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The 10 most common mistakes to avoid in PCB design

 Any electrical circuit's heart and soul is represented by the printed circuit board, or PCB. Being in charge of the electrical connections between the components and the device's interface with the outside world, it's obvious that even the tiniest design flaw might cause significant production delays or expenses, or even cause the circuit to malfunction or completely fail. The most up-to-date and cutting-edge design tools enable PCB producers to considerably lower production costs when compared to the past. However, faults committed during the PCB design phase can often cause manufacturing prices to rise.It should be noted, however, that even the most experienced PCB designers can make mistakes; our advise is to follow a few simple principles to prevent making the ten most common design blunders, which we will now detail.
 

1 – Incorrect tracing geometry

PCB traces are responsible for conveying electrical signals between the circuit's numerous components while adhering to strict speed, current intensity, and frequency requirements. In this setting, the geometry of each trace is critical; in particular, the width and thickness of each trace must be correctly sized. A power transmission line, often known as a high current line, is one in which the current flowing through a trace surpasses the indicative value of 0.5 A. In this scenario, the standard width used in low-power circuits is no longer applicable, but it must be sized appropriately using, for example, calculators based on the IPC-2221 standard or later for internal (stripline) and external (microstrip) traces. It's also worth noting that traces on the PCB's external layers can carry a larger current at the same thickness because they benefit from better air flow and heat dissipation. The amount of copper utilized for that layer determines the trace width. Each trace's thickness, in addition to its width, must be correctly sized. Copper thicknesses ranging from 0.5 oz/ft2 to 2.5 oz/ft2 and beyond are available from most PCB manufacturers. The standard thickness value, equating to one ounce of copper (1 oz, corresponding to 35 m), is typically chosen by many designers. However, for high-power tracks, where a thickness of 2 or 3 ounces is frequently utilized, this figure may not be sufficient. A higher thickness has the advantage of presenting a lower resistance to current flow, resulting in improved thermal dissipation. The disadvantages stem from the increased weight and requirement for higher trace isolation.
 

2 – Poorly designed layout

Designers are being compelled to use components with smaller footprints and reduce the spacing between components as the need for ever-smaller printed circuit boards grows. There is a danger of facing connectivity or non-compliance issues if an inefficient layout is employed. When employing components with a smaller pitch and a greater pin count, this is especially true. It is critical to choose a layout technique that is appropriate for the circuit's needs in order to achieve the intended functionality. Leaving adequate room on the PCB for extra components (or alternatives to the current ones) that may be required in the near future is a highly valuable expedient.
 

3 – The decoupling capacitors are in the wrong place.

On the PCB power supply lines, decoupling capacitors are necessary to ensure a stable power supply free of transients or oscillations to all board components. These capacitors must always be connected in parallel with the power supply and positioned as close as feasible to the pins of the power-requiring components. The power line from the power source must be routed properly on the PCB in order to reach the decoupling capacitor before reaching the pin that requires a constant voltage. Otherwise, the decoupling function will not perform properly; In fact, all voltage regulators use a feedback circuit that, if not properly stabilized, might oscillate.
 

4 – Errors in landing patterns

Despite the fact that the terms landing pattern and footprint are sometimes used interchangeably, there is a distinction to be made. More exactly, the landing patterns refer to the size of the pads, which should always be slightly larger than the matching footprint for each component. Even a half-millimeter inaccuracy in pad-to-pad spacing measurement can be deadly in soldering throughout the manufacturing process, resulting in component and PCB misalignment. The top PCB CAD software packages feature a vast library of libraries that contain both the schematic symbol and the landing pattern for each component. If a component not included in these libraries is utilized, both the electrical symbol and the PCB landing pattern must be added by hand. It's not uncommon to make mistakes at this point; for example, if the space between two pads is less than one millimeter, the pins will not line properly, rendering soldering impossible. Figure 1 displays the landing pattern dimensions for a component with the PG-TQFP-64-19 package, as taken directly from the datasheet.
 
Figure 1: Example of specification for a landing pattern (Source: NXP)
 

5 – An excessive dependence on automated routing

Some designers rely on the automatic routing functionality, which is now available in most PCB design programs, for simple PCBs. Automatic routing, on the other hand, tends to take up more space on the PCB and create via holes that are larger than those possible with manual routing. The number of PCB tracks, as well as the number of through holes, has a direct impact on PCB manufacturing costs.
 

6 – Vias that are either blind or buried

Via holes are useful because they can address a variety of difficult routing problems and increase PCB heat transfer; however, they must be utilized with caution and discretion.
To connect an exterior layer to an interior layer, blind vias (type “1” in Figure 2) must be used, whereas buried vias (type “2” in Figure 2) must be used to connect two internal layers together. Instead, through hole vias (type "3" in Figure 2) should only be utilized to connect the PCB's two external layers and potentially certain inside layers. It is important to provide the overall size, hole size, tolerances, and other attributes when creating a via hole. They can be defined on the fly or constructed using templates. It should also be mentioned that because blind and buried vias have a higher production cost, it is best to plan their use ahead of time in order to stay within the PCB budget.


Figure 2: Types of via holes (Source: Altium)
 

7 – Excessive trace length

High-speed signal traces should be as short and straight as possible. If the length is surpassed, major issues such as signal reflection (with direct ramifications for signal integrity), increased sensitivity to electromagnetic interference (EMI), and, of course, higher expenses may arise. A transmission line is defined as a trace that is longer than a tenth of the wavelength of the signal it spans.In this situation, in addition to the length, an impedance calculation must be performed (using one of the numerous specialist tools accessible online) to ensure impedance coupling and avoid signal power loss.
 

8 – Electromagnetic interference (EMI)

Electromagnetic interference (electromagnetic interference) (number 8) (EMI)
Improper PCB design is frequently the source of electromagnetic interference. It is advised that analog and digital blocks, power sections, low speed circuits, high speed circuits, and other parts on a PCB be grouped according to their usefulness to prevent EMI. In addition, right angles on the traces must be reduced, if not eliminated, and interference must be absorbed using metal containers and insulated wires.

 

9 – Incorrect antenna layout

 
 If the PCB contains antennas for wireless connection, designers must be very careful not to make any layout errors. It is first important to adjust the impedance between the transceiver and the antenna in order to maximize power transfer. The transmission connection between the transceiver and the antenna should normally have a 50 ohm impedance. A Pi (LC) tuner filter, or any other matching circuit, should be inserted between the antenna and the transceiver for correct impedance adjustment.
 

10 – The project was not sufficiently revised.

Design review is one of the most critical aspects of the PCB development process, and it is often overlooked. The project's periodic assessments enable for verification of compliance with the project's high-level criteria, the functions allocated to the PCB, and the interconnections between the various circuits. This allows designers to avoid or detect the most common design flaws in advance; a peer review by other members of the development team can often spot mistakes that the designer had missed. 

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