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Developing Prototype Circuits from Home: Part 4, Suggestions for Effectively Routing a Board

This blog is Part 4 in a series that initiates designing prototype circuits from home. Developing Prototype Circuits from Home: Part 1, Introduction introduced the concept whereas Developing Prototype Circuits from Home: Part 2, Schematic Capture addressed schematic capture (using National Instrument’s Multisim3 and LTSpice 5 ). In Developing Prototype Circuits from Home: Part 3 Board Layout, the emphasis was on board layout using National Instrument’s Ultiboard.

Routing a board effectively is the key to successful circuit operation, reliability, and safety. Effectively routing a board is a complicated subject that’s worthy of a book rather than a blog. Therefore this writing will focus more on introducing the subject and providing tips. There are references listed at the end for more detailed information.

Routing is the method by which you connect the board components for current and signal flow. One could picture it like the neighborhood you live in where all of the streets, walkways, piping, cabling, and wiring has to be “routed” without interfering with each other. A circuit board is much the same way however it is typically void of any overhead connections like power poles use. Instead, a circuit board uses layers similar to the buried unground layers in a neighborhood.

Routing of this nature uses paths called traces to carry the signals. In addition to traces, circuit boards will often employ planes for carrying current. Planes are similar to unmarked parking lots where anything goes. They do differ slightly as current will take the shortest path of least resistance. In that way planes are more like water draining in a parking lot than steered vehicles using the area. Beyond that, grounding is a subject all of its own. Grounding will be covered in a separate blog.

In the same way the schematic provides guidance for the physical layout of the components, the schematic is also a guide for routing signals. The board layout portion of this series of blogs 8 covered the physical placement of components in relation to each other. Components were oriented in a manner that favored signal flow using the “rubber band” connections implemented by the Ultiboard 4 software. Routing the traces is simply a matter of running these traces on the board without them crossing over or shorting each other. With our street analogy this would mean the eventual use of bridges and overpasses to replace intersections in a multilayer board. More realistically, it’s more like an underground road system.

From this 'Rubber Band' picture in Multisim, the Component Connections Overlap as the Software Maintains Connections When the Components are Moved.  This Would Result in Shorted Signals

From this “Rubber Band” picture in Multisim, the Component Connections Overlap as the Software Maintains Connections When the Components are Moved. This Would Result in Shorted Signals

Layers offer the option of separating signals that cross each other. They also can isolate signals by inserting ground planes in between layers. In the same manner, layers can protect inner signals from external noise by forming a faraday shield on the outer layer of a board. Reference 9 covers ground planes for signaling with a number of recommendations and images that explain the concept very well.

Layers offer the option of separating signals that cross each other. They also can isolate signals by inserting ground planes in between layers. In the same manner, layers can protect inner signals from external noise by forming a faraday shield on the outer layer of a board. Reference 9 covers ground planes for signaling with a number of recommendations and images that explain the concept very well.

As with components on a board, most traces are laid out along either the X or Y axis. However, use of traces that run in 45 degree angles is common and offers an advantage. Without getting into a deep discussion, signals, like water, tend to favor junction angles that are less severe. In some cases a sever angle in a trace will actually cause a signal to reflect back on itself. In other instances, heating can occur due to current crowding. Use of 45 degree traces reduces the impact in both cases.

Even with three options for routing directions, signals are bound to run into each other. This is where changing to another layer solves to problem. Layers are accessed through vias. Vias once populated boards in great numbers when through hole products were used. In more recent times, surface mount devices have reduced the number of through holes while adding complication at the component level. There are now many more leads vying for the same real estate on this level.

Before routing the signals, it’s best to understand their interactions. Signals couple noise capacitively as well as inductively. Isolation is also a key factor. Because of this, design rules are established in order to offset the effects of these potential interactions. Design rules define the spacing as well as many other things such as trace width. Trace width is determined by the amount of current flow. Each of these subjects will be addressed separately.

Signal coupling from an inductive standpoint is due to the loop area encompassed by the signal. The loop area is kept small in order to avoid encircling as many components and traces as possible. Capacitive coupling is proportional to the shared area of the trace and inversely proportional to the distance between the traces. Inductive coupling is mainly affected by external signals. Capacitive coupling can be a factor for external signals as well as the trace signal itself. Therefore effective layout must consider these interactions and the potential impact it will have on circuit performance.

Trace width affects the parasitics of the circuit. Traces have parasitic inductance and resistance that is proportional to the length of the trace. Resistance is inversely proportional to the width and thickness of the trace. Typically the trace thickness is determined by the overall copper thickness on the board. The thinner the copper, the less expensive the board is so copper is kept as thin as possible favoring finance over function. In general, it is a good idea to keep traces as short and as wide as possible.

Isolation is a key factor in many designs. Isolation prevents flash over of high voltages. Isolation barriers are created by increasing the separation distance of components, leads, and traces. This distance is known as creepage distance. UL and other safety agencies have creepage and clearance distances that must be adhered to in order for a design to be certified. For a prototype board, isolation considerations often don’t have to take into account environmental factors like dirt and humidity. Still, safety concerns due to arc over of high voltages can be a factor in lab settings especially in the more humid regions of the world.

The basics of routing have been introduced. Routing becomes more complicated based on the type of signals being transmitted. Power, digital, and RF signals all have their associated restrictions that drive the routing design rules. As speed increases, digital signals are rapidly approaching the needs of their faster RF counterparts. Delving into this subject any further is beyond the scope of this article. There are however some good references that explain the subject further. Reference 9 “Digital Isolator Design Guide,” has a lot of information that is useful when considering routing and layout of signals. Reference 10 “Considerations in Designing the Circuit Board of Embedded Power Supplies” also has some useful tips that are more focused on power applications.

Summary

Effective routing is crucial to creating a properly functioning prototype circuit board. An explanation of the basic fundamentals has been provided with references to more detailed information.

References

  1. Developing Prototype Circuits from Home: Part 1, Introduction
  2. Developing Prototype Circuits from Home: Part 2, Schematic Capture
  3. National Instruments Multisim schematic design and evaluation tool
  4. National Instruments Ultiboard board layout tools
  5. LTSpice
  6. Eagle Schematic Capture and Board Layout Software website Eagle Schematic Capture and Board Layout Software website
  7. You Need a Power Supply Designed by When?
  8. Developing Prototype Circuits from Home: Part 3 Board Layout
  9. Digital Isolator Design Guide (Rev. A), Texas Instruments, 2009
  10. Considerations in Designing the Circuit Board of Embedded Power Supplies

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