Why Running Traces Under SMT Components on a PCB is Considered a Bad Practice
When designing a printed circuit board (PCB), it is crucial to follow established best practices for optimal performance and reliability. One such practice is avoiding the placement of traces under SMT (Surface Mount Technology) components. This article explores the reasons behind this recommendation, highlighting the implications on thermal management, signal integrity, manufacturability, repairability, and design complexity.
Thermal Management and Heat Dissipation
One of the primary reasons for avoiding traces under SMT components is the impact on thermal management. SMT components can generate heat during operation, and if traces are placed underneath, they can trap this heat. This can lead to a significant increase in the component's temperature, thereby affecting its performance and potentially reducing its lifespan. Proper thermal isolation is essential to ensure that different components with varying thermal profiles do not create hotspots, leading to uneven thermal distribution. Effective heat dissipation is crucial for maintaining component reliability and performance.
Signal Integrity and Crosstalk
Another critical factor to consider is the impact on signal integrity. When traces run directly under SMT components, the electromagnetic fields from adjacent traces can couple with these bottom traces, leading to crosstalk. This interference can cause signal degradation, especially in high-speed signals, which can result in reflections and other signal integrity issues. Proper impedance control is necessary to mitigate these effects, and running traces under components can make this more challenging.
Manufacturing Process and Soldering Issues
The manufacturing process is another area where traces running under SMT components can pose problems. Reflow soldering is a common method used in PCB assembly, but the presence of bottom traces can complicate this process. It can lead to incomplete solder joints or solder bridging between pads, which can affect the overall quality of the solder. Additionally, traces under components can be difficult to inspect visually, making it challenging to detect manufacturing defects or issues during visual checks. This can lead to potential quality control issues and increased risk of field failures.
Repairability and Troubleshooting
From a repairability standpoint, running traces under SMT components can complicate the process. If a component needs to be replaced, the bottom traces can make it difficult to desolder the component without damaging the traces themselves. This can lead to longer repair times and increased costs. Furthermore, diagnosing issues can be more challenging when traces are hidden under components, which can extend troubleshooting times and increase the risk of misdiagnosis or incorrect repair steps.
Design Complexity and Layer Management
Finally, the design process can be more complex when traces are placed under SMT components. Managing additional layers in the PCB stack-up to accommodate these traces can increase the complexity and cost of the design. It can also limit routing options, making it harder to achieve an optimal layout, particularly in dense designs. Keeping traces clear of component footprints can help simplify the design process, allowing for more straightforward routing and easier manufacturing and repair.
While it is technically possible to run traces under SMT components, designers often avoid this practice to enhance thermal performance, maintain signal integrity, simplify manufacturing processes, and facilitate easier repairs and troubleshooting. By adhering to these best practices, designers can create more reliable and efficient PCBs that meet the needs of modern electronic systems.