What is routing in PCB manufacturing?

Overview of the PCB Manufacturing Process

To put routing into context, let’s first briefly review the overall PCB manufacturing process:

  1. Design: Creating the schematic diagram and PCB layout
  2. Fabrication: Printing and etching the copper traces onto the insulating substrate
  3. Drilling: Drilling holes for through-hole components and vias
  4. Plating: Applying finish plating to pads and through-holes
  5. Silkscreen: Printing component labels and identifiers
  6. Solder Mask: Applying solder resist over bare copper
  7. Surface Mount: Placing and soldering SMD components
  8. Testing: Verifying functionality and catching defects

Routing falls under the PCB design phase. The designer uses EDA (electronic design automation) software to lay out the components and map the connections between them. This produces the files needed for fabrication, with routing being one of the most complex and time-consuming aspects of PCB design.

What Exactly is PCB Routing?

PCB routing is the process of laying out the physical pathways for electrical signals to travel between components on a circuit board. These pathways are called traces.

On a typical PCB, the components (resistors, ICs, capacitors, etc.) are fixed in place, and the router’s job is to efficiently connect them according to the netlist without violating any design rules. It’s like solving a 2D puzzle with dozens or hundreds of pieces that all need to link together in very specific ways.

Some of the main considerations in PCB routing include:

  • Trace width: Determines current carrying capacity and impedance
  • Clearance: Minimum gap required between traces to prevent short circuits
  • Via placement: Location and number of vias needed to route between layers
  • Layer stackup: Arranging signal, power and ground layers for optimal EMC
  • Length matching: Ensuring equal trace lengths for timing-critical signals

Careful PCB routing is crucial for signal integrity, power delivery, electromagnetic compatibility, manufacturability, and reliability. A poorly routed board is at risk for issues like crosstalk, reflections, ground loops, RF interference and excessive heating.

Types of PCB Routing

There are two main types of routing used in PCB design:

1. Manual Routing

With manual routing, the designer has full control over the placement and pathways of every single trace. This is done by clicking node-to-node in the EDA tool to manually draw the desired connections.

Manual routing offers the highest degree of precision and customization. The designer can optimize parameters like trace length and geometry for sensitive analog circuits and high-speed signals. They can also work around complex RF elements like antennas and ensure sufficient clearance around mechanical features.

However, manual routing is very tedious and time-consuming, especially for dense, multi-layer boards. It requires a great deal of knowledge and experience to route a complex PCB entirely by hand.

2. Autorouting

Autorouting utilizes algorithms to automatically map out the connections on a PCB according to the netlist and specified design rules. The designer simply has to configure settings like trace width, via size, clearances, etc. and click a button to route the entire board.

Modern autorouters are quite sophisticated and can complete complex routing jobs in a fraction of the time it would take to do manually. They consider an enormous number of potential paths for each connection and optimize parameters like total trace length, number of vias, and number of layer transitions.

Autorouting is the go-to method for most digital PCB designs. It excels at boards with a high component density, strict length matching requirements, and relatively simple geometry.

However, autorouting has some significant limitations. The algorithms tend to struggle with non-standard trace widths, RF elements, and irregular board outlines. They also can’t interpret the intent behind a schematic, so they often make strange routing choices from an electronics point of view.

Comparing Manual Routing vs. Autorouting

Here is a summary of the key differences between manual PCB routing and autorouting:

Manual Routing Autorouting
Control Full manual control over trace placement Limited control, routes based on algorithms
Speed Slow, tedious process Extremely fast
Precision Allows precise optimization of critical traces Less precise, limited to defined rules
Difficulty Requires high skill and PCB design knowledge Minimal PCB knowledge required
Flexibility Handles complex geometry and non-standard elements Limited flexibility outside of normal parameters
Debugging Easier to debug and change individual routes Entire nets often re-routed when edited
Best For Analog/RF designs, power electronics, complex geometry Digital designs, high density, simple geometry

In practice, most PCB designs utilize a combination of autorouting and manual routing. A typical workflow is to autoroute the bulk of the simple connections, then manually clean up and optimize the sensitive and critical nets.

PCB Routing Guidelines and Best Practices

Regardless of the routing method used, there are some fundamental rules and best practices to follow for an optimized, manufacturable PCB design:

1. Know Your Foundry’s Capabilities

Every PCB fab house has different manufacturing capabilities in terms of minimum trace width, minimum clearance, maximum number of layers, drilling tolerances, etc. Be sure to obtain and adhere to your specific vendor’s design rules to avoid costly re-spins.

2. Use a Gridded System

Routing traces on a defined grid with consistent widths and clearances makes the design more understandable and maintainable. Always use the largest trace size possible for power and ground nets. Be consistent with pad sizes and via dimensions.

3. Avoid Acute Angles

Route traces with smooth curves or 45° corners rather than 90° bends whenever possible. Sharp angles can cause acid traps during etching and create signal reflections due to impedance discontinuities. Use teardrops and chamfers to strengthen the trace-to-pad transition.

4. Minimize Vias

While useful for routing between layers, vias add inductance and capacitance that can distort high-frequency signals. They also increase cost and decrease manufacturing yield. Use the minimum number of vias required to make the necessary connections.

5. Provide Good Grounding

A solid ground plane is critical for signal integrity and EMC. Use wide traces and multiple vias for low-impedance grounding. Carefully partition analog, digital, and power ground regions to prevent coupling. Stitch around the perimeter to provide shielding.

6. Length Match Critical Signals

Ensure consistent arrival times for timing-sensitive signals like clocks, strobes, and buses by matching the trace lengths. The propagation delay of a trace is determined by the dielectric constant (Dk) of the substrate material. Most modern EDA tools have length tuning features to help with this.

7. Separate Sensitive Signals

Identify and segregate sensitive nets like analog audio, radio frequencies, and high-speed digital to prevent interference. Route these traces on separate layers sandwiched between ground planes if possible. Avoid crossing over gaps in the ground fill.

8. Consider Electromagnetic Compatibility

Be mindful of potential emissions from current loops, which occur when the supply and return currents take different paths. Minimize the enclosed area to reduce EMI. Also avoid routing traces parallel to the board edge, as they can act as antennas.

9. Plan for Manufacturability

Incorporate design for manufacturing (DFM) principles like using standard hole sizes, avoiding blind and buried vias, and providing clear soldermask openings. Ensure adequate clearance around the board edge for panelization. Consider the assembly process when placing and rotating components.

10. Perform Design Rule Checks

Utilize your EDA software’s DRC functions to catch potential errors like overlapping traces, insufficient clearances, disconnected nets, and missing vias. Run a DRC before sending files to the manufacturer to avoid expensive and time-consuming revisions.

By applying these routing principles, you’ll be well on your way to producing reliable, high-performing PCBs while avoiding costly design mistakes. Of course, every design is unique and may require bending some rules, but these guidelines provide a solid foundation.

Frequently Asked Questions About PCB Routing

To wrap up, let’s address some common questions that come up regarding PCB routing:

What is the difference between a track and a trace on a PCB?

In PCB design, the terms “track” and “trace” are used interchangeably. They both refer to the copper pathways that route signals between pads. Traces or tracks can be on the outer or inner layers of a board.

How do you calculate PCB trace width?

PCB trace width is determined by the amount of current the trace needs to carry. There are several online calculators that will give you the width based on the conductor thickness, temperature rise, and maximum current.

As a rule of thumb, you can use the following formula for a 10°C rise:

Width (mils) = 0.5 x Current (amps)

So for example, a trace carrying 500mA would need to be at least 0.5 x 0.5 = 0.25 mils (6.35mm) wide. In practice, you would round up to the nearest standard trace size and use the maximum width possible.

What is the 3W rule in PCB design?

The 3W rule states that the distance between two traces should be at least 3 times the width of the traces. This limits crosstalk due to the mutual capacitance and inductance between nearby traces.

For example, if you’re using 8mil traces, the clearance between them should be at least 8 x 3 = 24 mils. Note that this is just a guideline, and the actual clearance required depends on factors like the substrate material, layer stackup, and edge rates.

What is a via in PCB?

A via is a plated hole that routes a signal from one layer of a PCB to another. There are several types of vias:

  • Through hole: goes through the entire board
  • Blind: starts on an outer layer and ends on an inner layer
  • Buried: goes between inner layers and doesn’t touch the surface

Vias are an essential part of PCB routing, but they do add capacitance, inductance and cost. High layer-count PCBs also require microvias, which are laser-drilled holes less than 6 mils in diameter.

How do I learn PCB routing?

Like any skill, PCB routing proficiency comes with study and practice. If you’re just starting out, here are some steps you can take to learn PCB routing:

  1. Master the fundamentals of electronics and PCB design
  2. Get familiar with EDA software like Altium, KiCad, or Eagle
  3. Study reference designs and follow routing tutorials
  4. Start with simple 1-2 layer boards and work up in complexity
  5. Learn one specific routing method or strategy at a time
  6. Seek feedback from experienced designers and manufacturers
  7. Practice, practice, practice!

With time and experience, you’ll develop an intuitive sense for optimal routing and be able to handle increasingly challenging designs. Just remember that every PCB design is a learning opportunity.

Conclusion

We’ve covered a lot of ground in this deep dive into PCB routing. To recap, we looked at:

  • The role of routing in the overall PCB manufacturing process
  • The difference between manual routing and autorouting
  • Routing guidelines and best practices
  • Answers to frequently asked routing questions

Routing is truly both an art and a science. It requires a blend of electronics knowledge, spatial reasoning, and design intuition. At the same time, it demands precision, attention to detail, and adherence to specific rules and constraints.

Mastering the skill of PCB routing is an ongoing journey, but one that is immensely rewarding. With each board you design, you’ll learn new techniques and encounter new challenges that will expand your capabilities as an engineer.

The PCB routing process may be complex, but the result – a well-routed, reliable circuit board – is a thing of beauty and a testament to your hard work and expertise. So embrace the challenge, keep learning, and enjoy the process of bringing your electronic visions to life through the power of PCB routing.