How to make a good PCB?
How to make a good PCB?
The two major difficulties in the field of microelectronics are high-frequency signal and weak signal processing, in this respect PCB production level is particularly important.
The same principle design, the same components, the PCB produced by different people has different results. then how to make a good PCB board?
Based on our past experience, we would like to talk about our views on the following aspects,
1, Clear design objectives
First of all, need to clear its design objectives, is the ordinary PCB board, high-frequency PCB board, small signal processing PCB board or both high frequency and small signal processing PCB board
If the ordinary PCB board, just make the wiring reasonable and tidy, mechanical size is accurate can be.
If there is a medium load and long line, it is necessary to use a certain means to deal with, reduce load, strengthen the drive to long line, the focus is to prevent long line reflection. When there are more than 40MHz signal lines on the board, special consideration should be given to these signal lines.
In addition there are other related factors to consider, such as board layer, the use of components of the package shape, the board's mechanical strength. Before making PCB board, need to know fairly well of design goals.
2, Understand the function of the components used in the layout and wiring requirements
For example, LOTI and APH analog signal amplifier, require the power supply should be smooth and ripple is small.Simulate small signal parts to try to stay away from power devices.
3, Consideration of component layout
One of the first factors to consider is the electrical performance, put the connected components together as closely as possible, especially for some high-speed lines, layout should make it as short as possible, power signal and small signal devices should be separated.
The transmission delay time of grounding and interconnect in high speed system is also the first factor to be considered in system design.
4, Consideration of wire layout
With the design of OTNI and star Optical fiber network, there will be more than 100MHz of high speed signal line of the board need to design. Here we will introduce some basic concepts of the high-speed line.
Any "long" signal path on a printed circuit board can be considered a transmission line.
For most MOS circuits at present, due to the increase of time on the line is much greater than the transmission delay time, so the line can be long meter without signal distortion. But for the faster logic circuit, especially ultra-high speed ECL, if there is no other measures, the length of the line must be greatly shortened to maintain the integrity of the signal.
1,Several types of transmission lines
（1）Coaxial cable and twisted pair.
They are often used for connections between systems and systems. The characteristic impedance of the coaxial cable usually has 50Ω and 75Ω, the twisted pair is usually 110Ω
（2）Microstrip lines on a printed board
The Microstrip line is a ribbon guide (signal Line), separated from the ground plane by an dielectric
（3）Strip lines in printed boards
A strip line is a copper strip in the middle of a dielectric between two layers of conductive plane.
2, End-connected transmission line
A resistor that is equal to the line characteristic impedance is terminated at the receiving end of a line. The transmission line is called the parallel end-connected line. It is mainly to obtain the best electrical properties, including driving the distributed load to use.
If a two-way delay of a non end-connected transmission line (The time that a signal travels back and forth on the transmission line.) is shorter than the pulse signal's rise time, the recoil caused by a non end-connected is about 15% of the logical swing.
When making a PCB, choose double layers or multilayer board, need to see the highest operating frequency and the complexity of the circuit system and the assembly density requirements to decide. Multi-layer board is preferred when the clock frequency exceeds 200MHZ.
If the working frequency is more than 350MHz, it is best to use PTFE as a dielectric layer of the printed circuit board, because of its high frequency decay is smaller, parasitic capacitors is smaller, and faster transmission, but also because of the Z0 is larger can save power.
The following are required for the layout of printed circuit boards
（1）All parallel signal lines should be kept to a large interval to minimize crosstalk
（2）Avoid sharp bends when designing signal transmission lines
（3）The width of the printed line can be calculated according to the characteristic impedance calculation formula of the microstrip line and strip line, and the impedance value of the general selection of 68Ω is more appropriate.
（4）For two-sided board (or six-layer board to walk four-ply line. The lines on both sides of the circuit board should be perpendicular to each other, in order to prevent mutual inductance generated crosstalk.
（5）If a large current device is installed on the printed board, such as relays, LED, horn and so on, their ground line best to separate walk, to reduce the noise on the ground. The ground wires of these large current devices should be connected to a separate ground bus on the board.
（6）If there's a small signal amplifier on the board, before amplifying, the weak signal line needs to stay away from the strong signal line, and the line should be as short as possible, if possible also to use ground to shield it.
VipCircuit, founded in 2004, Is a batch, sample and fast PCB - -oriented Enterprise, provide single-sided PCB, double-sided PCB, multilayer PCB, quick PCB prototyping and so on. Fourteen years PCB manufacturing experience in Shenzhen, China.