Circuit design is a basic skill for an electronic engineer. However, even if the circuit schematic diagram is perfect, if the common problems and challenges are not understood and prevented in the process of converting to PCB circuit board, the whole system will still be greatly compromised and will not work at all in serious cases. In order to avoid engineering design changes, improve efficiency and reduce costs, the author will explain the most prone to problems today. Finally, I will show you the DesignSpark PCB. You can download the software on the DesignSpark website and provide a large number of free resources, which will certainly bring you an extraordinary experience in PCB design.
Component selection and layout
The specification of each component is different, even if different manufacturers of the same product may not have the same component characteristics, so in the design of the choice of components, you must contact the supplier to understand the characteristics of the components, and know the impact of these characteristics on the design.
Nowadays, choosing the right memory is also very important for the design of electronic products. Due to the continuous updates of DRAM and Flash memory, it is a big challenge for PCB designers to design a new design that is not affected by the changing memory market outside. DDR3 now accounts for 85-90% of the current DRAM market, but DDR4 is expected to rise from 12% to 56% by 2014. So designers must keep a close eye on the memory market and keep close ties with manufacturers.
Components overheat and burn
In addition, the necessary calculation must be made for some components with large heat dissipation, and their layout also needs special consideration. A large number of components together can generate more heat, thus causing deformation and separation of soldering layer and even ignition of the whole board. So the design and layout engineers have to work together to make sure the components are properly laid out.
PCB size should be considered first when layout. When PCB size is too large, the length of printed line will increase, the impedance will increase, the anti-noise ability will decrease, and the cost will also increase. Too small, it is not good heat dissipation, and adjacent lines are easily disturbed. After determining the PCB size, determine the location of special components. Finally, according to the functional units of the circuit, the layout of all components of the circuit.
Two cooling system
The design of cooling system includes cooling method, cooling element selection and cold expansion coefficient consideration. At present, PCB heat dissipation is mainly through the PCB itself heat dissipation, heat sink and heat conduction plate.
In the traditional PCB board design, due to the use of copper clad/epoxy glass cloth or phenolic resin glass cloth substrate, and a small number of paper based copper clad plate, these materials have good electrical and processing properties, but poor thermal conductivity. As QFP, BGA and other surface mounting components are widely used in the current design, the heat generated by the components is largely transmitted to the PCB board. Therefore, the best way to solve the heat dissipation is to improve the heat dissipation capacity of the PCB in direct contact with the heating element, and conduct or send out through the PCB board.
When PCB has a few device calorific value bigger, can add radiator or conduct heat pipe on calorific device, when temperature still cannot come down, can use the radiator that takes fan. When the quantity of heating elements is high, a large heat dissipation cover can be used, and the whole heat dissipation cover is buckled on the element surface to contact with each element for heat dissipation. Professional computers for video and animation are even cooled by water cooling.
Humidity sensitivity class MSL
MSL: Moisure Sensitive Level, on the label outside a moisture-proof bag, it’s 1, 2, 2a, 3, 4, 5, 5a, 6. The components with special requirements for humidity or with the label of the humidity sensitive element on the package must be effectively managed to provide the temperature and humidity control scope of the material storage and manufacturing environment, so as to ensure the reliability of the performance of the temperature and humidity sensitive components. In baking, BGA, QFP, MEM, BIOS and other requirements of vacuum packaging perfect, high temperature resistant and high temperature resistant components are baked at different temperatures, pay attention to the baking time. PCB baking requirements first refer to PCB packaging requirements or customer requirements. The humidity sensor and PCB after baking shall not exceed 12H at room temperature, and the humidity sensor or PCB that is not used or has not been used for more than 12H at room temperature must be vacuum-packed or put into a drying box for storage.
The key techniques of PCB testability include: testability measurement, testability mechanism design and optimization, test information processing and fault diagnosis. In fact, testability design of PCB is to introduce some testability method that is convenient for testing into PCB and provide information channel to obtain test information of test object. Therefore, reasonable and effective design of testability mechanism is the guarantee of successfully improving the testability level of PCB. High product quality and reliability, reduce the cost of the whole life cycle of the product, the testability design technology is required to be able to quickly and conveniently obtain the feedback information during the test, can easily make fault diagnosis according to the feedback information. In PCB design, the detection position and the access path of DFT and other probe heads shall not be affected.
With the miniaturization of electronic products, the pitch of components will be smaller and the mounting density will be larger. There are less and less circuit nodes available for testing, so it is more and more difficult to test PCB assembly online. Therefore, the electrical and physical and mechanical conditions of the testability of PCB should be fully considered in the design, and appropriate mechanical and electronic equipment should be used for testing.