PCB layout style mistakes that can ruin your design

Infineon / Mitsubishi / Fuji / Semikron / Eupec / IXYS

PCB layout style mistakes that can ruin your design

Posted Date: 2024-01-18

Modern PCB layout software allows engineers, designers and hobbyists to design PCBs quickly and easily. The software offers creative freedom, but sometimes that's not a good thing. PCB designers can make sloppy design mistakes that don't affect the functionality of the product but can impact assembly, debugging, and yield because these sloppy mistakes create chaos. This article explains some basic sloppy PCB design style mistakes and how to avoid them.

Reference marks under solder pads

Reference marks placed on the 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 components will be difficult. In the picture below, the reference mark for R1 will not be completely printed on the PCB. "1" will be cut off. The reference number for R2 is correctly placed.

Reference mark under package

If you place a reference mark underneath the component, you or your contract manufacturer may be able to place the component, but if you need to remove or replace the component for repair or debugging, it will be difficult to locate the component on the PCB. In the picture below, U1's reference mark is hidden after it is placed on the PCB. Once U2 is placed, the reference mark for U2 will be clearly visible.

Reference tag not explicitly assigned to component

Place the reference mark as close to its component as possible. Place reference designators in a way so that it is clear which component they belong to. If you don't do this, it will be difficult to place the right component in the right place. This is important for assembly and debugging. In the picture below, it is not clear which resistor is R1 and which is R2.

Reference marks in small font

Use a reference designator font that is large enough to be read easily. The author has successfully used fonts that are at least 0.060" tall and 0.050" wide. There are no pictures for this tip because reference indicators of any size look good on a large high-resolution monitor, especially when you zoom in.

Components close to each other with ambiguous reference designators

Components placed next to each other with reference numbers that do not clearly indicate which components are placed on which pads can cause a number of problems, including the wrong components being placed on the wrong pads or components being placed in an unintended manner introducing shorts or opens. . The author has seen layouts with this error, as shown below. Some PCBs made using this layout assemble correctly with resistors placed horizontally. Other PCBs have resistors placed incorrectly vertically. This misalignment of parts renders the PCB inoperable. Using an outline around the component is one way to avoid this problem.

Reference markers with random orientation

Reference marks on the PCB should face one or more two directions. Randomly oriented reference designators make assembly and debugging more difficult because components are harder to find. The components on the left have appropriately placed reference designators. The component on the right has reference designators in different orientations, which is bad.

Pin 1 is not marked on the integrated circuit

The IC should have a clear indicator such as a dot or star next to pin 1 to ensure the IC is installed correctly. Improperly installed ICs may be damaged or destroyed. Debugging will be easier if the pin 1 indicator is not buried underneath the IC when it is on the PCB. In the picture below, U1 will be difficult to place correctly. Please note that the pin numbers you see in the picture will not appear on the PCB. U2 will be placed correctly because pin 1 is clearly labeled (square pin).

Do not mark the polarity of polarized components

Some two-terminal components such as LEDs and electrolytic capacitors are polarized. Improper installation of polarized components can result in circuit failure or component damage. LEDs only glow when installed correctly. If installed upside down, the LED will not conduct, and may even be damaged by voltage breakdown. Electrolytic capacitors can explode if reverse biased. Use footprints indicating polarity. Polarity markings should not be buried beneath components. In the picture below, C1 is poorly packaged because the polarity markings will be covered by the component. C2 is well packaged because the polarity markings are visible when the capacitor is on the PCB.

Placing components too close

Placing components too close together can cause problems such as solder bridges. If the components are too close together, it may 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 replacing components difficult. This is seen on PCBs because component spacing may look good on large monitors.

No cooling used

Use heat sinks on component pins to make soldering easier. You probably don't want to use thermal relief to lower resistance and thermal resistance, but not using thermal relief can make soldering very difficult, especially when the component pads are connected to large traces or copper fillets. If proper heat sinking is not used, large traces and copper fill will act as heat sinks, which can make heating the pads difficult. In the picture below, there is no heat sink on the source pin of Q1. This MOSFET may be difficult to solder and desolder. The source pin of Q2 is thermally relieved. The MOSFET is easy to solder and desolder. PCB designers can change the amount of heat dissipation to control the resistance and thermal resistance of connections. For example,

Understand PCB Manufacturers, Contract Manufacturers, and Your Limitations

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

Find out if you or your CM can handle a difficult package like a ball grid array. Packages with legs extending out to the sides of the IC package are easier to solder if you do it yourself, especially if the pitch is not too small (less than 0.100”).

in conclusion

This article covers some PCB layout style issues that can cause assembly and debugging problems. Remember to place reference markers carefully, use a good package with polarity and pin 1 marking, use heat sinking, and consider manufacturing constraints when designing. Following these good design practices will make assembly easier, improve yield, and make debugging easier. These benefits save time, money and give you peace of mind.

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