5 Myths about PCB Design-Busted!

 

Introduction

Printed circuit boards (PCBs) are an essential component of most electronic devices. They provide the mechanical support and electrical connections between components. While PCB design principles are well established, there are still some common myths and misconceptions floating around. This article will examine and debunk five of the most prevalent PCB design myths.

Myth #1: More layers are always better

Using more layers in a PCB allows for increased routing capability and flexibility. However, this comes at increased cost. More layers mean higher fabrication charges, more complex design issues, and greater risk of errors. So when is using more layers justified?

Fact: The optimal number of layers depends on the design requirements and constraints. Complex, high-density designs with many components and tight routing tolerances benefit from more layers. But many simpler, low component count designs can be implemented efficiently with just two to four layers. Blind and buried vias help minimize layer count without compromising performance. The key is to use the minimum number of layers required to route the board successfully within design rules. Just adding more layers arbitrarily is unnecessary and costly over-design.

Myth #2: Tighter trace spacing and widths are better

It seems intuitive that packing traces more densely and using the thinnest widths allowed by your fabrication process would be desirable. But this is not always true. There are several trade-offs involved.

Fact: Thinner trace widths do allow for higher component density and more available routing channels. But they also increase current density, resistance, and risk of overheating. Minimum spacing reduces crosstalk and capacitive coupling between traces but increases fabrication cost. It also limits the effectiveness of impedance control techniques like distance tuning. The optimal trace geometry balances these conflicting constraints for the particular design. Blind adherence to the thinnest and closest possible traces often leads to over-designed boards that are harder to manufacture.

Myth #3: 90 degree angles should always be avoided

Right angle trace bends are often singled out as poor design practice that should be avoided. It is true that they concentrate current density versus curved bends. But there are definitely situations where 90 degree angles are perfectly acceptable and even preferable.

Fact: For low frequency digital signals with moderate current, right angle bends perform just fine and allow for more efficient routing. They are also easier to manufacture than sharp curved traces. For higher frequency, higher power analog or RF traces, right angles can cause problems like impedance discontinuities and EMI issues. But even then, they may be unavoidable in dense designs. Intelligently weighing the trade-offs for each net rather than blind avoidance of 90 degree bends is key.

Myth #4: Vias should always be avoided when possible

Vias do disrupt signals passing through the different board layers. But just like with right angle traces, blind via avoidance at all costs is inefficient and unnecessary in many cases.

Fact: Well-designed vias with proper anti-pad dimensions behave similarly to short transmission line segments when used for digital signals. RF and other high frequency signals are more impacted, but even then intelligently chosen vias optimized for impedance matching can work well. Vias are also essential for routing flexibility in complex designs. The key is to understand their impact and model their effects with proper CAD tools when used for critical signals. Avoiding every via possible often leads to overly complex routing.

Myth #5: Ground planes must be unbroken

Ground planes provide low impedance return paths, shielding, and thermal heatsinking. So it would seem that contiguous ground planes are ideal and breaks should be avoided. But some exceptions apply here as well.

Fact: For low to moderate frequency digital circuits, breaks in the ground plane cause negligible impact. Slotting the ground plane to route signals on inner layers is a common technique to simplify routing. Cuts in the ground plane also help isolate noise between sections of analogue circuits. Simulations and measurements should be used to verify that any splits or slots do not compromise impedance control or coupling requirements. Blind adherence to contiguous ground planes often leads to inefficient layer usage and routing difficulties.

Summary

This examination of common PCB design myths illustrates that blindly following simplistic rules of thumb often leads to inefficient and over-designed boards. Instead, the savvy designer analyzes the specific requirements and trade-offs for each design and nets and uses good engineering judgment to make intelligent decisions, avoiding both under and over-design. Rigorously adhering to principles of impedance control, proper bypassing, and minimizing coupling for critical signals, while pragmatically managing density and complexity are key PCB design skills.

Frequently Asked Questions

Question 1: Are there any general rules of thumb to follow in PCB design?

Answer: While strict adherence to simplistic rules can lead to sub-optimal design as discussed above, some high-level guidelines are still useful:

• Match trace widths and spacing to target impedance values for critical signals
• Use 45 degree angled traces over 90 degree bends where possible for RF and high speed signals
• Limit trace length differences between critical signals to minimize skew
• Provide proper power supply bypassing and decoupling
• Use ground planes for shielding and current return paths
• Maintain at least 3X dielectric spacing between traces for typical low speed signals
• Minimize the number of vias per net, especially for high frequency traces

Question 2: How can I determine the optimal number of layers for my PCB design?

Answer: The ideal layer count depends on many factors:

• Component density - more components require more routing layers
• Signal trace routing requirements - high frequency signals require more layers to maintain isolation and impedance control
• Routing constraints - tight spacing requirements demand more layers
• Placement constraints - components needing isolation or shielding require partitions o more layers
• Cost - additional layers substantially increase fabrication cost

Analyze these factors in the context of your specific design requirements to right-size the layer count. Start with the minimum feasible and only add more if routing problems arise.

Question 3: When should I use blind and buried vias?

Answer: Blind and buried vias help reduce overall layer count by providing vertical routing without consuming outer layer space. Specific situations where they help:

• Routing between non-adjacent layers when few layers are available
• Accessing internal power/ground planes from outer layers
• Component pads requiring inner layer connections
• Maintaining component placement on outer layers while routing signals internally
• Minimizing via stubs by only penetrating the minimum set of layers

Question 4: How can I determine the optimal trace widths for my design?

Answer: The key considerations for trace width selection are:

• Current carrying capacity - wider for higher current levels
• Impedance - match widths to target impedance value
• Manufacturability - stay above minimum fabrication limit
• Power efficiency - minimize width to reduce conduction losses
• Density - minimize spacing between traces

Use impedance calculators to determine appropriate widths and simulate worst-case thermal rise to check current capacity. Manufacturing capabilities and density requirements provide practical lower and upper bounds.

Question 5: When should I use 45 degree traces instead of 90 degree angles?

Answer: For low-speed digital signals, 90 degree bends are generally acceptable. But 45 angles help avoid problems for:

• High frequency analog or RF signals - avoids impedance discontinuities
• Wide traces carrying significant current - reduces current crowding
• Traces subject to high mechanical vibration - reduces flex cracking risk
• Noise sensitive signals - decreases electromagnetic coupling

In general, use 45 degree bends for traces longer than 1/10 wavelength of the highest frequency component of the signal.

Conclusion

PCB design involves navigating many interdependent constraints and trade-offs. Rules of thumb can provide general guidance, but optimal design requires thorough analysis of each specific situation. The most successful PCB designers understand the underlying engineering principles and thoughtfully apply them to make intelligent trade-offs, avoiding the pitfalls of blind myth adherence. This allows creating boards that are high-performing, reliable, and cost-effective tailored exactly to the design requirements.