The essentials of PCB design – addressing the issues

Mark Cundle

by Mark Cundle, Global Head of Technical Marketing

Designing a printed circuit board means taking a schematic design and laying it out so that it can be made as cost effectively as possible. This used to be the preserve of expensive dedicated tools, but the increasing popularity of highly functional yet freely available software, such as DesignSpark PCB, as well as templates and sample designs, are enabling board designers to get up and running quickly.

But knowing the best practice is the best way to avoid problems in the design that can cost time and money to get right later on. Finding a problem during EMC testing for example can mean costly fixes or even going back to the original design and re-doing the layout, which can cost months.

 

Challenges

Placement is the first issue that designers have to face. This is determined to a certain extent by the schematic, as some devices logically sit next to each other. But it is important to make sure that temperature sensitive elements such as sensors are not close to heat-producing devices such as power converters. With multiple power domains in a design, 12V to 5V converters can sit in different locations on the board, generating heat and electrical noise that can affect other components and reliability and performance.

These components also influence the electromagnetic performance of the design, and this is important both for the performance and power consumption but also for final approvals. Any equipment that is sold in Europe has to have the CE mark, and one of the tests involved in getting the mark is to make sure the equipment does not interfere with other systems. Often this is from the power supply, but noise is emitted from many different devices, including the DC-DC converters on a board, and even in high-speed data converters. The noise can also be picked up by tracks and radiated as small antennas, creating noise in unexpected frequencies and areas as a result of the board design.

The far field EMI issues can be solved with shielding and metal enclosures, but paying attention to any EMI emitting devices on the board can help reduce the overall cost of the system by allowing cheaper enclosures to be used.

But EMI can be a factor within the board design as well. Crosstalk can couple with tracks to reduce the signal to noise ratio and slow down the performance. If the coupled noise is too high the signal can be lost altogether, requiring more expensive components such as amplifiers to be added, when more attention to critical signal paths in the board design would have avoided the problem in the first place. This is where template designs may not help, as the design being built will have different devices in different places with different thermal and EMI requirements.

Capacitance is also a key issue to look out for in the board design as it slows down the signals and increases the power consumption. Tracks can couple sideways or vertically through two layers, to all intents and purposes creating a capacitor. This can be relatively easily avoided by ensuring that lines do not run parallel for too long, breaking up the coupling by adding a kink into one of the lines. However this again has to take into account the manufacturing design rules to make it easy to build and avoid any noise radiating from too sharp a corner. Lines can also get too close, and this can lead to short circuits between the lines, particularly on corners where ‘whiskers’ of metal can grow over time. Design rule checks will usually show up where this risk is higher than normal.

The problem is most notable with ground planes. A single sheet of metal as a layer capacitively coupled with all the lines above and below. While it does shield the line from noise effectively, it also creates its own parasitic capacitance, reducing the speed of the line and increasing the power consumption.

Vias between layers are probably one of the most contentious issues on multi-layer boards, as they can create problems in manufacturing. Voids in the vias reduce the performance of the signals and the reliability of the design, and so have to be considered carefully.  

 

Solutions

There are a number of different techniques that can be used to tackle the various challenges of PCB design. Some come from the design itself, such as using differential lines to reduce noise, while others come from the board layout. These design elements can be implemented automatically by the layout tools, but being able to manually tweak the automatic placement and layout can help the quality of the design. Throughout this, the design rule checks will ensure that the design will meet the requirements of the board manufacturer via the technology file.

Split planes can help reduce the parasitic capacitance, although this can increase the number of layers in a board and so increase the cost and introduce more challenges with vias. Using an orthogonal grid of power and ground lines can actually create the effect of the ground plane in a two-layer board to reduce the capacitance and reduce the complexity of the manufacturing, although may increase the physical size of the board.

Design tools such as DesignSpark PCB are helpful in solving some of these problems right from the start, although they do require some understanding of the needs of a PCB design. For example the PCB editor starts off with a number of layers, for example two signal layers, a ground plane and a power plane. Automatic placement of the components is extremely helpful and allows time for the designer to focus on placing devices in areas that might be an issue such as power devices close to sensitive signal lines or in areas that may have higher temperature profiles. Similarly the signal lines can be automatically routed and will avoid the majority of problems, but a bit of analysis and manual intervention in high-risk areas can dramatically improve the quality of the PCB design, improving the yield and reducing the overall costs.

Design rule checking is also a very helpful tool, checking that lines are not too close with a risk of short-circuiting, but still provide an economical design. It is also possible to examine and edit the power and ground planes to avoid creating large areas of parasitic capacitance.

The tools will also help with the Gerber plots for the printing of the lines and pads, and the Excellon files for the drilling of holes to produce the final design. This has to link with the technology files of the board maker.

 

Conclusion

There are many issues to consider in designing a PCB, and tools such as DesignSpark PCB are a highly effective way of tackling the vast majority. By adding in some best practice guidelines designers can develop cost effective, reliable boards that meet the system specification and avoid wider problems with system certification that can end up being costly.