Basic concepts commonly used in PCB design

 Products designed by excellent engineers must meet design requirements and production processes at the same time. Any defect in one aspect cannot be considered a perfect product design. Standardize the product's circuit design, process design, PCB design, and other related process parameters, so that the production of physical products produced meets technical specifications such as productivity, measurability, and maintenance. In the process of product design, build the advantages of process, technology, quality, and cost. This article will allow you to quickly understand the basic concepts of PCB design.

1. FR4 board

FR-4 is a glass fiber oxygen resin covering a copper plate. A substrate on the circuit board can be divided into a general FR4 board and a high TG FR4 board. TG is the temperature of the glass conversion temperature, i.e. the melting point. The circuit board must be flammable, and it cannot be burned at a certain temperature. It can only be softened. At this time, the temperature point is called the glass conversion temperature (TG point), which is related to the size stability of the PCB board.

Generally, the TG plate is more than 130 degrees, the high TG is generally greater than 170 degrees, and the medium TG is about 150 degrees. PCB printed boards, which are usually TG ≥ 170 ° C, are called high TG printed boards. The TG of the substrate is improved, and the characteristics of heat resistance, moisture resistance, chemical resistance, and stability resistance of the printed board will be improved and improved. The higher the TG value, the better the board's temperature resistance, especially in the lead-free process. There are more high TG applications.

2, impedance matching

Impedance matching is mainly used for transmission lines to achieve the purpose of all high-frequency microwave signals that can be transmitted to the load point, and there is almost no signal reflection return source point, thus improving energy efficiency. The internal resistance of the signal source is equal to the characteristics of the transmission line and the same aspect, or the characteristic impedance of the transmission line is equal to the size of the load impedance and the same aspect. Increased resistance. The common impedance control values in PCB are 100-ohm differential impedance, 90-ohm differential impedance, and 50-ohm single-ended impedance.

3. Surface treatment

The surface treatment of PCB board is generally divided into several types. In order to better understand your PCB design problems, a brief introduction is now made:

1)Tin spray, the tin spray is the most common surface treatment process in circuit board lines. It has good weldability and can be used for most electronic products. Other surface treatments, it has the advantages of low cost and good weldability. Its shortcomings are that the surface is not as smooth as the surface, especially when the window is opened in large areas, it is more prone to uneven tin.

2)The difference between Shen Xi and Shenxi and tin spray is that its flatness is good, but the disadvantage is that it is very easy to oxidize black hair.

3)Shen Jin, as long as it is "Shen", its flatness is better than "spray". Shen Jin is not a leader. Shen Jin is usually used for gold fingers and keyboards. Because the resistance of gold is small, contact must be used, such as the keyboard light of the mobile phone. Shen Jin is soft gold. For those who often need to be inserted, they must use gold plating. Shen Jin is mainly nickel gold.

4) Gold plating

It was mentioned in Shen Jinzhong. When there is a fatal lack of gold plating, its weldability is poor, but its hardness is better than that of Shen. This process is usually not used in MID and VR design.

Small auxiliary tips: If there are flatness requirements, if the impedance circuit board (such as micro-line) requires the frequency of the frequency, try to use gold technology as much as possible; MID board cards with BGA generally use gold technology.

5) OSP

It mainly depends on the reaction between the part and the welded copper skin to produce weldability. The only advantage is that it is produced quickly and at a low cost. However, due to poor welding and easy oxidation, the circuit board is generally less used in the line.

4Core board (Core)/PP tablet (semi-curing tablet)

The reinforced material is immersed in resin, and a plate-shaped material made of copper foil on one or both sides is called a plate-shaped material made of hot pressing. It is the basic material for PCB, which is often called a substrate. When it is used for multilayer board production, it is also called a core board.

A piece-like adhesive material synthesized by resin and carrier is called PP tablets.

Core plates and semi-solidary chips are common materials for making multilayer boards.

5. Differential line

A differential signal is to send two equivalent and reverse signals to the driver's side. The receiver determines the logical state "0" or "1" by comparing the difference between the two voltages. The pair of wires carrying the differential signal is called differential wiring. Differential signals are also called differential signals. With two exactly the same, opposite signal transmission all the way, relying on two signal level differences to make a judgment. In order to ensure that the two signals are completely consistent, it is necessary to keep parallel when wiring and the line width and line spacing remain unchanged.

6. Signal Integrity

Signal integrity refers to the quality of the signal on the transmission line. The signal has good signal integrity. This means that when needed, there must be a voltage level value that must be reached. Poor signal integrity is not caused by a single factor, but by many factors in board design. The main signal integrity problems include reflection, oscillation, ground bomb, string disturbance, etc.

7, signal reflection

Reflection is the echo on the transmission line. Part of the signal power (voltage and current) is transmitted to the line and reaches the load, but some of it is reflected. If the source end has the same impedance as the load end, the reflection will not occur. Non-matching of the source end and the load end will cause online reflection, and the load will reflect some voltage reflection back to the source end. If the load impedance is less than the source impedance, the reflected voltage is negative, otherwise, if the load impedance is greater than the source impedance, the reflected voltage is positive. Changes in the geometry of the wiring, incorrect wire-end connection, transmission of connectors, and discontinuousness of the power plane will cause such reflections.

Reflection will cause the signal to overlap, the lower UNDERSHOOT, the ringing bell, and the slow edge of the ladder voltage wave.

8. Crosstalk

Stringing is a coupling between the two signal lines, and the interdependence between the signal lines and the mutual intermodulation causes online noise. Capacitive coupling causes coupling current, and perceptual coupling causes coupling voltage. The parameters of the PCB board layer, the signal line spacing, the driver and the electrical characteristics of the receiver, and the end-side connection method have a certain impact on string disturbance.

9.Internal layers (Inner layer)

The internal electrical layer is a PCB-negative sheet layer. The main role is to divide the power supply if necessary.

10, blind buried holes

1Blind holes: extend from the middle layer to the overhangs of a PCB surface layer. Common first-order, second-order, e.g., first-order blind holes refer to the perforated from the second layer to the top layer or the second layer to the bottom.

2Burning holes: From one middle to the other, the holes will not extend to the PCB surface layer.

11, testing point

Generally, independent PTH holes, SMT PAD, Golden Finger, Bonding finger, IC finger, BGA welding point, and customer test point after plug-in.

12, Mark point

Mark point is the position recognition point of the PCB applied to the automatic patch machine in the circuit board design, also known as the optical point site. The selection of Mark points directly affects the sewing efficiency of the automatic sewing machine. The selection of the Mark Point is related to the model of the automatic sewing machine. Mark is designed as a circular figure of 1 mm (40 mils). Considering the contrast between the color of the material and the environment, it is left-welding -a free area of 0.5 mm (19.7 mils) larger than the optical positioning reference symbol, and no characters are allowed, as shown in the figure. The optical positioning reference symbols on the same board are the same as those on its adjacent inner background. That is, the copper foil should be consistent or not under the three reference symbols. The surrounding 10mm insulated optical positioning symbols should be designed with a protective ring with an inner diameter of 2mm ring width and 1mm, and an 8-side insulated copper ring with a diameter of 2.8mm.

13PTH (metallic hole)/NPTH (non-metallic hole)

The hole with a metal layer of the hole is called a metal hole, which is mainly used for the electrical connection of the intervertebral conductive patterns. Conversely, it is a non-metal hole, which is generally used as a positioning hole or installation hole.