1.3 in the mobile phone PCB board design, the following aspects should be given great attention

1.3.1 power, ground handling
Even if the wiring in the entire PCB board is done very well, but due to power, ground considerations are not thoughtful interference caused by the performance of the product will decline, and sometimes even affect the success rate of the product. So the electricity, ground wiring to be taken seriously, the electricity, ground noise generated by the interference to a minimum, to ensure product quality. For each engineer engaged in the design of electronic products are aware of the noise between the line and the power supply of the reasons, only to reduce the type of suppression noise to express:
(1), it is well known to add a decoupling capacitor between the power supply and ground.
(2), as far as possible to widen the power, ground width, preferably ground than the power line width, their relationship is: ground> power line> signal line, usually signal line width: 0.2 ~ 0.3mm, Fine width of up to 0.05 ~ 0.07mm, power line 1.2 ~ 2.5mm. The PCB of the digital circuit can use a wide wire to form a loop, that is, to form a ground to use (analog circuit can not be used)
(3), with a large area of ​​copper for the ground line, in the printed circuit board has not been used on the ground are connected with the ground as a ground. Or made of multi-layer board, power, ground each occupied a layer.

1.3.2 digital circuit and analog circuit common ground processing
Now there are many PCB is no longer a single functional circuit (digital or analog circuit), but by the digital circuit and analog circuit mix. So when wiring need to consider the interference between them, especially on the ground noise interference. Digital circuit frequency is high, the sensitivity of the analog circuit is strong, the signal line, the high-frequency signal line as far as possible away from the sensitive analog circuit devices, the ground, the whole PCB on the outside world only one node, so Must be handled within the PCB number, the problem of the common ground, and in the board of digital and analog ground is actually separated from each other between them are not connected, but in the PCB and the external interface (such as plugs, etc.). Digital and analog to a little short, please note that there is only one connection point. There are also on the PCB on the ground, which is determined by the system design.

1.3.3 signal lines in the electrical (ground) layer
In the multi-layer printed circuit board wiring, because the signal line layer is not finished cloth line left is not much, and then add more layers will cause waste will also increase the production of a certain amount of work, the cost has increased accordingly To solve this contradiction, you can consider wiring in the electrical (ground) layer. First consider the power layer, followed by the formation. Because it is best to preserve the integrity of the formation.

1.3.4 Treatment of connecting legs in large area conductors
In a large area of ​​the ground (electricity), the commonly used components of the legs connected to the connection legs to deal with the need for comprehensive consideration, the electrical performance, the component legs of the pad and copper surface is better, but the Components of the welding assembly there are some bad risks such as: ① welding need high-power heater. ② easy to cause virtual solder joints. So taking into account the electrical performance and process needs, made of cross-shaped pad, known as the heat isolation (heatshield) commonly known as the thermal pad (Thermal), so that the welding can be too much heat due to cross-section of the potential decrease very much. Multilayer board of the power (ground) layer of the same treatment.

1.3.5 The role of the network system in the wiring
In many CAD systems, cabling is determined by the network system. Grid is too dense, although the channel has increased, but the pace is too small, the figure of the amount of data is too large, which must have a higher storage requirements of the equipment, but also the object of computer electronic products operating speed Great influence. And some passages are ineffective, such as those occupied by the pads of the component legs or by the holes, which are occupied by the holes. The grid is too sparse, the passage of too little impact on the cloth rate. So there must be a dense and reasonable grid system to support the wiring. The distance between the legs of the standard component is 0.1 inch (2.54 mm), so the basis of the grid system is generally set to an integer multiple of 0.1 inch (2.54 mm) or less than 0.1 inch, such as 0.05 inch, 0.025 inch, 0.02 Inch and so on.

1.4 techniques and methods for high-frequency PCB design are as follows:

1.4.1 Transmission line corner to use 45 ° angle to reduce the return loss

1.4.2 to use the insulation constant value strictly controlled by the level of high-performance insulation circuit board. This method is advantageous for the effective management of the electromagnetic field between the insulating material and the adjacent wiring.

1.4.3 To improve the PCB design specifications for high-precision etching. Consider the specified line width total error of +/- 0.0007 inches, manage the undercut and cross section of the wiring shape and specify the wiring sidewall plating conditions. The overall management of the wiring (wire) geometry and the coating surface is important to address the skin effect associated with microwave frequencies and to implement these specifications.

1.4.4 highlight the presence of tap inductance, to avoid the use of lead components. In high frequency environments, it is best to use surface mount components.

1.4.5 For signal vias, avoid using the pth process on the sensitive plate, as this process results in lead inductance at the vias.

1.4.6 To provide a rich ground layer. These ground layers are to be connected using molded holes to prevent the effect of 3-dimensional electromagnetic fields on the circuit board.

1.4.7 to choose non-electrolytic nickel plating or gold plating process, do not use HASL method for plating. This electroplated surface can provide a better skin effect for high frequency currents (Figure 2). In addition, this high solderable coating requires fewer leads and helps reduce environmental pollution.

1.4.8 solder mask can prevent the flow of solder paste. However, due to the thickness of the uncertainty and the insulation properties of the unknown, the entire surface of the board are covered with solder mask material will lead to microstrip design of the electromagnetic energy in the larger changes. Generally use solder dam (solderdam) as a solder mask. Of the electromagnetic field. In this case, we manage the transition between the microstrip and the coaxial cable. In the coaxial cable, the ground layer is circularly intertwined, and the interval is uniform. In the microstrip, the ground plane is below the active line. This introduces some edge effects that need to be understood, predicted and taken into account at design time. Of course, this mismatch can also cause a return loss, which must be minimized to avoid noise and signal interference.

1.5 Electromagnetic compatibility design

Electromagnetic compatibility is the ability of electronic equipment to be able to work effectively and effectively in various electromagnetic environments. Electromagnetic compatibility design aims to make electronic equipment can not only inhibit a variety of external interference, so that electronic equipment in a specific electromagnetic environment can work properly, while reducing the electronic equipment itself on other electronic equipment, electromagnetic interference.

1.5.1 Select a reasonable wire width
As the transient current on the printed line generated by the impact of interference is mainly caused by the inductance of the printed wire, and therefore should minimize the inductance of the printed wire. The inductance of the printed wire is proportional to its length, inversely proportional to its width, so short and fine wires are advantageous for suppressing interference. Clock lines, line drivers, or bus drivers' signal lines often carry large transients, and the printed leads are as short as possible. For the discrete component circuit, the printed wire width of 1.5mm or so, you can fully meet the requirements; for integrated circuits, printed wire width can be selected between 0.2 ~ 1.0mm.

1.5.2 using the correct routing strategy
The use of equal alignment can reduce the wire inductance, but the wire between the mutual inductance and distributed capacitance increases, if the layout allows, it is best to use well-shaped mesh fabric structure, the specific approach is printed on the side of the horizontal wiring, the other side of vertical wiring, And then connected at the cross hole with a metal hole.

1.5.3 In order to suppress the crosstalk between the printed circuit board wire, in the design of wiring should try to avoid long distance equal alignment, as far as possible to pull the line between the line and the distance between the signal line and the ground and power lines as much as possible Not cross. It is possible to effectively suppress crosstalk by setting a grounded printed line between some signal lines that are sensitive to interference.

1.5.4 In order to avoid the high frequency signal through the printed wire when the electromagnetic radiation generated in the printed circuit board wiring, should also note the following:
(1) to minimize the continuity of the printed wire, such as wire width without mutation, the corner of the wire should be greater than 90 degrees forbidden ring alignment.
(2) the clock signal lead most likely to produce electromagnetic radiation interference, the alignment should be close to the ground circuit, the drive should be next to the connector.
(3) The bus driver should be close to the bus to be driven. For leads that leave the printed circuit board, the drive should be next to the connector.
(4) The wiring of the data bus should be sandwiched between two signal lines. It is advisable to place the ground circuit next to the least important address pins, since the latter often carries high frequency currents.
(5) When arranging high-speed, medium-speed and low-speed logic circuits on printed circuit boards, the devices should be arranged in the manner shown in Fig.

1.5.5 Suppression of reflection interference
In order to suppress the reflection interference that appears at the end of the printed line, the length of the printed wire and the slow circuit should be used as much as possible, except for special needs. If necessary, can be added terminal matching, that is, at the end of the transmission line to the ground and the power side of the same resistance to the same matching resistance. According to experience, the general speed of the TTL circuit, the printed line longer than 10cm should be used when the terminal matching measures. The resistance of the matching resistor should be determined according to the output drive current of the integrated circuit and the maximum value of the absorption current.

1.5.6 circuit board design process using differential signal line routing strategy
Wiring is very close to the differential signal pairs will be closely coupled with each other, this coupling between each other will reduce the EMI emission, usually (of course there are some exceptions) differential signal is high-speed signal, so high-speed design rules are usually applicable This is especially true for wiring of differential signals, especially when designing signal lines for transmission lines. This means that we must design the wiring of the signal lines with great care to ensure that the characteristic impedance of the signal lines is continuous and maintains a constant along the signal lines. In the process of routing and routing of differential pairs, we hope that the two pairs of PCB lines in the differential line are exactly the same. This means that the best effort should be made in practical applications to ensure that the PCB lines in the differential line pair have exactly the same impedance and that the length of the wiring is exactly the same. Differential PCB lines are usually always in pairs, and the distance between them is kept constant at any position along the direction of the pair. In general, the layout and routing of differential pairs is always as close as possible.