Technology

The most common PCB design mistakes: the drama certainly doesn't end with the schematic

The foundation of any PCB design is a properly executed circuit diagram. In our last article, we wrote tips on how to get it right and what to look out for. But the work is far from over with the schematic. We'll show you how to design a working PCB - the first time, without mistakes.

Footprinting mistakes, libraries

The design of every PCB starts with the creation of the elements (components) in the library. And it is in this initial phase that mistakes are often made. We can draw each footprint from scratch, or we can use one of the tools that facilitate footprint design. For example, the Altium Designer environment includes a PCB Component Wizard function that generates most footprints, including the 3D model, automatically. This way the developer doesn't have to keep an eye on the formal side of things (layers, line thicknesses, technological issues...). All they need to do is check the part.

What to look for during the inspection?  First, check the exact case names (watch out for confusion between SOIC Narrow and SOIC Wide or TSSOP and TQFP). At the same time, verify that the dimensions match the datasheet. Also pay attention to the correct pitch of the components. A common problem when creating footprints without tools like PCB Component Wizard is the so-called "mirror error". This is a mirror-rotated or rotated footprint. All of us have certainly encountered ambiguous or hard to read descriptions in datasheets. And so we know how much inconvenience such a seemingly trivial mistake can bring.

Another problem is the wrong polarity (rotation of the housing) of components. These errors are caused by ambiguous markings in the assembly drawing, or directly by a manufacturing error (the assembly does not conform to the assembly drawing). The only defence against this error is to indicate the polarity with a dot in the printing and in the fitting drawing.  At the same time, it is advisable to provide the manufacturer with a 3D view or a document in *.3dpdf format. For example, in our ASN+ we generate all these files automatically using Altium Designer. In this way, the risk of an error made by the manufacturer during the fitting process is minimised.

Layout of components

The design of a printed circuit board (PCB) should be preceded by a consideration of its layout. Look for a compromise between the mechanical, electrical and thermal side of things. It is the only way to achieve proper PCB functionality.

PCB dimensions and shape, connector positions and mounting holes are often pre-specified. Nowadays, paper models are no longer used for inspection. A better solution is to use the export of PCBA models directly to AutoCAD type programs.  Conversely, it is also possible to import mechanical models into a PCB development environment. Once you have obtained the shape of the PCB and have placed the connectors and possibly key integrated circuits on it, you can start the electrical design of the board. However, the latter is not so simple.

Don't forget about the thermal side of things either. For the latter, it's crucial to ensure that the powerful components are well cooled. It is desirable to improve the heat distribution as much as possible (air, copper on the PCB, heat sinks, etc.).

Tips on how to properly create a schematic and what to look out for is here

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asn pcb

System errors and EMC

When designing a PCB, constantly consider the PCB layout. Consider the layout of the components used according to the supply voltages (or signal voltage levels) and observe the correct isolation distances, i.e. clearance and creepage. This is often not only about correct functionality, but also about safety. Follow the usual professional recommendations. Minimize loop lengths, minimize frequency spectrum, filter the power supply and use appropriate blocking capacitors.

It is essential that you also think about the vertical structure of the PCB. The stack-up gives an accurate description of all layers of the PCB, including the interconnects used. Stack-up obviously affects, for example, the impedance of differential pairs. If possible, always start from the PCB manufacturer's quotation, then calculate the parameters of the used leads yourself (or with the help of the development environment).

Design rules, development environment

Do not forget the silkscreen (e.g. signal names on the connectors). Set up a "release procedure", use "design rules" and the associated automatic check (DRC or Design Rule Check) as much as possible. Generate entire packages of production data automatically in one click.

Keep working and improving.  A good choice of development environment and perfect knowledge of it is an essential part of efficient and quality PCB design.

Get inspired. Use development kits and open-source projects

Want to take a step forward in PCB design? Follow the work of others, analyse designs and critically evaluate them. Finally, we have selected some useful links for you, links from which you can download interesting projects in Altium Designer.  Are you a beginner? Then you will definitely learn something from the designs. If you consider yourself advanced in PCB design, you will at least get an idea of how projects are created elsewhere.

Altium Designer projects can be downloaded at:

https://developer.toradex.com/carrier-board-design/reference-designs

https://www.imx6rex.com/

https://project.turris.cz/en/hardware-documentation

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