drilling. If drilling is necessary, the aperture should be marked as “zero diameter.” Otherwise, the drilling data will generate unnecessary hole coordinates, causing confusion during manufacturing.
1. **Drilled Single-Sided Pads**: If drilling is required for single-sided pads, they must be explicitly marked to avoid confusion during production. This will ensure the correct pad is processed without issues.
### 4. Use of Filler Blocks in Pad Design
Filler blocks are often used in designs to pass Design Rule Checks (DRC). However, this approach is not ideal for manufacturing:
– **Solder Mask Generation Issues**: Pads with filler blocks cannot directly generate solder mask data. When the solder resist is applied, these areas may get blocked by the resist, making it difficult to solder components correctly.
### 5. Ground and Power Planes as “Flower Pads”
Designing power supply planes as “flower pads” or using isolated ground layers can cause connectivity issues in the PCB. If not properly connected, this results in unreliable signal transmission or power delivery.
– **Avoid Isolation Gaps**: Care should be taken when drawing isolation lines between power supplies or ground connections to ensure no gaps that could cause short circuits. Properly blocking the connection areas is also essential for maintaining clear signal paths.
### 6. Unclear Processing Levels
Clearly defining processing levels for your design is essential, especially for multi-layer boards:
1. **Single-Sided Boards**: On single-sided boards, ensure that the TOP layer is clearly specified. Failing to do so could result in difficulties during assembly, as manufacturers may not know which side to mount components.
2. **Multi-Layer Boards**: In a multi-layer board, proper layer stack-up should be defined (e.g., TOP, Mid1, Mid2, Bottom). Any ambiguity in layer order can lead to confusion during fabrication, requiring detailed clarification in the design files.
### 7. Overuse of Filler Blocks or Thin Lines
Using too many filler blocks or excessively thin lines to create pads and traces can lead to various issues:
1. **Lost Gerber Data**: Filler blocks drawn with thin lines can lead to incomplete or lost Gerber data, causing potential failures in manufacturing or assembly.
2. **Processing Complications**: Drawing filler blocks individually with thin lines generates large amounts of light drawing data, which increases the complexity of data processing and can slow down the overall workflow.
### 8. Inadequate Surface-Mount Device (SMD) Pad Size
When designing SMD pads, especially in dense layouts, the pad size should be adequate to support testing and soldering:
– **Continuity Test Issues**: If the pads are too small, it will be challenging to install test pins or perform continuity tests. This could lead to issues during both the manufacturing and quality inspection phases.
### 9. Excessively Small Grid Spacing in Large Areas
When designing grids for large-area copper fills, ensure that the spacing between adjacent lines is sufficient:
– **Risk of Film Breakage**: If the spacing is too small (less than 0.3mm), the copper fill could cause broken films during the image transfer process, leading to potential wiring failures.
### 10. Insufficient Distance Between Large Copper Areas and Outer Frame
It is important to maintain a minimum gap between large copper areas and the outer frame of the PCB:
– **Warping and Solder Mask Issues**: A gap of at least 0.2mm should be maintained to prevent the copper from warping during milling and to avoid issues with the solder resist peeling off.
### 11. Undefined or Inconsistent Outline Design
Inconsistent or unclear contour lines can create confusion during PCB fabrication:
– **Overlap of Contour Layers**: Contour lines such as Keep-out, Board outline, and Top overlay should be aligned. Any misalignment can make it difficult for manufacturers to determine the correct cutting path, leading to errors in the final product.
### 12. Uneven Pattern Design
Uneven design patterns, especially in areas with copper plating, can lead to inconsistent plating quality:
– **Plating Issues**: If pattern plating is not uniform, the resulting plating layer may be inconsistent, affecting the overall electrical and mechanical performance of the PCB.
### 13. Copper Area Size and Grid Use
For large copper areas, the use of grid lines can prevent potential issues during the soldering process:
– **Blistering During SMT**: Without grid lines, large copper areas may cause thermal issues during surface-mount technology (SMT) assembly, leading to blistering or other defects.
**Conclusion**
By understanding these common design pitfalls and implementing best practices, PCB designers can ensure a smoother manufacturing process, reduce errors, and improve the overall quality of their designs. Each design choice, from pad size to copper area management, plays a critical role in the functionality and reliability of the final product. Attention to detail, thorough testing, and clear communication with the manufacturing team are all essential steps in creating a successful PCB design.
