PCB Technical - Experience summary of mobile phone RF PCB layout and wiring

With the advent of a round of Bluetooth devices, cellular phones and 3G and 4G era, engineers pay more and more attention to the design skills of RF circuits. Radio frequency (RF) circuit board design is often described as a "black art" because there are still many uncertainties in theory, but this view is only partially correct. RF circuit board design also has many guidelines that can be followed and rules that should not be ignored.

However, in practical design, the real practical skill is how to compromise these guidelines and rules when they cannot be accurately implemented due to various design constraints. Of course, there are many important RF design topics worth discussing, including impedance and impedance matching, insulating layer materials and laminates, wavelength and standing wave, so these have a great impact on EMC and EMI of mobile phones. The following summarizes the conditions that must be met when designing RF layout of mobile phone PCB:

1.1 isolate the high-power RF amplifier (HPA) and low-noise amplifier (LNA) as far as possible. In short, keep the high-power RF transmitting circuit away from the low-power RF receiving circuit. The mobile phone has many functions and many components, but the PCB space is small. At the same time, considering the highest limit of the wiring design process, all these requirements for design skills are relatively high. At this time, it may be necessary to design four to six layers of PCBs to make them work alternately rather than at the same time. High power circuits may sometimes also include RF buffers and voltage controlled oscillators (VCOs). Ensure that there is at least a whole piece of land in the high-power area on the PCB. It is best that there are no vias on it. Of course, the more copper sheets, the better. Sensitive analog signals should be as far away from high-speed digital signals and RF signals as possible.

1.2 design zoning can be divided into physical zoning and electrical zoning. Physical zoning mainly involves the layout, orientation and shielding of components; Electrical partitions can be further divided into partitions of power distribution, RF wiring, sensitive circuits and signals, grounding, etc.

1.2.1 we discuss physical partitioning. Component layout is the key to an excellent RF design. The most effective technology is to first fix the components on the RF path and adjust its orientation to minimize the length of the RF path, keep the input away from the output, and separate the high-power circuit and low-power circuit as far as possible.

The most effective circuit board stacking method is to arrange the main ground (main ground) on the second layer below the surface layer, and walk the RF line on the surface layer as much as possible. Minimizing the via size on the RF path can not only reduce the path inductance, but also reduce the false solder joints on the main ground, and reduce the chance of RF energy leakage to other areas in the laminate. In physical space, linear circuits such as multistage amplifiers are usually enough to isolate multiple RF regions from each other, but duplexers, mixers and if Amplifiers / mixers always have multiple RF / if signals interfering with each other, so this effect must be carefully minimized.

1.2.2 the routing of RF and if shall cross as far as possible and be separated by a piece of land as far as possible. The correct RF path is very important for the performance of the whole PCB, which is why the component layout usually accounts for most of the time in the design of mobile phone PCB. In the design of mobile phone PCB, it is usually possible to place the low-noise amplifier circuit on one side of the PCB and the high-power amplifier on the other side, and finally connect them to the antenna at the RF end and baseband processor end on the same side through the duplexer. Some skills are needed to ensure that the through holes do not transfer RF energy from one side of the board to the other. The common technology is to use blind holes on both sides. The adverse effect of the through-hole can be minimized by arranging the through-hole in the area where both sides of the PCB are free from RF interference. Sometimes it is impossible to ensure sufficient isolation between multiple circuit blocks. In this case, it must be considered to use metal shield to shield RF energy in the RF area. The metal shield must be welded to the ground and keep an appropriate distance from components. Therefore, it needs to occupy valuable PCB space. It is very important to ensure the integrity of the shield as much as possible. The digital signal line entering the metal shield should go through the inner layer as much as possible, and the PCB layer below the wiring layer is the layer. The RF signal line can go out from the small gap at the bottom of the metal shield and the wiring layer at the ground gap, but more ground should be distributed around the gap as much as possible, and the ground on different layers can be connected together through multiple vias.

1.2.3 proper and effective chip power decoupling is also very important. Many RF chips integrated with linear lines are very sensitive to power supply noise. Usually, each chip needs up to four capacitors and an isolation inductor to ensure that all power supply noise is filtered out. An integrated circuit or amplifier often has an open drain output, so a pull-up inductor is required to provide a high impedance RF load and a low impedance DC power supply. The same principle also applies to decoupling the power supply at the inductor end. Some chips need multiple power supplies to work, so you may need two or three sets of capacitors and inductors to decouple them respectively. The inductors are rarely close together in parallel, because it will form a hollow core transformer and induce interference signals to each other, so the distance between them should be at least equal to the height of one of the devices, Or arranged at right angles to minimize mutual inductance.

1.2.4 the principle of electrical zoning is basically the same as that of physical zoning, but it also includes some other factors. Some parts of the mobile phone adopt different working voltages and are controlled by software to prolong the service life of the battery. This means that the phone needs to run multiple power supplies, which brings more problems to isolation. The power supply is usually introduced from the connector and immediately decoupled to filter out any noise from the outside of the circuit board, and then distributed after passing through a group of switches or regulators. The DC current of most circuits on the mobile phone PCB is quite small, so the routing width is usually not a problem. However, a large current line as wide as possible must be routed separately for the power supply of the high-power amplifier to minimize the transmission voltage drop. In order to avoid too much current loss, multiple vias need to be used to transfer current from one layer to another. In addition, if it cannot be fully decoupled at the power pin end of the high-power amplifier, the high-power noise will radiate to the whole board and bring all kinds of problems. The grounding of high power amplifier is very important, and it is often necessary to design a metal shield for it. In most cases, it is also critical to ensure that the RF output is away from the RF input. This also applies to amplifiers, buffers and filters. In the worst case, if the outputs of amplifiers and buffers are fed back to their inputs with appropriate phase and amplitude, they may produce self-excited oscillation. At best, they will be able to operate stably at any temperature and voltage. In fact, they may become unstable and add noise and intermodulation signals to the RF signal. If the RF signal line has to be wound back from the input to the output of the filter, it may seriously damage the bandpass characteristics of the filter. In order to isolate the input and output well, a circle of ground must be arranged around the filter, and then a piece of ground must be arranged in the lower area of the filter and connected with the main ground around the filter. It is also a good way to keep the signal line that needs to pass through the filter as far away from the filter pin as possible.

In addition, the grounding of all parts on the whole board should be very careful, otherwise a coupling channel will be introduced. Sometimes single ended or balanced RF signal lines can be selected. The principles of cross interference and EMC / EMI also apply here. Balancing RF signal lines can reduce noise and cross interference if they are routed correctly, but their impedance is usually relatively high, and it may be difficult to maintain a reasonable linewidth to obtain an impedance matching signal source, routing and load. The buffer can be used to improve the isolation effect because it can divide the same signal into two parts and drive different circuits. In particular, the local oscillator may need a buffer to drive multiple mixers. When the mixer reaches the common mode isolation state at RF frequency, it will not work properly. The buffer can well isolate the impedance changes at different frequencies, so that the circuits will not interfere with each other. Buffers are very helpful to the design. They can closely follow the circuit to be driven, so that the high-power output line is very short. Because the input signal level of buffers is relatively low, they are not easy to cause interference to other circuits on the board. Voltage controlled oscillators (VCOs) can convert changing voltage into changing frequency, which is used for high-speed channel switching, but they also convert trace noise on the control voltage into small frequency change, which adds noise to the RF signal.

1.2.5 to ensure no increase in noise, the following aspects must be considered: first, the expected bandwidth of the control line may range from DC to 2MHz, and it is almost impossible to remove such wide-band noise through filtering; Secondly, the VCO control line is usually part of a feedback loop that controls the frequency. It may introduce noise in many places. Therefore, the VCO control line must be handled very carefully. Ensure that the ground under the RF wiring is solid, and all components are firmly connected to the main ground and isolated from other wiring that may cause noise. In addition, to ensure that the power supply of the VCO has been fully decoupled, because the RF output of the VCO is often a relatively high level, the VCO output signal is easy to interfere with other circuits, so special attention must be paid to the VCO. In fact, VCO is often placed at the end of the RF area, and sometimes it needs a metal shield. The resonant circuit (one for the transmitter and the other for the receiver) is related to VCO, but it also has its own characteristics. In short, the resonant circuit is a parallel resonant circuit with capacitive diodes, which helps to set the VCO operating frequency and modulate voice or data to the RF signal. The design principles of all VCOs also apply to resonant circuits. Because the resonant circuit contains a considerable number of components, a wide distribution area on the board and usually operates at a high RF frequency, the resonant circuit is usually very sensitive to noise. The signals are usually arranged on the adjacent pins of the chip, but these signal pins need to cooperate with relatively large inductors and capacitors to work, which in turn requires that the positions of these inductors and capacitors must be close and connected back to a control loop sensitive to noise. It is not easy to do this.

Automatic gain control (AGC) amplifier is also an easy problem. There will be AGC amplifier in both transmitting and receiving circuits. AGC amplifiers usually can effectively filter out noise. However, because mobile phones have the ability to deal with the rapid change of transmitted and received signal strength, AGC circuits are required to have a fairly wide bandwidth, which makes it easy for AGC amplifiers in some key circuits to introduce noise. The design of AGC circuit must comply with good analog circuit design technology, which is related to the short input pin of operational amplifier and short feedback path, both of which must be far away from RF, if or high-speed digital signal routing. Similarly, good grounding is also essential, and the power supply of the chip must be well decoupled. If it is necessary to take a long line at the input or output, it is better to take it at the output. Usually, the impedance of the output is much lower and it is not easy to induce noise. Generally, the higher the signal level, the easier it is to introduce noise into other circuits. In all PCB designs, it is a general principle to keep digital circuits away from analog circuits as far as possible, which is also applicable to RF PCB design. Common analog ground and ground used to shield and separate signal lines are usually equally important. Therefore, in the early stage of design, careful planning, comprehensive component layout and thorough layout * estimation are very important. Similarly, RF lines should be kept away from analog lines and some key digital signals. All RF wiring, pads and components should be filled with grounding copper as much as possible, And connect with the main ground as much as possible. If the RF routing must pass through the signal line, try to lay a layer of ground connected to the main ground along the RF routing. If it is impossible, make sure they are crossed, which can minimize capacitive coupling. At the same time, distribute more ground around each RF line as much as possible and connect them to the main ground. In addition, minimizing the distance between parallel RF routes can minimize inductive coupling. The isolation effect is best when a solid whole grounding plate is placed directly on the first layer below the surface layer, although other methods also work when designing with care. On each layer of PCB board, lay as much ground as possible and connect them to the main ground. Place the routing as close as possible to increase the number of plots in the internal signal layer and power distribution layer, and adjust the routing appropriately so that you can arrange the ground connection vias to the isolated plots on the surface. Generation of free ground on PCB layers should be avoided because they will pick up or inject noise like a small antenna. In most cases, if you can't connect them to the main place, you'd better remove them.

1.3 in the design of mobile phone PCB, great attention should be paid to the following aspects

1.3.1 treatment of power supply and ground wire

Even if the wiring in the whole PCB board is well completed, the interference caused by the thoughtless consideration of power supply and ground wire will degrade the performance of the product, and sometimes even affect the success rate of the product. Therefore, the wiring of electricity and ground wire should be taken seriously to minimize the noise interference generated by electricity and ground wire, so as to ensure the quality of products. Every engineer engaged in electronic product design understands the causes of noise between ground wire and power line. Now only reduced noise suppression is described:

(1) It is well known to add coupling capacitance between power supply and ground wire.

(2) The width of power supply and ground wire shall be widened as much as possible, preferably the ground wire is wider than the power line width, and their relationship is: ground wire > power line > signal line. Generally, the signal line width is 0.2 ~ 0.3mm, the thinnest width can reach 0.05 ~ 0.07mm, and the power line is 1.2 ~ 2.5mm. For the PCB of digital circuit, a wide ground wire can be used to form a circuit, that is, to form a ground grid (the ground of analog circuit cannot be used in this way)

(3) Use a large area of copper layer as the ground wire, and connect the unused places on the printed board with the ground as the ground wire. Or it can be made into a multilayer board, and the power supply and ground wire occupy one floor respectively.

1.3.2 common ground processing of digital circuit and analog circuit

Nowadays, many PCBs are no longer single functional circuits (digital or analog circuits), but are composed of a mixture of digital circuits and analog circuits. Therefore, it is necessary to consider the mutual interference between them, especially the noise interference on the ground wire. The frequency of digital circuit is high and the sensitivity of analog circuit is strong. For the signal line, the high-frequency signal line is far away from the sensitive analog circuit devices as far as possible. For the ground wire, the whole PCB has only one node to the outside, so the problem of digital and analog common ground must be handled inside the PCB. In fact, the digital ground and analog ground inside the board are separated, and they are not connected to each other, Only at the interface between PCB and the outside world (such as plug, etc.). There is a short circuit between digital ground and analog ground. Please note that there is only one connection point. Some are not common on the PCB, which is determined by the system design.

1.3.3 the signal line is arranged on the electric (ground) layer

When wiring multi-layer printed boards, there are not many lines left in the signal line layer. Adding more layers will cause waste, increase certain workload for production, and increase the cost accordingly. In order to solve this contradiction, wiring on the electrical (ground) layer can be considered. The power layer should be considered first, followed by the stratum. Because it is best to preserve the integrity of the formation.

1.3.4 treatment of connecting legs in large area conductors

In large-area grounding (electricity), the legs of common components are connected with them, and the treatment of the connecting legs needs to be comprehensively considered. In terms of electrical performance, the bonding pad of the component legs is fully connected with the copper surface, but there are some adverse hidden dangers in the welding and assembly of components, such as: 1. welding requires a high-power heater. 2. It is easy to cause false solder joints. Therefore, taking into account the electrical performance and process needs, a cruciform pad is made, which is called heat shield and commonly known as thermal. In this way, the possibility of false solder joints due to excessive heat dissipation of the section during welding can be greatly reduced. The treatment of the grounding (grounding) leg of the multilayer board is the same.

1.3.5 function of network system in wiring

In many CAD systems, routing is determined according to the network system. Although the grid is too dense and the path has increased, the step is too small and the amount of data in the map field is too large, which will inevitably have higher requirements for the storage space of the equipment, and also have a great impact on the operation speed of the object computer electronic products. Some paths are invalid, such as those occupied by the pad of the component leg or by the mounting hole and fixing hole. Too sparse grids and too few paths have a great impact on the distribution rate. Therefore, there should be a dense and reasonable grid system to support the wiring. The distance between the legs of standard components is 0.1 inch (2.54 mm), so the basis of grid system is generally set as 0.1 inch (2.54 mm) or an integral multiple less than 0.1 inch, such as 0.05 inch, 0.025 inch, 0.02 inch, etc.

1.4 the skills and methods of high frequency PCB design are as follows:

1.4.1 45 ° angle shall be adopted for the corner of transmission line to reduce back loss

1.4.2 high performance insulated circuit board with insulation constant value strictly controlled by level shall be adopted. This method is conducive to the effective management of the electromagnetic field between the insulating material and the adjacent wiring.

1.4.3 PCB design specifications for high-precision etching shall be improved. Consider specifying a total line width error of + / - 0.0007 inches, managing undercut and cross section of wiring shapes, and specifying wiring sidewall plating conditions. The overall management of wiring (wire) geometry and coating surface is very important to solve the problem of skin effect related to microwave frequency and realize these specifications.

1.4.4 the protruding lead has tap inductance, and the components with leads shall be avoided. In high frequency environments, surface mount components are preferred.

1.4.5 for signal vias, it is necessary to avoid using via machining (PTH) process on sensitive board, because this process will lead to lead inductance at vias.

1.4.6 abundant ground plane shall be provided. Molded holes shall be used to connect these grounding layers to prevent the influence of 3D electromagnetic field on the circuit board.

1.4.7 non electrolytic nickel plating or gold dipping process shall be selected, and HASL method shall not be used for electroplating. This electroplated surface can provide better skin effect for high-frequency current (Fig. 2). In addition, this highly weldable coating requires fewer leads, which helps to reduce environmental pollution.

1.4.8 the solder resist layer can prevent the flow of solder paste. However, due to the uncertainty of thickness and the uncertainty of insulation performance, the whole plate surface is covered with solder resist material, which will lead to great changes in electromagnetic energy in microstrip design. Generally, the solder dam is used as the solder resist layer. Electromagnetic field. In this case, we manage the conversion from microstrip to coaxial cable. In coaxial cable, the ground wire layer is ring interleaved and evenly spaced. In a microstrip, the ground plane is below the active line. This introduces some edge effects that need to be understood, predicted and considered in the design. Of course, this mismatch will also lead to back loss. This mismatch must be minimized to avoid noise and signal interference.

1.5 EMC design

Electromagnetic compatibility refers to the ability of electronic equipment to work coordinately and effectively in various electromagnetic environments. The purpose of EMC design is to make the electronic equipment not only suppress all kinds of external interference, make the electronic equipment work normally in the specific electromagnetic environment, but also reduce the electromagnetic interference of the electronic equipment itself to other electronic equipment.

1.5.1 select a reasonable conductor width

Since the impact interference caused by transient current on the printed wire is mainly caused by the inductance of the printed wire, the inductance of the printed wire should be reduced as much as possible. The inductance of printed wire is directly proportional to its length and inversely proportional to its width. Therefore, short and precise wire is beneficial to suppress interference. The signal lines of clock leads, row drivers or bus drivers often carry large transient currents, and the printed wires should be as short as possible. For discrete component circuits, when the printed wire width is about 1.5mm, it can fully meet the requirements; For integrated circuits, the width of printed wires can be selected between 0.2 ~ 1.0mm.

1.5.2 adopt correct wiring strategy

The use of equal routing can reduce the conductor inductance, but the mutual inductance and distributed capacitance between conductors increase. If the layout allows, it is best to use the well shaped network wiring structure. The specific method is that one side of the printed board is wired horizontally and the other side is wired longitudinally, and then connected with metal holes at the cross holes.

1.5.3 in order to suppress the crosstalk between printed board conductors, long-distance equal routing shall be avoided as far as possible in the design of wiring, the distance between wires shall be opened as far as possible, and the signal wire, ground wire and power wire shall not cross as far as possible. A grounded printed line is set between some signal lines that are very sensitive to interference, which can effectively suppress crosstalk.

1.5.4 in order to avoid electromagnetic radiation generated when high-frequency signals pass through printed wires, pay attention to the following points when wiring printed circuit boards:

(1) The discontinuity of printed wires shall be minimized. For example, the width of wires shall not change suddenly, the corners of wires shall be greater than 90 degrees, and circular routing is prohibited.

(2) The clock signal lead is most likely to generate electromagnetic radiation interference. When routing, it shall be close to the ground circuit, and the driver shall be close to the connector.

(3) The bus driver shall be next to the bus to be driven. For those leads leaving the printed circuit board, the driver shall be close to the connector.

(4) The wiring of data bus shall be clamped with a signal ground wire between each two signal wires. It is best to place the ground loop next to the least important address lead, because the latter often carries high-frequency current.

(5) When high-speed, medium speed and low-speed logic circuits are arranged on the printed board, the devices shall be arranged in the manner of Figure 1.

1.5.5 suppression of reflection interference

In order to suppress the reflection interference at the terminal of printed line, in addition to special needs, the length of printed line shall be shortened as much as possible and slow circuit shall be adopted. If necessary, terminal matching can be added, that is, a matching resistance with the same resistance value can be added to the ground and the power supply terminal at the end of the transmission line. According to experience, terminal matching measures should be adopted for TTL circuits with general high speed when the printed line is longer than 10cm. The resistance value of matching resistance shall be determined according to the maximum value of output driving current and absorption current of integrated circuit.

1.5.6 differential signal line wiring strategy shall be adopted in circuit board design

Differential signal pairs with very close wiring will also be closely coupled with each other, which will reduce EMI emission. Usually (of course, there are some exceptions) differential signals are also high-speed signals, so high-speed design rules are usually applicable to the layout of differential signals, especially when designing the signal lines of transmission lines. This means that we must carefully design the wiring of the signal line to ensure that the characteristic impedance of the signal line is continuous throughout the signal line and maintains a constant. During the layout and wiring of the differential pair, we hope that the two PCB lines in the differential pair are completely consistent. This means that in practical application, we should try our best to ensure that the PCB lines in the differential line pair have exactly the same impedance and the wiring length is exactly the same. Differential PCB lines are usually wired in pairs, and the distance between them remains a constant at any position along the line. In general, the layout and routing of differential line pairs are always as close as possible.