From PCB Design to Fabrication & Assembly
The General Introduction of PCB Design
PCB design is significant and complex knowledge. The first printed circuit board and its design originated from a radio designed by Mr. Eisler. It has been 90 years since then, during which PCB has developed rapidly and matured step by step.
As we know, PCB design is greatly important for an electronic product in the application. Its basic structure is derived from the schematic diagram of the final product. An integrated circuit board is formed through reasonable and ingenious circuit layout design and component layout design.
At the same time, if the schematic diagram is properly designed and excellent, the circuit performance and heat dissipation design are good, and the product performance and life can be further improved, including saving the cost of subsequent product production, etc.
In addition to the layout of lines and components mentioned above, it also includes the layout of connecting lines and vias, especially the impact of layout on heat dissipation, which will directly affect the functionality and life of finished products. Like single panels and HDI printed circuit boards, different end products also use different design methods. Simple single-panel and double-sided board products can usually be realized by manual design.
If we encounter high-frequency printed circuit boards, HDI printed circuit boards, military PCBs, and other products that are too complex and require too strong functionality and heat resistance, we need to use computers as design tools.
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The PCB Design Guide
As the preface says, heat dissipation greatly affects the functional stability of the final product, including life. In addition, the distance between components is too close, which may also produce integrity interference. Therefore, the first problem is to confirm the size of the PCB. After confirming the size, the next step is to place and allocate the specific positions of components. Dealing with different types of components need to pay attention to different points. For example, high-frequency components, high-voltage components, and high-temperature components, including the male and female nature of some components, all need special attention. Their positions should be allocated reasonably, otherwise, if they are placed too close, they will interfere with each other, including compatibility problems between some components.
The distribution of components is very important for a successful design. In addition, not only the design of functionality and compatibility but also the beautiful layout will improve the recognition and success of the product. Therefore, a PCB layout with both function compatibility and appearance will make the final PCBA more successful.
Some Problems with the Layout of the Components
1. As mentioned before, too small spacing between components is easy to affect each other. Whether it is stability or compatibility, especially for some high-frequency and high-voltage components, a long distance should be maintained to protect the stability and safety of the whole line. In addition, low-voltage components and high-voltage components should also be kept away from high-voltage components as far as possible, otherwise, the gap is too large, and it is easy to form a short circuit or damage the circuit in use, and the high-voltage itself is dangerous. For the safety of users, it should also be distributed in a more remote location.
2. Components with too heavy weight should be fixed and supported by auxiliary means. Components with a weight of more than 14g should be welded and supported by auxiliary means as far as possible, to avoid damage to components and circuit boards during transportation and use. Secondly, components with high heat should also be kept away from heat-sensitive components to prevent high-heat components from affecting and damaging other components. If the heating of components is too high, it can be considered to be externally connected to other parts of the hardware and separated from the circuit board to further prevent accidental heating from causing damage to components and circuit boards.
3. Following the above suggestions, the layout of the PCB should not only ensure functionality and safety but also pay attention to practical layout and aesthetic layout. Except for some components that need special placement for safety, high-voltage components should be far away from the frequently touched position to ensure the safety of users, and other adjustable components, such as switches, etc. It is suggested that these commonly used components can be placed close to the place of use so that it is convenient to operate.
4. According to Viasion’s PCBA production experience, there is usually a general order in the layout of components. Usually, priority will be given to the design of key components, such as power supplies, switches, etc., and then some high-voltage, high-temperature, and heavy-weight components that need to be supported by auxiliary supports, including chips, etc. Finally, arrange the smallest components, such as small resistors and capacitors.
5. After the above layout is completed, the final inspection is carried out next. First, check whether the actual size is completely consistent with the size of the drawing. Secondly, check whether all components are set on the PCB by comparing the BOM list. In addition, check whether the layout of components is reasonable to avoid mutual influence and conflict between the layout of components. Check whether the high-voltage components are placed in a reasonable position to avoid safety problems and whether the commonly used equipment is in a position that is easy to operate and convenient to use. Check whether the heat dissipation of the circuit board meets the requirements to avoid overheating, damage to the circuit board and components, or short circuits in use.
6. The above is about PCB size design and some small tips for component layout. The summary is that the size should be appropriate. If the PCB size is too large size, it will be wasteful. Moreover, components are not safe enough when placed in the board with too small size. Try to ensure functionality and practicality, but also pay attention to the aesthetics of the PCB layout. Secondly, high-level components need to be placed independently away from some sensitive components, and finally pay attention to the design of commonly used components in a convenient position.
Regarding the Common Software for PCB Design
There are many kinds of software used in PCB layout. According to different regions and needs, the commonly used software is also different. Some are easy to use, and some as complex. Here we take Protel series software as an example to list the general operation steps.
First, start the Protel series software and enter the editor window. Then set the size of the PCB first, and then put the components in the BOM list on the reasonable position of the PCB to complete the layout, and then connect the components with lines. Finally, adjust the layout rationality of lines and components to prevent conflicts and unreasonable placement. After completion, you can save it, and finally print out the drawing we just designed, so you can use Protel software to complete the design of a PCB.
Some Points Concerning Different Aspects of PCB Design
The silkscreen mainly displays the date, UL, company name, etc. in PCB. These data are helpful for PCB applications and after-sales, etc. Most novice PCB designers will focus too much on the font design and position placement of the silkscreen. The priority setting is too high, and the design looks beautiful and generous. However, after the actual operation, they will find that the size and position of the silkscreen are unreasonable, which will conflict with the position of the components. Either they are covered by components, or the characters are too large, which will extend to the welding area. The final character is neither complete nor beautiful.
Therefore, when designing the silkscreen layer, the priority setting should not be too high. Try to find a suitable location for imitation after the core components and circuit layout, to make it beautiful and reasonable.
The Main Process of PCB Design
I will use the RF PCB designed by Protel as an example to explain the main process of design.
Types of PCB Design
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Frequently Asked Questions
The following layout requirements for PCB components should be taken into consideration.
High, medium, and low-speed logic circuits should be arranged in different areas on the PCB.
The circuit should be arranged in such a way that the signal line length is minimized.
Ensure that there are no excessively long parallel signal lines between adjacent boards, between adjacent layers of the same board, or between adjacent routing on the same layer.
EMI filter should be as close as possible to the EMI source and placed on the same printed circuit board.
DC/DC converters, switching elements, and rectifiers should be placed as close to the transformer as possible to minimize their wire length.
Place voltage-regulating components and filter capacitors as close as possible to the rectifier diode.
Noise-sensitive routing should not parallel high current, and high-speed switching lines.
In general, the basic process of PCB design is as follows: pre-preparation → designing the structure of PCB →the layout of PCB→ routing→ routing optimization and silkscreen → the DRC check and the structure check → outputting PDF file
In general, the services PCB design companies can offer include PCB design (or flex board design), high-speed backplane design, circuit simulation, EMC design, PCB design technology training, schematic component library design, PCB package library design, and the technology consulting of PCB processing and inspection, etc.
When the company’s design team cannot meet the demands of the design schedule and the quality of the design, it is useful to use a PCB design company.
Using a PCB design company shortens the time to develop products and get them to market.
For cost reasons, most small and medium-sized companies do not hire or just hire a few full-time PCB design engineers. These companies do not have many projects, and recruiting 1-2 full-time PCB design engineers is a waste of resources. In this case, it is more cost-effective and efficient for them to choose a PCB design company.
Know more about the strength and backgrounds of a PCB design company.
Focus on the quality and effect of the design.
Choose a PCB design company with high professionalism and a good service attitude.
Be sure to choose a company that can meet your needs, taking into account cost-effectiveness.
The following are some special considerations for PCB design.
Avoid high-frequency interference: Minimize interference from high-frequency signal electromagnetic fields, i.e., crosstalk.
Solve the problem of signal integrity: Basically, it is the problem of impedance matching. Factors affecting impedance matching include the signal source architecture and output impedance, the characteristic impedance of the routing, the characteristics of the load side, and the topology of the routing.
Implement differential wiring: There are two points to note regarding the wiring of differential pairs. One is that the length of the two lines should be as long as possible, and the other is that the spacing between the two lines (determined by the differential impedance) should be kept the same, i.e., parallel.
Matching resistor between the differential pairs at the receiver: Usually, a matching resistor is added between the differential pairs at the receiver, and its value should be equal to the value of the differential impedance for better signal quality.
The wiring of the differential pair should be closely parallel: The wiring of the differential pair should be properly closed and parallel. The proper proximity is because that spacing affects the value of the differential impedance, which is an important parameter for designing differential pairs. Parallelism is also required because the consistency of the differential impedance is maintained.
Any type of PCB can be designed. Without being designed, PCBs can not be manufactured.
The following summarizes the common mistakes in PCB design.
Characters designed to be too small cause difficulties in silkscreen. Characters designed to be too large make characters overlap each other and difficult to distinguish.
The design does not conform to conventionality. For example, the component side is designed on the bottom layer while the solder side is designed on the top layer, causing inconvenience.
Some useless wiring was done on some graphics layers. For instance, a 4-layer PCB was designed with more than five layers of circuits, which caused misunderstanding.
Using filler blocks to draw pads in the PCB design can pass the DRC check, but is not conducive to PCBA processing, resulting in the difficulty of soldering components.