When starting a new design, because most of the time is spent on the circuit board design and the selection of components, the PCB layout and wiring stage is often due to lack of experience and insufficient consideration. If adequate time and effort is not provided for the design of the PCB layout layout phase, it may lead to problems at the manufacturing stage when the design is transformed from the digital domain to the physical reality, or defects in function.
So what's the key to designing a board that's real 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, functionally reliable PCB.
The component placement phase 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 your electronic components will determine how easy your board is to manufacture and how well it meets your original design requirements.
While there are general general sequences for component placement, such as sequential placement of connectors, mounting devices for printed circuit boards, power circuits, precision circuits, critical circuits, etc., there are also specific guidelines to keep in mind, including:
Orientation - Ensuring that similar components are positioned in the same direction will contribute to an efficient and error-free welding process.
Layout - Avoid placing smaller components behind larger components, which may be affected by the welding of larger components and cause mounting problems.
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 - when using mixed technology components (through hole and surface mount components), manufacturers may require additional processes to assemble the board, which will increase your overall cost.
Good chip component orientation (left) and bad chip component orientation (right)
Good component arrangement (left) and bad component arrangement (right)
After placing the component, you can next place power, ground, and signal wiring to ensure that your signal has a clean, trouble-free path. At this stage of the layout process, keep the following guidelines in mind:
1. Locate the power supply and grounding plane
It is always recommended to place the power and ground plane layers inside the board while remaining symmetrical and centered. This helps prevent your board from bending, which is also related to whether your components are positioned correctly.
For power supply to the IC, it is recommended to use a common channel for each power supply, ensure a strong and stable line width, and avoid Daisy chain power connections from component to component.
2. Signal cable connection
Next, connect the signal lines as designed in the schematic diagram. It is recommended to always take the shortest possible path and direct path routing between components.
If your components need to be fixed in the horizontal direction without deviation, it is recommended that the components of the circuit board be basically horizontally routed, and then vertical routed after the outgoing line.
In this way, as the solder migrates during welding, the component will be fixed in the horizontal direction. As shown in the top half of the following figure. The signal routing mode of the lower half of the following figure may cause deflection of the components with the flow of solder during welding.
Recommended wiring (arrow indicating solder flow direction)
Not recommended wiring (arrow indicates solder flow direction)
3. Define the network width
Your design may require different networks that will carry a variety of currents, which will determine the desired network width. With this basic requirement in mind, a 0.010 "(10mil) width is recommended for low current analog and digital signals. When your line current exceeds 0.3 amps, it should be widened. There is a free line width calculator to make this conversion process easy.
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, follow these guidelines:
Isolation - Ensure that each power supply is kept power and control 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 in the middle layer, make sure to place a small impedance path to reduce the risk of interference from any power circuits and help protect your control signals. You can follow the same guidelines to keep your digital and analog separate.
Coupling - To reduce capacitive coupling due to placing a large ground plane and running lines above and below it, try to cross analog ground only through analog signal lines.
Have you ever experienced circuit performance degradation or even circuit board damage due to heat issues? Due to the lack of consideration of heat dissipation, there have been many problems that have plagued many designers. Here are some guidelines to keep in mind to help solve heat dissipation problems:
1. Identify troublesome components
The first step is to start thinking about which components dissipate the most heat from the board. This can be achieved by first finding the "thermal resistance" rating in the element's data table and then following the recommended guidelines to transfer the heat generated. Of course, radiators and cooling fans can be added to keep the component temperature down, and also remember to keep key components away from any high heat sources.
2. Add hot air pad
The addition of hot air pads is very useful for the production of fabricable circuit boards, and they are essential for wave soldering applications on high copper content components and multilayer circuit boards. Because of the difficulty of maintaining process temperature, it is always recommended to use hot air pads on through-hole elements in order to make the welding process as simple as possible by slowing down the rate of heat dissipation at the element pins.
As a general rule, always use hot air pad connections for any through or through holes connected to the ground or power plane. In addition to hot air pads, you can also add teardrops where the pads connect wires to provide additional copper foil/metal support. This will help reduce mechanical and thermal stress.
Many engineers responsible for 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 welding. No matter how the process conditions are changed or how the furnace temperature of reflow welding is adjusted, there is a certain proportion of tin that cannot be welded. What the hell is going on here?
Leaving aside the problem of oxidation of components and circuit boards, it is found that a large part of such poor welding actually comes from the lack of layout design of the circuit board, and the most common is that a few welding feet of the components are connected to a large area of copper skin, resulting in poor welding after reflow welding of these components. Some hand welded components may also cause problems with false welding or cladding due to similar situations, and some may even weld the components due to excessive heating.
In general PCB circuit design often need to lay a large area of copper foil as a power supply (Vcc, Vdd or Vss) and Ground (GND, Ground). These large areas of copper foil are generally connected directly 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 melting tin, it usually takes more time than the independent welding pad (that is, the heating will be slower), and the heat is 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 welding problems because of the inconsistent time of melting tin and solidification; If the temperature curve of reflow welding is not adjusted well and the preheating time is insufficient, the welding feet of these components connected to large copper foils are easy to cause the problem of false welding because they cannot reach the melting temperature.
When manual welding (Hand Soldering), these components connected to a large piece of copper foil welding feet will be too fast to complete the welding within the specified time. The most common adverse phenomenon is blanket welding, virtual welding, solder only welded to the component of the welding foot and not connected to the circuit board of the pad. From the appearance, the whole solder joint will form a ball; What is more, in order to weld the welding foot on the circuit board, the operator constantly raises the temperature of the soldering iron, or heats it for too long, causing the component to exceed the heat resistance temperature and be damaged without realizing it.
Since we know the problem point, we can have a solution, generally we will require the so-called Thermal Relief pad (hot air welding pad) design to solve this kind of welding problems caused by large copper foil connection elements. As shown in the following figure, the wiring on the left does not use the hot air pad, while the wiring on the right has adopted the connection mode of the hot air pad. It can be seen that the contact area between the pad and the large copper foil is only a few small lines, which can greatly limit the loss of temperature on the welding pad and achieve the best welding effect.
When you're constantly chugging all the pieces together for manufacturing, it's easy to find problems and be overwhelmed by the end of a design project. Therefore, double and triple checking your design work at this stage can mean the difference between success and failure in manufacturing.
To help complete 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. With these two systems, you can easily check for clearance width, line width, common manufacturing Settings, high speed requirements and short circuits.
When your ERC and DRC produce error-free results, it is recommended that you examine the wiring of each signal, from schematics to PCBS, one signal line at a time in a way that carefully ensures that you are not missing any information. Also, use the detection and shielding features of your design tools to ensure that your PCB layout materials match your schematics.
When you have this - our top 5 PCB design guidelines that PCB designers all need to know - by following these tips, you will soon be able to design powerful and capable, and have a truly quality printed circuit board in your hands. Good PCB design practices are critical to success, and these design rules lay the foundation for building and cementing hands-on experience in continuous improvement across all design practices.
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