At the beginning of the new PCB outsourcing design, because most of the time is spent on circuit design and component selection, in the PCB layout stage, due to lack of experience, the consideration is not comprehensive enough.
If enough time and energy are not provided for the design of PCB layout, it may lead to problems in the manufacturing stage or functional defects when the design is transformed from digital domain to physical reality.
So, what is the key to designing a circuit board that is true and reliable on paper and in physical form? Let’s explore the first six PCB design guidelines that we need to know when designing a manufacturable and reliable PCB.
1. Fine-tune your component layout.
The component placement stage of PCB layout process is both scientific and artistic, so it is necessary to consider the main components available on the PCB strategically. Although this process may be challenging, the way you place electronic components will determine how easy it is to manufacture your circuit board and how it meets your original design requirements.
Although there is a general order of component placement, such as placing connectors, mounting devices of printed circuit boards, power circuits, precision circuits, key circuits, etc. in turn, there are some specific guidelines to keep in mind, including:
Orientation-Ensure that similar components are positioned in the same direction, which will help to achieve an efficient and error-free welding process.
Layout-Avoid placing smaller components behind larger components, which may cause mounting problems due to the welding of larger components.
Organization-It is recommended to place all surface mount (SMT) components on the same side of the circuit board and all through-hole (TH) components on the top of the circuit board to minimize assembly steps.
One last PCB design guide-that is, when using mixed technology components (through holes and surface mount components), manufacturers may need additional processes to assemble circuit boards, which will increase your overall cost.
2. Properly place power supply, grounding and signal wiring.
After the components are placed, the power supply, grounding and signal wiring can be placed next to ensure that your signal has a clean and trouble-free path. At this stage of the layout process, please keep the following guidelines in mind:
1) Position the power supply and ground plane layer.
It is recommended that the power and ground planes should always be placed inside the circuit board, while maintaining symmetry and centering. This helps to prevent your circuit board from bending, which is also related to whether your components are positioned correctly.
For supplying power to IC, it is recommended to use a common channel for each power supply, to ensure a firm and stable trace width, and to avoid daisy-chain power connection from component to component.
2) Wiring connection of signal lines
Next, connect the signal lines according to the design in the schematic diagram. It is recommended to always take the shortest possible path and direct route between components.
If your components need to be fixed in the horizontal direction without any deviation, it is recommended that the components of the circuit board be routed basically horizontally, and then routed vertically.
In this way, with the migration of solder during soldering, the components will be fixed in the horizontal direction. As shown in the lower half of the figure. The signal routing mode in the lower part of the figure below may cause the deflection of components with the flow of solder during welding.
3) Define the network width.
Your design may require different networks, which will carry various currents, which will determine the required network width. Considering this basic requirement, it is recommended to provide a width of 0.010”(10mil) for low current analog and digital signals. When your line current exceeds 0.3 amp, 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 supply circuit can interfere with your low-voltage current control circuit. To minimize such interference problems, please follow the following guidelines:
Isolation-Ensure that each power supply is kept separate from the control ground. If you have to 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 in the middle layer, make sure to place a small impedance path to reduce the risk of any power circuit interference and help protect your control signals. You can follow the same guidelines to keep your digital and analog separate.
Coupling-In order to reduce the capacitive coupling due to the placement of a large ground plane and the wiring above and below it, try to cross the analog ground only through analog signal lines.
4. Solve the heat problem.
Have you ever suffered from thermal problems, resulting in circuit performance degradation or even circuit board damage? Because heat dissipation is not considered, many problems have appeared, which have troubled many designers. Here are some guidelines to keep in mind to help solve the heat dissipation problem:
1) Identify troublesome components.
The first step is to start thinking about which components will dissipate the most heat on the circuit board. This can be achieved by first finding the “thermal resistance” level in the data sheet of the component, and then transferring the generated heat according to the suggested guidelines. Of course, radiators and cooling fans can be added to keep the temperature of components down, and remember to keep key components away from any high heat sources.
2) Add hot air pad.
Adding hot air pads is very useful for producing manufacturable circuit boards, and they are essential for wave soldering applications on high copper content components and multilayer circuit boards. Because it is difficult to maintain the process temperature, it is always recommended to use hot air pads on through-hole components, so as to make the soldering process as simple as possible by slowing down the heat dissipation rate at the pins of the components.
As a general rule, always connect any through hole or via hole connected to the ground plane or power plane with hot air pad. In addition to the hot air pad, you can also add tears at the pad connecting line to provide additional copper foil/metal support. This will help to reduce mechanical stress and thermal stress.
5. Popular Science of Hot Air Pad
Many engineers in charge of Process or SMT technology in the factory often encounter the problem of non-wetting of circuit board components, such as solder empty, de-wetting or cold solder. No matter how the process conditions are changed or the temperature of reflow soldering is adjusted, there is a certain ratio of non-wetting. What the hell is going on here?
Leaving aside the problem of oxidation of components and circuit boards, it is found that most of these welding defects are actually due to the lack of layout design of circuit boards, and the most common one is that some solder feet of components are connected to a large area of copper skin, which causes the solder feet of these components to be soldered poorly after reflow soldering. Some hand-welded components may also cause false soldering or package soldering due to similar situations, and some even cause the components to be soldered out because of too long heating.
Generally, in PCB circuit design, it is often necessary to lay a large area of copper foil for power supply (Vcc, Vdd or Vss) and Ground (GND). These large-area copper foils are usually directly connected to pins of some control circuits (IC) 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 (that is, the heating will be slower) than the independent solder pads, and the heat dissipation will be faster. When one end of such large-area copper foil wiring is connected to small components such as small resistance and small capacitance, while the other end is not, it is easy to cause welding problems due to inconsistent time of tin melting and solidification; 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 large copper foils will easily cause the problem of virtual soldering because they can’t reach the melting temperature of tin.
During manual welding, the welding feet of these components connected to a large piece of copper foil will be unable to complete the welding within the specified time because the heat dissipation is too fast. The most common bad phenomena are package soldering and virtual soldering. The solder is only soldered on the solder pins of the components but not connected to the pads of the circuit board. From the outside, the whole solder joint will form a sphere; What’s more, the operator keeps increasing the temperature of the soldering iron in order to solder the solder feet on the circuit board, or heats it for too long, so that the components exceed the heat-resistant temperature and are damaged without knowing it.
6. Check your work.
When you keep humming and humming all the parts together for manufacturing, it is easy to find the problem at the end of the design project, and you are overwhelmed. Therefore, the double and triple inspection of your design work at this stage may mean the success or failure of manufacturing.
To help complete the quality control process, we always recommend that you start with the Electrical Rule Check (ERC) and Design Rule Check (DRC) to verify whether your design fully meets all the rules and constraints. Using these two systems, you can easily check the gap width, line width, common manufacturing settings, high-speed requirements, short circuit and so on.
When your ERC and DRC produce error-free results, it is recommended that you check the wiring of each signal, from the schematic diagram to the PCB, one signal line at a time to carefully confirm that you have not missed any information. In addition, use the detection and shielding functions of your design tools to ensure that your PCB layout materials match your schematic diagram.
