[Experience sharing] PCB engineers teach you step by step how to lay out a perfect PCB

When starting a new design, most of the time is spent on circuit design and component selection. In the PCB layout and routing stage, due to lack of experience, insufficient consideration is often given.

Failure to provide sufficient time and energy for the design during the PCB layout and routing phase may result in problems during the manufacturing phase or functional defects when the design is transformed from the digital realm into physical reality.

So what’s the key to designing a board that’s authentic 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.

Fine-tune your component layout

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

While there is a general general order of component placement, such as placing connectors, printed circuit board mounting devices, power circuits, precision circuits, critical circuits, etc. in order, there are also specific guidelines to keep in mind,

including:

• Orientation - Making sure that similar components are positioned in the same direction will help achieve an efficient and error-free soldering process.
• Placement - Avoid placing smaller components behind larger components where the smaller component may be affected by the soldering of the larger component and cause mounting problems.
• Organization - It is recommended that all surface mount (SMT) components be placed on the same side of the board and all through-hole (TH) components be placed 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.

Good chip component orientation (left) and faulty chip component orientation (right).

Good component placement (left) and bad component placement (right)

Properly place power, ground and signal traces

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

▶ 1) Position 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 is also important for your components to be positioned correctly.

For powering the IC, it is recommended to use common channels for each power supply, ensure strong and stable trace widths, and avoid daisy-chaining power connections from component to component.

▶ 2) Signal cable routing connection
Next, connect the signal lines as shown in the schematic. It is recommended to always take the shortest possible paths and direct routing between components.

If your components need to be placed in a horizontal direction without any deviation, it is recommended that the components on the circuit board are routed basically horizontally, and then vertically routed after the traces are routed.

In this way, the components will be fixed in the horizontal direction as the solder migrates during soldering. This is shown in the upper part of the picture below. The signal routing pattern in the lower half of the figure below may cause component deflection as the solder flows during soldering.

建议的布线方式 (箭头指示焊料流动方向)

Not recommended routing (arrow indicates direction of solder flow)

▶ 3) Define network width

Your design may require different nets that will carry a variety of currents, which will determine the required net width. With this basic requirement in mind, a 0.010'' (10mil) width is recommended for low current analog and digital signals. When your line draws more than 0.3 amps, it should be widened. Here's a free line width calculator to make this conversion easy.

Effective isolation

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

• Isolation - Ensure that power and control grounds are kept separate for each power source. If you must connect them together in the PCB, make sure it is as close to the end of the power path as possible.
• Placement - If you have placed a ground plane in the middle layer, be sure to place a small 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 running traces above and below it, try crossing analog grounds only with analog signal lines.

Component Isolation Example (Digital and Analog)

Solving heat problems

Have you ever experienced thermal problems that resulted in reduced circuit performance or even circuit board damage? Due to the failure to consider heat dissipation, many problems have occurred that troubled many designers. Here are some pointers to keep in mind to help resolve cooling issues:

▶ 1) Identify troublesome components
The first step is to start considering which components will dissipate the most heat from the board. This can be accomplished by first finding the "thermal resistance" rating in the component's data sheet, and then following the recommended guidelines for transferring the heat generated. 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 pad
Adding hot air pads is useful for producing manufacturable boards, and they are critical for wave soldering applications on high copper content components and multi-layer boards. Since 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 easy as possible by slowing down the rate of heat dissipation at the component pins.

As a general guideline, always use hot air pad connections for any vias or vias that are connected to a ground or power plane. In addition to the 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.

Typical hot air soldering pad connection method

Hot air soldering pad science

Engineers responsible for process or SMT technology in many factories often encounter problems such as solder empty, de-wetting or cold soldering of circuit board components. No matter how the process conditions are changed or the reflow oven temperature is adjusted, there will always be a certain rate of tin failure. What is going on?

Leaving aside the problem of oxidation of components and circuit boards, after investigating the root cause, we found that a large number of such poor soldering actually comes from the lack of layout design of the circuit board. The most common thing is that certain soldering feet of the component are connected to a large area of copper, causing the soldering feet of these components to undergo poor soldering after reflow soldering. Some hand-soldered components may also suffer from false welding or over-welding problems due to similar situations, and some may even cause component welding damage due to excessive heating.

In general, PCB circuit design often requires laying a large area of copper foil to serve as power supply (Vcc, Vdd or Vss) and ground (GND, Ground). These large areas of copper foil are generally directly connected to the pins of some control circuits (ICs) and electronic components.

Unfortunately, if we want to heat these large areas of copper foil to the temperature of molten tin, it usually takes more time (that is, heating will be slower) than independent pads, and the heat will dissipate faster. When one end of such a large-area copper foil wiring is connected to small components such as small resistors and small capacitors, but the other end is not, welding problems may easily occur due to inconsistent tin melting and solidification times. If the reflow soldering temperature curve is not adjusted well and the preheating time is insufficient, the solder legs of these components connected to the large copper foil will easily fail to reach the melting tin temperature, causing a virtual soldering problem.

During hand soldering, the solder legs of these components connected to large pieces of copper foil will dissipate heat too quickly and cannot be completed within the specified time. The most common undesirable phenomena are over-welding and false soldering. The solder is only soldered to the soldering feet of the components but not connected to the pads of the circuit board. From the appearance, the entire solder joint will form a ball shape. A more serious situation is when the operator constantly raises the temperature of the soldering iron in order to solder the solder legs to the circuit board, or heats for too long, causing the component to exceed the heat-resistant temperature and be damaged without realizing it. As shown in the picture below.

cover solder、cold solder、solder empty

Now that we know the problem, we can find a solution. Generally, we will require the use of the so-called Thermal Relief pad design to solve this type of soldering problem caused by the soldering feet of large copper foil connecting components. As shown in the figure below, the wiring on the left does not use hot air pads, while the wiring on the right has used hot air pads for connection. There are only a few small lines left in the contact area between the pad and the large copper foil, which can greatly limit the temperature loss on the pad and achieve better welding results.

Comparison using Thermal Relief pad (hot air soldering pad)

Check your work

When all the pieces come together for manufacturing, it’s easy to get overwhelmed at the end of a design project only to discover problems. So double and triple checking your design work at this stage can mean the difference between a manufacturing success or a failure.

To help with the quality control process, we always recommend that you start with an Electrical Rule Check (ERC) and a Design Rule Check (DRC) to verify that your design fully meets all rules and constraints. Using both systems, you can easily inspect gap widths, line widths, common manufacturing setups, high-speed requirements, short circuits, and more.

When your ERC and DRC produce error-free results, it is recommended that you check the routing of each signal, from the schematic to the PCB, one signal line at a time to carefully confirm that you have not missed any information. Also, use your design tool's probing and masking capabilities to ensure your PCB layout materials match your schematic.

Double check the design, PCB and constraint rules

Conclusion

The top 5 PCB design guidelines every PCB designer needs to know. By following these suggestions, you'll soon be comfortable designing functional and manufacturable circuit boards and have a truly high-quality printed circuit board.

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

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