6 Guidelines to Know When Designing a PCB

When starting a new design, most of the time is spent on circuit design and component selection, which often leads to inexperience and lack of consideration during the PCB layout stage.

Failure to provide sufficient time and effort for the design during the PCB layout stage can lead to problems during the manufacturing stage or defects in functionality when the design is translated from the digital realm to physical reality.

So what is the key to designing a circuit board that is both realistic and reliable on paper and in physical form? Let’s explore the top 6 PCB design guidelines you need to know when designing a manufacturable, functional and reliable PCB.

1.Fine-tune your component placement

The component placement stage of the PCB layout process is both a science and an art, requiring strategic consideration of the major components available on the board. While this process can be challenging, the way you place the electronic components will determine how easy your board is to manufacture and how well it meets your original design requirements.

While there is a general order for component placement, such as placing connectors, PCB mounting devices, power circuits, precision circuits, critical circuits, etc. in order, there are also specific guidelines to keep in mind, including:

Orientation – Make sure to position similar components in the same direction, which will help achieve an efficient and error-free soldering process.

Layout – Avoid placing smaller components behind larger components, which may cause problems with the soldering of large components.

Organization – It is recommended to place all surface mount (SMT) components on the same side of the board and all through-hole (TH) components on the top of the board to minimize assembly steps.

One last PCB design guideline to note – when using mixed technology components (through-hole and surface mount components), the manufacturer may require additional processes to assemble the board, which will increase your overall cost.

2.Proper placement of power, ground, and signal traces

After placing the components, you can next place the power, ground, and signal traces to ensure that your signals have a clean and trouble-free path to travel. At this stage of the layout process, keep in mind some of the following guidelines:

1) Positioning the power and ground plane layers

It is always recommended to place the power and ground plane layers inside the board while keeping them symmetrical and centered. This helps prevent your board from bending, which also relates to whether your components are properly positioned.

For powering ICs, it is recommended to use a common channel for each power supply, ensure a solid and stable trace width, and avoid daisy-chain power connections from component to component.

2) Signal line trace connection

Next, connect the signal lines as designed in the schematic. It is recommended to always take the shortest possible path and direct path between components.

If your components need to be fixed in the horizontal direction without deviation, it is recommended to basically run horizontally where the components come out of the circuit board, and then run vertically after the lines come out.

In this way, as the solder migrates during soldering, the components will be fixed in the horizontal direction. As shown in the upper half of the figure below. The signal routing method in the lower half of the figure below may cause the components to deflect as the solder flows during soldering.

3) Define network width

Your design may require different networks that will carry various currents, which will determine the required network width. Considering this basic requirement, it is recommended to provide a 0.010” (10mil) width for low-current analog and digital signals. When your line current exceeds 0.3 amps, it should be widened. Here is a free line width calculator to make this conversion process simple.

3.Effective Isolation

You may have experienced how large voltage and current spikes in the power circuit can interfere with your low-voltage current control circuit. To minimize such interference problems, follow these guidelines:

Isolation – Make sure each power supply keeps the power ground and control ground separate. If you must connect them together in the PCB, make sure it is as close to the end of the power path as possible.

Layout – If you have placed a ground plane on the middle layer, make sure to place a low impedance path to reduce the risk of any power circuit interference and help protect your control signals. The same guidelines can be followed to keep your digital and analog separate.

Coupling – To reduce capacitive coupling due to placing a large ground plane and routing above and below it, try to cross the analog ground only with analog signal lines.

4.Solve Heat Problems

Have you ever experienced a reduction in circuit performance or even board damage due to heat problems? Many designers have encountered many problems due to not considering heat dissipation. Here are some guidelines to keep in mind to help solve thermal issues:

1) Identify troublesome components

The first step is to start thinking about which components will dissipate the most heat on the board. This can be done by first finding the “thermal resistance” rating in the component’s datasheet, and then following the recommended guidelines to move the generated heat. Of course, heat sinks and cooling fans can be added to keep component temperatures down, and also remember to keep critical components away from any high heat sources.

2) Add hot air pads

Adding hot air pads is very useful for producing manufacturable boards, and they are essential for wave soldering applications on high copper content components and multi-layer boards. Because it is difficult to maintain process temperatures, it is always recommended to use hot air pads on through-hole components to make the soldering process as simple as possible by slowing the heat dissipation rate at the component pins.

As a general guideline, always use hot air pads for any through-hole or vias that connect to the ground plane or power plane. In addition to hot air pads, you can also add teardrops where the pads connect to the wires to provide additional copper foil/metal support. This will help reduce mechanical and thermal stress.

5.Popularization of hot air soldering pad

Many engineers in charge of process or SMT technology in factories often encounter problems such as empty soldering, de-wetting or cold soldering on circuit board components. No matter how the process conditions are changed or how the reflow oven temperature is adjusted, there is a certain rate of non-wetting. What is going on?

Apart from the problem of component and circuit board oxidation, the root cause is that a large part of this type of poor soldering actually comes from the lack of layout design of the circuit board. The most common one is that some of the solder pins of the component are connected to a large area of copper foil, causing these component solder pins to have poor soldering after reflow soldering. Some hand-soldered components may also cause de-wetting or package soldering problems due to similar situations. Some components are even damaged due to excessive heating.

Generally, when designing PCB circuits, it is often necessary to lay a large area of copper foil as power supply (Vcc, Vdd or Vss) and ground (GND, Ground). These large areas of copper foil are generally directly connected to some control circuits (IC) and pins of electronic components.

Unfortunately, if we want to heat these large areas of copper foil to the temperature of melting tin, it usually takes more time than independent solder pads (that is, the heating will be slower), and the heat dissipation is also faster. When one end of such a large area of copper foil wiring is connected to small components such as small resistors and small capacitors, and the other end is not, it is easy to cause welding problems due to inconsistent melting and solidification time; if the temperature curve of reflow soldering is not adjusted well and the preheating time is insufficient, the solder feet of these components connected to the large piece of copper foil are prone to cause cold soldering problems because they cannot reach the melting temperature.

During manual soldering (Hand Soldering), the solder feet of these components connected to the large piece of copper foil will not be able to complete the soldering within the specified time due to too fast heat dissipation. The most common bad phenomena are solder wrap and cold soldering, where the solder is only soldered on the solder pins of the component but not connected to the solder pads of the circuit board. From the appearance, the entire solder joint will form a ball shape; even worse, the operator keeps raising the temperature of the soldering iron in order to solder the solder pins to the circuit board, or heats it for too long, causing the component to exceed the heat resistance temperature and be damaged without knowing it. As shown in the figure below.

Solder wrap, cold soldering or cold soldering

Since we know the problem point, we can have a solution. Generally, we will require the so-called Thermal Relief pad design to solve this type of welding problem caused by large copper foil connecting the solder pins of components. As shown in the figure below, the wiring on the left does not use the hot air solder pad, while the wiring on the right has already used the hot air solder pad connection method. It can be seen that the contact area between the solder pad and the large copper foil is only a few small lines, which can greatly limit the loss of temperature on the solder pad and achieve a better welding effect.

6.Check your work

When you’re humming and humming all the pieces together for manufacturing, it’s easy to find problems at the end of a design project and feel overwhelmed. Therefore, double and triple checking your design work at this stage can mean the difference between success or failure.

To help with the quality control process, we always recommend starting with electrical rule checking (ERC) and design rule checking (DRC) to verify that your design fully meets all rules and constraints. With these two systems, you can easily check for gap widths, line widths, common manufacturing settings, high-speed requirements, shorts, and more.

When your ERC and DRC produce error-free results, it’s recommended that you check the routing of each signal, one signal line at a time from schematic to PCB to double-check that you haven’t missed anything. Also, use the probing and shielding capabilities of your design tool to ensure that your PCB layout materials match your schematic.

Conclusion

There you have it – the top 5 PCB design guidelines that all PCB designers need to know. By following these recommendations, you will soon be well on your way to designing functional and manufacturable boards and having truly quality printed circuit boards.

Good PCB design practices are essential to success, and these design rules lay the foundation for building and solidifying a practical experience of continuous improvement in all design practices.