How To Avoid Traps In PCB Design

- Dec 28, 2018-

For a PCB designer, PCB design is a basic effort. But even if the circuit schematic is perfect, if you don't understand and prevent common problems and challenges in the process of converting to PCB board, the whole system will still be greatly reduced, and it will not work at all. In order to avoid engineering design changes, improve efficiency and reduce costs, today I will explain the most prone to problems. Finally, we will show you the DesignSpark PCB, which can be downloaded from the DesignSpark website, and a large number of free resource libraries will bring you an extraordinary experience in PCB design.

First, component selection and layout

     The specifications of each component are different. Even if the characteristics of components produced by different manufacturers in the same product may be different, the choice of components in design must be in contact with the supplier to understand the characteristics of the components, and know the characteristics. The impact of the design.

     In today's world, choosing the right memory is also very important for electronic product design. Due to the continuous updating of DRAM and Flash memory, PCB designers want new designs to be free from the ever-changing memory market. It is a big challenge. DDR3 now accounts for 85%-90% of the current DRAM market, but in 2014 it is expected that DDR4 will rise from 12% to 56%. Therefore, designers must aim at the memory market and maintain close contact with manufacturers.

Components overheated and burned

     In addition, for some components with large heat dissipation, necessary calculations must be made. Their layout also needs special consideration. When a large number of components are together, more heat can be generated, which causes deformation and separation of the solder resist layer, and even ignites the entire board. . So design and layout engineers must work together to ensure that the components have the right layout.

     The layout size must first consider the PCB size. When the PCB size is too large, the printed lines are long, the impedance is increased, the anti-noise ability is lowered, and the cost is also increased; if the size is too small, the heat dissipation is not good, and adjacent lines are susceptible to interference. After determining the PCB size, determine the location of the particular component. Finally, all the components of the circuit are laid out according to the functional unit of the circuit.

Second cooling system

      The design of the cooling system includes cooling methods and heat sink component selection, as well as consideration of the coefficient of cold expansion. At present, the main heat dissipation of the PCB is through the heat dissipation of the PCB board itself, plus the heat sink and the heat conducting board.

      In the traditional PCB board design, since the board is mostly made of copper/epoxy glass cloth substrate or phenolic resin glass cloth substrate, and a small amount of paper-based copper-clad board is used, these materials have good electrical properties and processing properties, but thermal conductivity. Very poor. Since the surface mount components such as QFP and BGA are widely used in the current design, the heat generated by the components is largely transmitted to the PCB. Therefore, the best way to solve the heat dissipation is to improve the heat dissipation capability of the PCB itself in direct contact with the heat generating component. The PCB board is conducted out or emitted.

     When there are a few devices in the PCB that generate a large amount of heat, a heat sink or a heat pipe can be added to the heat generating device. When the temperature cannot be lowered, a heat sink with a fan can be used. When the amount of the heat generating device is large, a large heat dissipating cover can be used, and the heat dissipating cover is integrally buckled on the component surface to be in contact with each component to dissipate heat. For professional computers for video and animation, even water cooling is required to cool down.

Three moisture sensitivity grade MSL

     MSL: Moisure Sensitive Level, which is a moisture sensitive grade, is stated on the label on the outside of the moisture-proof packaging bag. It is divided into: 1, 2, 2a, 3, 4, 5, 5a, and 6 levels. Components that have special requirements for humidity or have moisture-sensitive components marked on the package must be effectively managed to provide temperature and humidity control in the material storage and manufacturing environment to ensure the reliability of temperature and humidity sensitive components. When baking, BGA, QFP, MEM, BIOS, etc. require vacuum packaging to be perfect, and the components with high temperature resistance and high temperature resistance are baked at different temperatures, pay attention to the baking time. PCB baking requirements first refer to PCB packaging requirements or customer requirements. The moisture sensitive component and the PCB after baking should not exceed 12H at normal temperature. The humidity sensor or PCB that has not been used or not used at room temperature and does not exceed 12H must be sealed in a vacuum package or placed in a dry box.

Four testability design

      Key technologies for PCB testability include: measurability of testability, design and optimization of testability mechanisms, and processing and troubleshooting of test information. The testability design of the PCB is actually to introduce a testable method that can be easily tested into the PCB, and provide an information channel for obtaining the internal test information of the measured object. Therefore, the reasonable and effective design of testability mechanism is the guarantee to successfully improve the testability of PCB. High product quality and reliability, reducing product life cycle costs, requiring testability design technology to quickly and easily obtain feedback information during testing, and can easily make fault diagnosis based on feedback information. In PCB design, it is necessary to ensure that the detection position and the path of the entrance of the DFT and other probes are not affected.

      With the miniaturization of electronic products, the pitch of components is getting smaller and smaller, and the installation density will become larger and larger. There are fewer and fewer circuit nodes to be tested, so it is more and more difficult to test the printed circuit board assembly online. Therefore, the electrical conditions and physical and mechanical conditions of the testability of the printed board should be fully considered in the design. Test with appropriate mechanical and electronic equipment.

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