PCB Layout Style Mistakes That Can Ruin Your Design

Modern PCB layout software allows engineers, designers, and hobbyists to quickly and easily design PCBs. The software provides creative freedom, but sometimes this is not a good thing. PCB designers can make sloppy design mistakes that do not affect the functionality of the product, but can affect assembly, debugging, and yield because these sloppy mistakes create confusion. This article describes some basic sloppy PCB design style mistakes and how to avoid them.

Modern PCB layout software allows engineers, designers, and hobbyists to quickly and easily design PCBs. The software provides creative freedom, but sometimes this is not a good thing. PCB designers can make sloppy design mistakes that do not affect the functionality of the product, but can affect assembly, debugging, and yield because these sloppy mistakes create confusion. This article describes some basic sloppy PCB design style mistakes and how to avoid them.

Reference Markers Under Pads

Reference markers placed on copper will appear in the PCB layout software, but not on the physical PCB. If your reference designators are placed on pads in the layout, they will be lost when you get the PCB and placing the components will be difficult. In the image below, R1’s reference designator will not be fully printed on the PCB. The “1” will be cut off. R2’s reference designator is placed correctly.

Reference Designators Under the Package

If you place reference designators under a component, you or your contract manufacturer may be able to place the component, but it will be difficult to find the component on the PCB if you need to remove or replace the component for repair or debugging. In the image below, U1’s reference designator will be hidden after it is placed on the PCB. U2’s reference designator will be clearly visible after it is placed.

Reference Designators That Are Not Clearly Assigned to Components

Place reference designators as close to their components as possible. Place reference designators in a way that makes it clear which components they belong to. If you don’t, it will be difficult to place the right component in the right place. This is important for assembly and debugging. In the image below, it is unclear which resistor is R1 and which is R2.

Reference Designators with Small Fonts

Use a reference designator font that is large enough to be easily read. The author has had success using a font that is at least 0.060” tall and 0.050” wide. There are no pictures for this tip, as reference designators of any size look good on large, high-resolution displays, especially when you zoom in.

Components Close to Each Other with Ambiguous Reference Designators

Components placed next to each other without reference designators clearly indicating which components go on which pads can cause a number of problems, including the wrong component being placed on the wrong pad or components being placed in an unintended manner that introduces shorts or opens. The author has seen layouts with this error, such as the one shown below. Some PCBs made with this layout assembled correctly with resistors placed horizontally. Other PCBs had resistors placed incorrectly vertically. This misalignment of parts caused the PCB to not work. Using packages with outlines around the components is one way to avoid this problem.

Reference Markers with Random Orientations

Reference markers on a PCB should face one or more of two directions. Randomly oriented reference designators make assembly and debugging more difficult because the components are harder to find. The components on the left have properly placed reference designators. The components on the right have reference designators with different orientations, which is bad.

Not Marking Pin 1 on Integrated Circuits

Integrated circuits should have a clear indicator like a dot or star next to pin 1 to ensure the IC is installed correctly. An improperly installed IC can be damaged or ruined. Debugging will be easier if the pin 1 indicator is not buried under the IC when it is on the PCB. In the image below, U1 will be difficult to place correctly. Notice that the pin numbers you see in the picture do not appear on the PCB. U2 will be placed correctly because pin 1 is clearly marked (square pin).

Not Marking Polarity of Polarized Components

Some two-terminal components such as LEDs and electrolytic capacitors are polarized. Installing polarized components incorrectly can cause circuit failure or component damage. LEDs will only light up when installed correctly. If installed the wrong way around, the LED will not conduct and may even be damaged by voltage breakdown. Electrolytic capacitors can explode if reverse biased. Use footprints that indicate polarity. Polarity markings should not be buried under components. In the image below, C1 has a bad footprint because the polarity marking would be covered by the component. C2 has a good footprint because the polarity marking can be seen when the capacitor is on the PCB.

Placing Components Too Close

Placing components too close together can cause problems such as solder bridges. If components are too close together, it can be difficult to probe with an oscilloscope or multimeter because the probe may short multiple components together. Placing components too close together can also make it difficult to replace components. This is seen on the PCB because component spacing may look good on a large display.

Not Using Heatsinks

Use heatsinks on component pins to make soldering easier. You may not want to use heat releases to reduce electrical resistance and thermal impedance, but not using heat releases can make soldering very difficult, especially when component pads are connected to large traces or copper fills. Large traces and copper fills act as heat sinks without proper heatsinks, which can make heating the pads difficult. In the following figure, Q1 has no heatsink on its source pin. This MOSFET can be difficult to solder and desolder. Q2 has a heat release on its source pin. This MOSFET is easy to solder and desolder. PCB designers can change the amount of heatsinking to control the electrical resistance and thermal impedance of the connection. For example,

Understand the Limitations of the PCB Manufacturer, Contract Manufacturer, and You

Have a good set of design rules that your PCB manufacturer can meet.

Understand if you or your CM can handle difficult packages like ball grid arrays. If you are doing the soldering yourself, packages with legs sticking out to the side of the IC package are easy to solder, especially if the pitch is not too small (less than 0.100”).

Conclusion

This article covered some PCB layout style issues that can cause assembly and debug issues. Remember to place reference marks carefully, use a good package with polarity and pin 1 markings, use heat sinks, and consider manufacturing constraints when designing. Following these good design practices will make assembly easier, improve yields, and make debugging easier. These benefits can save time, money, and peace of mind.