Best Practices in Layout

Best Practices in Layout

Having effectively used capture to build a design, and simulation to validate its performance, it is time to build a physical prototype to test the real-world performance of the design in the format in which it will be eventually used. Layout is generally done in a CAD environment in which the symbols that represented the design in capture are now seen in the format of the actual component physical dimensions. The final form design from the layout application is exported to a Gerber format which can be used by board manufacturers to turn a physical representation of the board.

During the layout stage the actually integrated circuits (ICs) and components are placed onto the board, connected via a current carrying conduit, called a copper route (or copper trace). The final necessary step is creating a board outline which defines the form factor of the PCB (the form factor is important as it will ensure that the board fits the chassis, system or physical environment in which it will be eventually placed and operated from).

The layout process has been enhanced by the EDA industry to include many advanced tools that will automatically place and route a board for the engineer. Despite the advent of such tools it is important that an engineer use appropriate judgment on when and how to use automated tools. Devices and elements of critical importance should be addressed with a higher degree of scrutiny, and such a manual process of placement and trace definitions are important to ensure performance. Therefore as a best practice it is necessary to leverage manual tools to ensure that this part of the design is effectively completed. Automated tools (such as an autorouter) can be used to connect together the less critical parts of the design.

Routing of Copper Traces

In Printed Circuit Board (PCB) design, there are three fundamental tasks that allow you to prepare a board for prototype and manufacture. First the board outline must be created for the form factor of the design.

Second in consideration is part placement. In part placement various landpatterns (or footprints) of design devices are configured on the board. Each placed part consists of pins which are terminals that need to be connected in order to complete the design. A PCB design tool represents the necessary connections between parts with a wire. These wires are called nets. 

Therefore the third fundamental task in board design is to route these net connections between various parts. The routing process turns these various net connections into copper traces which connect parts in the physical prototype with current carrying connections. The net acts as a design guide indicating that two pins must be connected, while the copper trace is the actual physical connection which will be made as a part of your PCB.

NI Ultiboard allows you to define copper traces using a number of different methods. Each method provides varying degrees of control that allow an engineer to balance precise copper definition with automated speeds in order to effectively design a PCB. The routing methods available to engineers are:

1. Manual Trace Placement
2. Follow-Me Router
3. Connection Machine
4. Autorouter

 Manual Trace Placement

What is Manual Trace Placement?

Placing traces with the line drawing tool allows the user to completely control every aspect of a copper trace. This drawing tool follows your mouse cursor and creates a copper route according to your exact specifications.

Other than following your mouse it is important to note that since you have complete design control you must be careful not to create routes that will cause design rule errors or put into questions the validity of your design. This means that sharp or obtuse angles in your routes should be avoided that will cause you to lose signal integrity.

 When to use Manual Trace Placement

The manual trace placement is recommended when you have a part that requires a very specific routing, particularly when you have a surface mounted connector (with a high pin count), FPGA or very restrictive spacing between adjacent pins, the manual trace tool will give you the needed accuracy to properly define your route. Other methods such as the autorouter (discussed later in this article) may not be able to mathematically define how to route suitably in these situations.

 Autorouter

What is the Autorouter?

The autorouter can potentially be the fastest way in which to configure the routing of all the copper on your board. The router goes through a number of steps, most notably:

• Determining the approximate direction and route of each net on a board
• Selecting a sequence in which the nets are to be routed
• Routing each net in the sequence previously determined

If the routing cannot be completed based upon these previous steps:

• The router iteratively remove net routings
• The router tries to re-route the board with either alternate routes or in a different net order

It should be noted that an autorouter cannot always route a complete board. It is important to understand that the above steps are based upon a mathematical routing method for the board and that there are times that a router will not be able to resolve an appropriate solution.

A router can also possibly create routes that are not acceptable for your board. An angle maybe too acute for your application, causing issues with signal integrity, and therefore should be taken into consideration when defining the board.

When to use the Autorouter

The autorouter is certainly a strong tool for defining a board, however should be used when the nets that need to be routed are not critical. Critical nets should be manually defined using either manual trace placement or the follow-me router.

Also when you have a part such as an FPGA or connector, where you have multiple pins on the underside of a surface mounted component the autorouter may not be able to define the correct routes. In this situation again, we can consider these critical nets and a manual technique should be applied.

Best Practices: Maximizing the Use of Your Routing Methods

As discussed throughout this article we have a number of tools which allow us to effectively route our board. The autorouter should not be considered as the only routing option. In fact it is suggested that if one is to begin defining a board, you follow a procedure such as the following:

1. Consider a component of high importance with a trace routing that can be considered critical.
2. Route the nets for this critical component using either manual or follow-me routing
3. Find components such as connectors or FPGA components which require routing beneath/between multiple surface mount pins.
4. Route the nets for this component using either manual trace placement  (or follow-me routing if convenient/possible)
5. Lock all nets in the design that have been routed using steps 1 to 4 above.
6. Use a combination of the autorouter or connection machine to route the rest of your board.

Using this simple methodology, you can be comfortable in knowing that you can maximize the use of your time in the layout and routing stages of your design.

Best Practices

As the engineer moves through the design flow, from part selection to layout, integration becomes of paramount importance in order to help streamline the creation of PCBs, reduce iterations, correct common errors and have a faster transition to market. This integration, as discussed in this article, breaks down the walls which generally disrupts the traditional design flow and is a true best practice in board-level design.

The combination of National Instruments software, namely NI Multisim, NI Ultiboard and NI LabVIEW are the core foundation of an integrated board-level design and test platform. The seamless integration ensures the proper transfer of measurement data, easy transfer to prototyping and manufacturing. This also creates advantages for the engineer, particularly with the advanced simulation capabilities provided by the concept of virtual prototyping.

Therefore the best practices in PCB design are:

1. Integration of part selection, schematic capture, simulation, board layout and design validation is the core practice for an improved PCB design flow This integration allows the feedback of iterative changes easily from one stage to another to ensure design performance.
2. The NI Multisim schematic capture environment can be effectively utilized to test and validate component/device choices for a design topology.
3. The intuitive capture environment of Multisim allows an engineer to easily design with the use of a large component database of commonly used devices and modeless wiring.
4. The combination of the easy-to-use Multsim simulation interface and NI LabVIEW virtual instrumentation create an iterative design simulation platform; Interactive simulation, advanced analyses and virtual prototyping allow you to reduce common design errors and build a better simulation model of any board-level design.
5. By leveraging manual and automatic layout tools, the engineer can effectively use theNI Ultiboard layout environment to export industry standard Gerber data to prototype for final test and manufacturing.
6. Again the integration of Multisim and LabVIEW provides a powerful platform for board-level design. The design process is integrated with the testing of the final prototype to effectively leverage simulation data to benchmark the prototype performance acquired through virtual instrumentation.