05 Dec 2016

Designing your PCB: a review of the key functions required

Wojciech Giergusiewicz, DesignSpark PCB Programme Manager, RS Components looks at some of the key issues and function of designing a printed circuit board, PCB.

A fundamental component to any electronics design, the printed circuit board (PCB) needs to be a faithful and accurate representation of the circuit’s schematic design.

Getting it right first time is crucial to the success of a product where time to market is an all-important metric. Whether you are a maker wanting to prototype a PCB for a new design project, or a professional engineer tasked with translating the initial circuit design into a PCB, the process of creating a PCB that is accurate and doesn’t require any rework is key.

In today’s space-constrained product designs, managing to squeeze the PCB into the available space, while having to take care of factors such as containing EMI, ground plane and other position-sensitive components, has become the norm.

PCB design requirements

Accommodating all these design requirements, and more, is the job of a PCB design application, of which there is a wide variety. Some of them are free, some require high performance graphic workstations, while others are cloud-based, needing constant internet connectivity.

While the basic function of a PCB design is that of producing a PCB layout ready for etching, the software typically takes on a lot more of the chores associated with the process, many of them further contributing to saving time.

Use of component libraries such as the PCB Part library, which provides accurate pin-out, dimensions and lead positions, speed not only the whole PCB layout process but also provision creation of a bill of materials (BOM) report for production planning. In turn, integration of the BOM to a supplier’s database provides detailed component costing and purchasing information.

PCB design application selection

When selecting a PCB design application for the first time what are the core features an engineer should look for? The first is the schematic editor. This allows the entry of all the components required and for the circuit diagram to be built up through the use of “nets”: the connections between all the components.

Each component should be accessed from a library or model source function that contains all the associated meta-data available such as component symbol, footprint dimensions, manufacturer, parametric values and other key electronic and physical characteristics. Tight integration with a supplier’s database makes this process a lot easier and further aids other steps such as those mentioned above.

Schematic editor of DesignSpark PCB

Figure 1 – Schematic editor of DesignSpark PCB (source RS Components)

Figure 1 illustrates an example view of the schematic editor available in the DesignSpark PCB software from RS Components. This software is a good example of a package that suits a broad range of PCB design requirements, for makers and professional engineers, and is available for free download from the DesignSpark web site.

Individual components, including connectors and other ancillary components, are added one by one using the component library until all are displayed in the editor. Each component then needs to be laid out in the editor in the approximate relative position in relation to others prior to manually joining up the net connections between them. In this way the circuit schematic is created.

PCB schematic files

Should a schematic file be available that has been previously created in different PCB software then an import function is desired. For example, DesignSpark PCB can import files generated in CadSoft Eagle.

Engineers should check that the PCB design software they review does not have limitations with regards to final PCB and/or schematic size, number of PCB layers or number of components and/or net connections. DesignSpark PCB, for example, has no limitations in this regard, the only criteria being that the maximum PCB size is 1 x 1 m.

Check the PCB design

Once the schematic design has been finished and fully checked, the PCB design process can commence. Typically most applications support both manual and automatic routing of the connections.

Autorouting set-up usually allows defining the number of attempts to achieve a successful routing and a number of configuration parameters such as use of vias. Figure 2 below shows the result of an autorouting process. Red tracks are on top of the PCB and blue on the bottom, with vias connecting them as needed.

Autorouted PCB design achieved using DesignSpark PCB

Figure 2 – Autorouted PCB design achieved using DesignSpark PCB (source RS Components)

Should the design be too complex for autorouting, or a manual routing is preferred due to complex space envelope restrictions, this can be selected, the process starting with a “rats nest” of components with “air wires” that are then dragged into place on the PCB. Figure 3 shows an example of this process, with two vias created for illustration purposes.

Manual routing of connections using DesignSpark PCB

Figure 3 – Manual routing of connections using DesignSpark PCB (source RS Components)

PCB planes

DesignSpark PCB also provides for flooding PCB areas with copper as required. This might be to accommodate thermal, grounding or power factors, and the layout process will ensure that voids are created around tracks, components pads, vias and other occupied board space. This function, termed ‘Pour Copper’, has a number of configuration parameters such as separation distance and area occupied.

Figures 4 illustrates finished PCB layout available within DesignSpark PCB.

PCB layout, track colour denoting the PCB layer

Figure 4 – PCB layout, track colour denoting the PCB layer (source RS Components)

Exporting the PCB design

Once the PCB layout is complete the next step will be to export the layout using an industry standard file format such as Gerber. PCB drill files are also needed for which the ExcellonNC format is popular. DesignSpark PCB provides for these and other industry standard formats such as ODB+, DXF and IDF. RS has taken this process a stage further by collaborating with a number of fast-turnaround PCB manufacturers so that short-run PCBs can be ordered from directly within DesignSpark PCB.

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About the author

Wojciech Giergusiewicz joined the DesignSpark team at RS Components in January 2016 and is responsible for developing technical marketing programmes for the company’s PCB design software. Prior to this he spent more than 13 years in a variety of electronic and electrical engineering roles, including positions at Woodward Inc. and TRW Automotive, where he gained experience in schematic design and PCB layout capture. Wojciech holds a Master’s Degree in Electronics and a Bachelor’s Degree in Measuring and Control Systems from the Warsaw University of Technology, Poland.

RS Components and Allied Electronics are the trading brands of Electrocomponents plc, the global distributor for engineers. With operations in 32 countries, we offer more than 500,000 products through the internet, catalogues and at trade counters to over one million customers, shipping more than 44,000 parcels a day. Our products, sourced from 2,500 leading suppliers, include electronic components, electrical, automation and control, and test and measurement equipment, and engineering tools and consumables. Electrocomponents is listed on the London Stock Exchange and in the last financial year ended 31 March 2016 had revenues of £1.29bn.

DesignSpark can be found at: http://www.rs-online.com/designspark

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