PCB Blog - Mobile phone PCB board reliability design scheme

RF PCB board design is often described as a "black art" due to theoretical uncertainties, but this view is only partially true. RF board design has many rules that can be followed and rules that should not be ignored. However, in practical design, the really useful technique is how to compromise these principles and laws when they cannot be implemented precisely because of various design constraints. Of course, there are many important RF design topics worth discussing, including impedance and impedance matching, insulating layer materials and laminates, and wavelength and standing wave, so these have a great impact on the EMC and EMI of mobile phones.

1. Isolate high power RF amplifier (HPA) and low noise amplifier (LNA) as far as possible. In short, let high power RF transmitting circuit away from low power RF receiving circuit. Mobile phone features more, a lot of components, but the PCB board space is small, taking into account the design process of wiring limit, all these requirements for design skills are relatively high. At this point, you might want to design four to six PCB layers to work alternately, rather than simultaneously. High power circuits may also sometimes include RF buffers and voltage controlled oscillators (VCO). Make sure there is at least one whole floor of the high power area on the PCB with no holes in it. Of course, the more copper skin the better. Sensitive analog signals should be kept as far away from high-speed digital signals and RF signals as possible.

2. Design zones can be divided into physical zones and electrical zones. Physical partitioning mainly involves components layout, orientation and shielding, etc. Electrical partitions can continue to be decomposed into partitions for power distribution, RF wiring, sensitive circuits and signals, and grounding.
2.1 We discuss physical partitioning. The component layout is the key to implementing an RF design. The effective technique is to first fix the components on the RF path and Orient them so that the length of the RF path is reduced to such that the input is far away from the output and the high-power and low-power circuits are separated as far as possible. An efficient way to stack circuit boards is to place the main ground floor (the main ground) on the second layer below the surface, with RF lines on the surface as much as possible. Reducing the size of the through-holes in the RF path not only reduces path inductance, but also reduces virtual solder joints on the main ground and the chance of RF energy leakage to other areas within the laminate. In physical space, linear circuits such as multistage amplifiers are usually sufficient to isolate multiple RF regions from each other, but diplexers, mixers, and IF amplifiers/mixers always have multiple RF/IF signals interfering with each other, so this effect must be carefully reduced to.

2.2 RF and IF should be crossed as far as possible, and separated as far as possible between them. The correct RF path is very important to the performance of the entire PCB, which is why component layout usually takes up most of the time in mobile PCB design. In cell phone PCB designs, 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 eventually connect them to the RF and baseband processor antenna on the same side through a dipper. Some tricks are required to ensure that straight through holes does not transfer RF energy from one side of the board to the other, and a common technique is to use blind holes on both sides. The adverse effects of straight through holes can be reduced to a minimum by arranging straight through holes in areas on both sides of the PCB that are free from RF interference. Sometimes it is not possible to ensure adequate isolation between multiple circuit blocks, in which case a metal shield must be considered to shield RF energy within the RF region. The metal shield must be sold-to the ground and kept at a reasonable distance from the components, thus taking up valuable PCB space. It is very important to ensure the integrity of the shielding cover as much as possible. The digital signal line entering the metal shielding cover should go through the inner layer as much as possible, and the PCB board below the wiring layer is the stratum. RF signal line can go out from the small gap at the bottom of the metal shield cover and the wiring layer of the gap, but around the gap as much as possible to cloth some ground, the ground on different layers can be connected through a plurality of holes.

2.3 Proper and effective chip power decoupling is also very important. Many RF chips with integrated linear circuits are very sensitive to power source noise, and typically each chip requires up to four capacitors and an isolating inductor to ensure that all power source noise is filtered. 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 applies to decoupling the power supply at the inductor end. Some chip need more power to work, so you may need two or three sets of capacitance and inductance to decoupling on them respectively, few inductance parallel together, because this will form a tubular transformer and mutual induction interference signal, so the distance between them must be at least equal the height of one of the devices, or a right Angle to the mutual inductance to.

2.4 The principles for electrical zoning are generally the same as for physical zoning, but there are some other factors involved. Some parts of the phone operate at different voltages and are controlled by software to extend battery life. That means the phone needs to run on multiple power sources, which creates more problems for isolation. Power is usually brought in from the connector, immediately decoupled to filter out any noise coming from outside the circuit board, and then distributed through a set of switches or regulators. The DC current of most circuits on cell phone PCBS is fairly small, so wiring width is usually not an issue, however, a separate high-current line as wide as possible must be run for the high-power amplifier's power supply to reduce the transmission voltage drop to. To avoid too much current loss, a plurality of holes are used to transfer current from one layer to another. In addition, if it is not sufficiently decoupled at the power pin end of the high power amplifier, the high power noise will radiate throughout the board and bring all kinds of problems. Grounding of high power amplifiers is critical and often requires the design of a metal shield. In most cases, it is also critical to ensure that the RF output is kept away from the RF input. This also applies to amplifiers, buffers, and filters. In the bad case, amplifiers and buffers can generate self-excited oscillations if their outputs are fed back to their inputs with the right phase and amplitude. In this case, they will be able to operate stably under any temperature and voltage conditions. In fact, they can become unstable and add noise and intermodulation signals to RF signals. If the RF signal line has to wind back from the input to the output of the filter, this can seriously impair the bandpass characteristics of the filter. In order to achieve good isolation of input and output, a field must first be laid around the filter, and then a field must be laid in the lower region of the filter, and connected to the main ground surrounding the filter. It is also a good idea to place signal lines that need to pass through the filter as far away from the filter pins as possible.

2.5 To ensure no increase in noise, the following aspects must be considered: First, the expected bandwidth range of the control line may be from DC to 2MHz, and it is almost impossible to remove such wide band noise through filtering; Second, the VCO control line is usually part of a feedback loop that controls the frequency, and it can introduce noise in many places, so the VCO control line must be handled with great care. Make sure the RF floor is solid and all components are securely connected to the main floor and isolated from other wires that may cause noise. In addition, to ensure that the power supply of the VCO is sufficiently decoupled, special attention must be paid to the VCO because its RF output tends to be at a relatively high level and the VCO output signal can easily interfere with other circuits. In fact, the VCO is often placed at the end of the RF region, and sometimes it requires a metal shield. Resonant circuits (one for the transmitter, the other for the receiver) are related to the VCO, but have their own characteristics. Simply put, a resonant circuit is a parallel resonant circuit with capacitive diodes that help set the VCO operating frequency and modulate voice or data to RF signals. All VCO design principles also apply to resonant circuits. Resonant circuits are usually very sensitive to noise because they contain a large number of components, have a wide distribution area on the board and usually operate at a high RF frequency. Signals are usually arranged on adjacent pins of the chip, but these pins need to be paired with relatively large inductors and capacitors to work, which in turn requires that these inductors and capacitors be located close together and connected back to a noise-sensitive control loop. It's not easy to do that.

3. Great attention should be paid to the following aspects in the design of mobile phone PCB board
3.1 Processing of power supply and ground cable
Even if the wiring in the whole PCB board is completed well, but the interference caused by the power supply and ground wire is not considered well, the performance of the product will decline, and sometimes even affect the success rate of the product. So the wiring of electricity, ground wire should be treated seriously, the noise interference that electricity, ground wire place produces falls to limit, in order to ensure the quality of the product. For every engineer who is engaged in the design of electronic products, it is clear that the reason for the noise between ground wire and power line is generated. Now, the reduced noise suppression is only described as follows:
(1) It is well known that the decoupling capacitor is added between the power supply and ground wire.
(2) as far as possible to widen the width of power supply, ground wire is wider than the power line, their relationship is: ground wire > power line > signal line, usually signal line width is 0.2 ~ 0.3mm, the fine width can reach 0.05 ~ 0.07mm, power line is 1.2 ~ 2.5mm. The PCB board of digital circuit can be made up of a loop of wide ground conductors, that is, a ground network for use (analog circuit ground cannot be used in this way).
(3) with a large area of copper layer for ground, in the printed board is not used in the place are connected with the ground as ground. Or make it multi-layer board, power supply, grounding line each occupy a layer.

3.2 Common ground processing of digital circuit and analog circuit
Many PCBS are no longer single-function circuits (digital or analog), but a mixture of digital and analog circuits. Therefore, when wiring, we need to consider the interference between them, especially the noise interference on the ground line. The digital circuit has high frequency and the analog circuit has strong sensitivity. For the signal line, the high frequency signal line should be as far away from the sensitive analog circuit device as possible. For the ground line, the whole PCB board has only one node to the outside world, so the problem of digital and analog common ground must be dealt with inside the PCB board. And in the board inside the digital ground and analog ground are actually separate from each other, only in the PCB board and the external connection interface (such as plug, etc.). There is a bit of a short connection between the digital ground and the analog ground. Note that there is only one connection point. There are also incommon on the PCB board, which depends on the system design.

3.3 Signal Cables Are laid on electrical (ground) layers
In the multi-layer PCB wiring, because there is no finished line left in the signal line layer, and then add layers will cause waste will also increase the production of a certain amount of work, the cost also increased accordingly, in order to solve this contradiction, you can consider wiring in the electrical (ground) layer. The power zone should be considered first, and the formation second. Because it preserves the integrity of the formation.

3.4 Processing of connecting legs in large area conductor
In the large-area grounding (electricity), the legs of common components are connected with them. The processing of the connecting legs needs to be considered comprehensively. In terms of electrical performance, the pads of component legs are fully connected with the copper surface, but there are some hidden dangers for the welding assembly of components, such as: 1) the welding needs a high-power heater. 2) Easy to cause virtual solder joints. Therefore, taking into account the electrical performance and process needs, make a cross welding pad, called heat shield, commonly known as Thermal, so that the possibility of virtual welding spot due to excessive heat dissipation in the welding section is greatly reduced, and the electrical connection (ground) layer leg of the multilayer board is the same.

3.5 Role of network system in cabling
In many CAD systems, wiring is determined by the network system. The grid is too dense, the path is increased, but the step is too small, the data volume of the graph field is too large, which will inevitably have higher requirements for the storage space of the equipment, but also has a great impact on the computing speed of computer electronic products. Some paths are invalid, such as those occupied by the pads of component legs or by mounting holes, setting holes, etc. Too sparse grid and too few paths have a great influence on the distribution rate. Therefore, it is necessary to have a reasonably dense grid system to support the wiring. The legs of standard components are 0.1 inch (2.54mm) apart, so the base of grid systems is usually 0.1 inch (2.54mm) or integral multiples of less than 0.1 inch (e.g. 0.05 inch, 0.025 inch, 0.02 inch, etc.).

4. Techniques and methods for hf PCB board design are as follows:
4.1 The transmission line corner shall adopt 45° Angle to reduce return loss
4.2 High performance insulating circuit board with insulation constant value strictly controlled according to levels shall be used. This method is beneficial for effective management of electromagnetic field between insulating material and adjacent wiring.
4.3 PCB board design specifications for high precision etching should be improved. Consider specifying a total line width error of +/-0.0007 inches, managing undercut and cross sections of wiring shapes and specifying wiring side wall plating conditions. Overall management of wiring (wire) geometry and coating surfaces is important to address skin effects related to microwave frequencies and to implement these specifications.
4.4 Salient lead with tap inductance, avoid using lead components. In high frequency environments, use surface mounted components.
4.5 For signal through holes, the use of PTH on sensitive plates should be avoided as this process can cause lead inductance at the through holes.
4.6 Abundant ground strata should be provided. Moulded holes are used to connect these grounding layers to prevent 3d electromagnetic fields from affecting the circuit board.
4.7 Choose non-electrolysis nickel plating or immersion gold plating process, do not use HASL method for plating. This electroplated surface provides a better skin effect for high-frequency currents (Figure 2). In addition, this highly weldable coating requires fewer leads, helping to reduce environmental pollution.
4.8 Solder resistance layer can prevent solder paste from flowing. However, due to the uncertainty of thickness and unknown insulation performance, covering the entire plate surface with solder resistance material will lead to a large change in electromagnetic energy in microstrip design. Generally, solder dam is used as solder resistance layer. The electromagnetic field of. In this case, we manage the conversion from microstrip to coaxial cable. In coaxial cables, the ground layers are interlaced in rings and evenly spaced. In microbelts, the grounding layer is below the active line. This introduces certain edge effects that need to be understood, predicted, and considered at design time. Of course, this mismatch also leads to backloss and must be minimized to avoid noise and signal interference on PCB board.