When we start a new design, because most of the time is spent on circuit design and component selection, we often have insufficient experience and insufficient consideration in the PCB layout and routing stage.
Failure to provide sufficient time and effort to design in the PCB layout phase can lead to problems in the manufacturing phase or defects in functionality when the design is transformed from the digital realm to physical reality.
So what's the key to designing a circuit board that's both real and reliable on paper and in physical form?
Let's explore the following 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 phase of the PCB layout process is both science and art, requiring strategic consideration of the major components available on the board. While the process can be challenging, the way you place your electronic components will determine how easy your board will be to manufacture, and how well it will meet your original design requirements.
While there is a general general order for component placement, such as sequentially placing connectors, printed circuit board mounts, power circuits, precision circuits, critical circuits, etc., there are specific guidelines to keep in mind, including:
Orientation - Ensuring that similar components are positioned in the same direction will facilitate an efficient and error-free soldering process.
Placement – Avoid placing smaller components behind larger components, which may cause placement problems due to the influence of soldering on larger 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 top of the board to minimize assembly steps.
One final PCB design guideline to note - that when using mixed technology components (both through-hole and surface mount components), the manufacturer may require additional processes to assemble the board, which will add to your overall cost.
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Good chip component orientation (left) and bad chip component orientation |
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Good component placement (left) and bad component placement (right) |
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2. Proper placement of power, ground and signal traces
After placing components, power, ground, and signal traces can be placed next to ensure your signals have a clean and trouble-free path of travel. At this stage of the layout process, keep the following guidelines in mind:
1) Locate 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 warping, which is also a matter of properly positioning your components.
For powering the ICs, it is recommended to use common channels for each supply, ensure a solid and consistent trace width, and avoid daisy-chaining power connections from component to component.
2) Signal line wiring connection
Next, connect the signal lines according to the design situation in the schematic diagram. It is recommended to always take the best possible path and direct path routing between components.
If your components need to be placed in a horizontal direction without deviation, it is recommended that the components on the circuit board be routed basically horizontally, and then vertically routed after the components are routed out.
In this way, as the solder migrates during soldering, the components will be fixed in the horizontal direction. This is shown in the top half of the figure below. The signal routing method in the lower part of the figure below may cause component deflection as the solder flows during soldering.
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Suggested Layout (Arrows Indicate Direction of Solder Flow) |
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Suggested Layout (Arrows Indicate Direction of Solder Flow) |
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3) Define network width
Your design may require different nets that will carry various 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 that makes this conversion process easy.
3. Effective isolation
You've probably experienced how large voltage and current spikes in your power circuits can interfere with your low-voltage current control circuits. To minimize such interference problems, follow these guidelines: Isolation – Make sure each power supply keeps power and control grounds separate. If you must connect them together in the PCB, make sure it's as close to the end of the power path as possible.
Layout - If you have placed a ground plane on an intermediate layer, make sure to place a low impedance path to reduce the risk of interference from any power circuits 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 traces above and below it, try crossing analog ground only with analog signal lines.
4. Solve heat problems
Have you ever degraded circuit performance or even damaged a circuit board due to thermal problems? There have been many problems that have plagued many designers due to failure to consider heat dissipation. Here are some guidelines to keep in mind to help with thermal issues:
1) Identify troublesome components
The first step is to start thinking about which components will dissipate the most heat from the board. This can be achieved by first finding the "thermal resistance" rating in the component's datasheet, 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 very useful for producing manufacturable boards, and they are critical for wave soldering applications on high copper content components and multilayer boards. Due to the difficulty in maintaining process temperature, hot air pads are always recommended 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 a hot air pad connection for any vias or vias that connect to ground or power planes. In addition to the hot air pads, you can also add teardrops where the pads connect to the wires to provide additional copper/metal support. This will help reduce mechanical and thermal stress.
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Typical Hot Air Pad Connection |
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5. Hot air pad science popularization
Engineers in charge of process (Process) or SMT technology in many factories often encounter solder empty, de-wetting or cold solder on circuit board components, etc. The bad problem of tin (non-wetting), no matter how the process conditions are changed or how the temperature of the reflow oven is adjusted, there is a certain ratio of non-wetting soldering. what on earth is it?
Leaving aside the problem of component and circuit board oxidation, after investigating the root cause, it is found that a large part of this type of poor soldering actually comes from the lack of layout design of the circuit board, and the most common one is that some solder joints of components are missing. The feet are connected to a large area of copper skin, resulting in poor soldering of the solder feet of these components after reflow soldering. Some hand-soldered components may also cause false soldering or cladding due to similar situations. Components are damaged by soldering.
Generally, PCBs often need to lay a large area of copper foil for power supply (Vcc, Vdd or Vss) and grounding (GND, Ground) during circuit design. These large-area copper foils are generally directly connected to the pins of some control circuits (ICs) and electronic components.
Unfortunately, if we want to heat these large-area copper foils to the temperature of melting tin, it usually takes more time than independent pads (that is, the heating will be slower), and the heat dissipation is also faster. When one end of such a large-area 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 soldering problems due to inconsistent melting and solidification times;
If the temperature curve of reflow soldering is not well adjusted and the preheating time is insufficient, the solder feet of these components connected to large pieces of copper foil will easily cause the problem of virtual soldering due to the failure to reach the melting temperature.
During manual soldering (Hand Soldering), the soldering legs of these components connected to a 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 cladding and virtual 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; what's more, the operator constantly increases the temperature of the soldering iron in order to solder the solder leg to the circuit board, or heats it for too long, so that the component exceeds the heat-resistant temperature and dies. Damaged without knowing it. As shown below.
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Welding, cold welding or virtual welding |
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Now that we know the problem, we can find a solution. Generally, we will require the so-called Thermal Relief pad (hot air pad) design to solve this type of soldering problem caused by the large piece of copper foil connected to the solder leg of the component.
As shown in the figure below, the wiring on the left does not use hot air pads, while the wiring on the right has already adopted the connection method of hot air pads. It can be seen that there are only a few small lines left in the contact area between the pads and the large copper foil. In this way, the loss of temperature on the pad can be greatly limited, and a better soldering effect can be achieved.
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Comparison with Thermal Relief pad (hot air pad) |
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6. Check your work
When you're humbling non-stop to bring all the pieces together for fabrication, it's easy to find problems at the end of a design project and get overwhelmed. So double and triple checking your design work at this stage can mean the difference between manufacturing success or failure.
To help with the quality control process, we always recommend that you start with Electrical Rule Checks (ERC) and Design Rule Checks (DRC) to verify that your design fully complies with all rules and constraints. With both systems, you can easily check for gap widths, line widths, common manufacturing setups, high speed requirements and shorts, 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 make sure your PCB layout materials match your schematic.
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Double check your design, PCB and constraint rules |
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7. Conclusion
When you master these design guidelines that PCB designers need to know, by following these suggestions, you will soon be able to design powerful and manufacturable circuit boards with real high-quality printed circuits. plate.
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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