1. Reduce the impact of temperature on circuit board stress

Since “temperature” is the main source of circuit board stress, as long as the temperature of the reflow oven is lowered or the speed of heating and cooling of the circuit board in the reflow oven is slowed down, the bending and warping of the board can be greatly reduced. occur. However, other side effects may occur, such as solder short circuits.

2. PCB uses high Tg board

Tg is the glass transition temperature, which is the temperature at which the material changes from glass to rubber. The lower the Tg value of the material, the faster the circuit board begins to soften after entering the reflow oven and becomes soft and rubbery. The time will also become longer, and the deformation of the circuit board will of course become more serious. Using a higher Tg board can increase its ability to withstand stress deformation, but the price of high Tg PCB boards is relatively high.

3. Increase the thickness of the circuit board

In order to make many electronic products thinner and lighter, the thickness of the circuit board has been reduced to 1.0mm, 0.8mm, or even 0.6mm. Such a thickness is really a bit difficult to keep the circuit board from deforming after passing through the reflow oven. It is difficult to force people to do so. It is recommended that if there is no requirement to be thin and light, the circuit board should be 1.6mm thick, which can greatly reduce the risk of board bending and deformation.

4. Reduce the size of the circuit board and reduce the number of panels

Since most reflow ovens use chains to drive the circuit board forward, circuit boards with larger PCB design sizes will be dented and deformed in the reflow oven due to their own weight, so try to treat the long side of the circuit board as the board edge. Putting it on the chain of the reflow furnace can reduce the dent deformation caused by the weight of the circuit board itself. This is also the reason for reducing the number of panels. That is to say, when passing through the furnace, try to use the narrow edge perpendicular to the furnace passing direction. Achieve the lowest amount of dent deformation.

5. Use oven tray jig

If the above methods are difficult to achieve, the last option is to use an oven tray to reduce the amount of deformation. The reason why the oven tray can reduce the bending of the board is because whether it is thermal expansion or cold contraction, the tray is expected to be able to fix the circuit board. When the temperature of the circuit board drops below the Tg value and begins to harden again, the original size can be maintained.

If the single-layer pallet cannot reduce the deformation of the circuit board, you must add a layer of cover and clamp the circuit board with the upper and lower pallets. This can greatly reduce the problem of deformation of the circuit board after passing through the reflow oven. However, these oven trays are quite expensive, and labor is required to place and recycle the trays.

Today’s PCB technology mainly changes according to the characteristics and requirements of electronic products. In recent years, electronic products have become increasingly multi-functional, sophisticated and comply with environmental regulations. Therefore, the precision of PCB is getting higher and higher, and its application of combining soft and hard boards will also increase.

PCB is the foundation of the information industry. From computers to portable electronic devices, circuit boards are found in almost all electronic and electrical products. With the development of communication technology, handheld wireless radio frequency circuit technology is used more and more widely. One of the biggest characteristics of these devices (such as mobile phones, wireless PDAs, etc.) is: first, they include almost all portable subsystems; second, they are small Miniaturization means that the density of components is very high, which makes the mutual interference of components (including SMD, SMC, bare chips, etc.) very prominent. Therefore, it has become a very important topic to design a perfect PCB for RF circuits and audio circuits to prevent and suppress electromagnetic interference and thereby improve electromagnetic compatibility.

Because the performance indicators of the same circuit and different PCB design structures will vary greatly. Especially as the audio functions of today’s handheld products continue to increase, more attention must be paid to the PCB layout of the audio circuit. Based on this, this article focuses on the ingenious design of the PCB of the RF circuit and audio circuit of the handheld product (that is, including component layout, component arrangement, wiring and Grounding and other techniques) for analysis and explanation.

1. Component layout

First, let’s talk about the general principles of layout: components should be arranged in the same direction as much as possible, and the phenomenon of poor welding can be reduced or even avoided by choosing the direction in which the PCB enters the tin melting system; as known from practice, there must be at least 0.5mm spacing between components to meet the requirements of the components. Regarding the soldering requirements of the device, if the space of the PCB board allows, the spacing between components should be as wide as possible. For double-sided panels, one side should generally be designed with SMD and SMC components, and the other side should be designed with discrete components.

1.1 Divide the PCB into digital areas and analog areas.

The first step in any PCB design is, of course, choosing the PCB placement of each component. We call this step “layout consideration”. Careful component placement can reduce signal interconnections, ground separation, noise coupling, and occupied circuit board area.

Electromagnetic compatibility requires that each circuit module PCB is designed to produce as little electromagnetic radiation as possible and to have a certain anti-electromagnetic interference capability. Therefore, the layout of components also directly affects the interference and anti-interference capabilities of the circuit itself, which is also directly related to performance of the designed circuit.

Therefore, when designing an RF circuit PCB, in addition to considering the layout of ordinary PCB designs, you must also consider how to reduce the mutual interference between various parts of the RF circuit, how to reduce the interference of the circuit itself to other circuits, and the circuit itself. anti-interference ability.

It is known from experience that the quality of the RF circuit effect not only depends on the performance indicators of the RF circuit board itself, but also largely depends on the interaction with the CPU processing board. Since RF circuits contain digital circuits and analog circuits, in order to prevent digital noise from interfering with sensitive analog circuits, they must be separated. Dividing the PCB into digital areas and analog areas can help improve the layout of such circuits, which is particularly important. important.

1.2 Need to prevent RF noise from coupling into audio circuits

Although the RF portion of handheld products is often treated as analog circuitry, a common concern in many designs is RF noise. It is necessary to prevent RF noise from coupling into the audio circuitry, because the RF noise is demodulated to produce audible noise. In order to solve this problem, the RF circuit and the audio circuit need to be separated as much as possible. After dividing the PCB into analog and digital, the component layout of the analog part needs to be considered. The component layout should make the path of the audio signal the shortest, and the audio amplifier should be placed as close as possible to the headphone jack and speaker to minimize the EMI radiation of the Class D audio amplifier and the coupling noise of the headphone signal. The analog audio signal source must be as close as possible to the input of the audio amplifier to minimize input coupling noise. All input leads act as an antenna for RF signals, and shortening the lead length helps reduce the antenna radiation effect in the corresponding frequency band.

2. Issues that should be paid attention to in component layout and application examples

2.1 Issues that should be paid attention to in layout:

Carefully analyze the circuit structure. Divide circuits into blocks (such as high-frequency amplification circuits, mixing circuits and demodulation circuits, etc.) to separate strong current signals and weak current signals as much as possible. After separating digital signal circuits and analog signal circuits, attention should also be paid to Circuits that perform the same function should be arranged within a certain range as much as possible to reduce the signal loop area; the filter network of each part of the circuit must be connected nearby, which can not only reduce radiation, but also reduce the chance of interference and improve the efficiency of the circuit. anti-interference ability.

Unit circuits are grouped according to their different sensitivity to electromagnetic compatibility during use. For components in the circuit that are susceptible to interference, the layout should also try to avoid interference sources (such as interference from the CPU on the data processing board, etc.).

2.2 Examples of the impact of component placement on audio signals

The impact of unreasonable component layout on audio signal quality

An unreasonable layout of audio components has two serious problems: one is that the audio amplifier is too far away from the audio signal source, because the leads pass near noisy digital circuits and switching circuits, thereby increasing the probability of noise coupling. Longer leads also enhance the RF antenna effect. If a mobile phone uses GSM technology, these antennas can pick up the GSM transmitted signal and feed it into the audio amplifier. Almost all amplifiers can demodulate the 217Hz envelope to some extent, producing noise at the output. In the worst case, the noise may completely drown out the audio signal. Shortening the length of the input lead can effectively reduce the noise coupled to the audio amplifier. The second audio amplifier is placed too far away from the speaker and headphone jack.

If the audio amplifier uses a Class D amplifier, longer headphone leads will increase the EMI radiation of the amplifier. This radiation may cause the device to fail testing standards set by local governments. Longer headphone and microphone leads also increase lead impedance, reducing the power the load can draw. Finally, because the components are so spread out, the wires between them will have to pass through other subsystems. This will not only increase the wiring difficulty of the audio part, but also increase the wiring difficulty of other subsystems.

The impact of unreasonable component layout on audio signal quality

Reasonable component layout improves audio signal quality

Arrangement of the same components, rearranged components can use space more efficiently and shorten lead lengths. Note that all audio circuits are distributed near the headphone jack and speakers, the audio input and output leads are much shorter than the above solution, and no audio circuits are placed in other areas of the PCB. This design can comprehensively reduce system noise, reduce RF interference, and simplify wiring.

Reasonable component layout indicates the improvement of audio signal quality

3. Wiring principles and techniques

After the layout of the components is basically completed, wiring can begin.

3.1 Basic principles of wiring

When the assembly density permits, try to use a low-density wiring design, and keep the signal traces as consistent in thickness as possible to facilitate impedance matching.

For RF circuits, unreasonable design of the direction, width, and line spacing of signal lines may cause cross-interference between signal transmission lines. The signal path has a very limited impact on audio output noise and distortion, which means there are very few compromises that need to be made to ensure performance. Audio amplifiers are usually powered directly from batteries and require considerable current. If you use long, thin power supply leads, the power supply ripple will be increased. Compared with short and wide leads, long and thin leads have greater impedance, and the current changes generated by the lead impedance are converted into voltage changes and fed into the device. To optimize performance, amplifier power supplies should use the shortest possible leads. Differential signaling should be used whenever possible. Differential inputs have higher noise rejection, allowing the differential receiver to reject common-mode noise on both the positive and negative signal lines. To take full advantage of a differential amplifier, it is important to keep the differential signal pairs the same length when routing them, so that they have the same impedance, and that they are as close to each other as possible so that they couple the same amount of noise. The amplifier’s differential inputs are very effective at rejecting noise from the system’s digital circuitry. In addition, the system power supply itself also has noise interference, so comprehensive considerations must be taken when designing the RF circuit PCB and the wiring must be reasonable.

3.2 Wiring skills

When wiring, all traces should be kept away from the frame of the PCB board (about 2mm) to avoid wire breakage or potential risk of wire breakage during PCB board production. The power lines should be as wide as possible to reduce loop resistance. At the same time, the direction of the power lines and ground wires should be consistent with the direction of data transmission to improve anti-interference capabilities. The signal lines should be as short as possible and minimize excessive interference. The number of holes; the shorter the connections between components, the better, in order to reduce distribution parameters and mutual electromagnetic interference; incompatible signal lines should be kept away from each other, and try to avoid parallel wiring, and on both sides of the forward direction The signal lines should be perpendicular to each other; when wiring, the angle of 135° should be used at addresses that require corners, and avoid turning at right angles.

4. Grounding

In the design of RF circuit PCB, the correct wiring of power lines and ground wires is particularly important. Reasonable design is the most important means to overcome electromagnetic interference. A considerable number of interference sources on PCB are generated through power and ground wires, among which the ground wire causes the largest noise interference. The main reason why ground wires easily cause electromagnetic interference is that there is impedance in the ground wire. When a current flows through the ground wire, a voltage will be generated on the ground wire, resulting in a ground loop current and causing loop interference in the ground wire. When multiple circuits share a ground wire, common impedance coupling is formed, resulting in so-called ground noise. Therefore, when routing the ground wire of the RF circuit PCB, you should do the following:

The circuit is divided into blocks. The radio frequency circuit can basically be divided into high-frequency amplification, mixing, demodulation, local oscillator and other parts. It is necessary to provide a common potential reference point for each circuit module PCB design, that is, the respective ground wires of each module circuit. , so that signals can be transmitted between different circuit module PCB designs. Then, it is summarized at the place where the RF circuit PCB is connected to the ground wire, that is, it is summarized in the main ground wire. Since there is only one reference point, there is no common impedance coupling, and thus there is no mutual interference problem.

The digital area and analog area should be isolated as much as possible, and the digital ground and analog ground should be separated, and finally connected to the power ground.

When space permits, it is best to isolate each module with a ground wire to prevent signal coupling effects between each other.

For audio circuits, grounding is critical to achieving the performance requirements of the audio system. There are two important considerations for grounding in any system: first, it is the return path for current flowing through the device, and second, it is the reference potential for digital and analog circuits. Here are tips that apply to all systems:

Establish a continuous ground plane for digital circuits. The digital current from the ground plane returns through the signal path, and the area of this loop should be kept to a minimum to reduce antenna effects and parasitic inductance. Make sure that all digital signal leads have corresponding ground paths. This layer should cover the same area as the digital signal leads, with as few breakpoints as possible. Breaks in the ground plane, including vias, cause ground current to flow through larger loops, thus producing greater radiation and noise.

Ensure ground galvanic isolation. The ground currents of digital circuits and analog circuits must be kept isolated to prevent digital current from interfering with analog circuits. To achieve this goal, the components need to be arranged correctly. If you place analog circuits in one area of the PCB and digital circuits in another area, the ground currents will be naturally isolated. It’s best to have separate PCB layering for analog circuits.

The analog circuit uses star grounding. Star grounding regards one point of the PCB as a common grounding point, and only this point is regarded as ground potential. In cellular phones, the battery ground terminal is usually used as a star grounding point. The current flowing into the ground plane will not automatically disappear. All ground currents will flow into this ground point. Audio amplifiers draw considerable current, which affects the circuit’s own ground reference and the ground reference of other systems. To solve this problem, it is best to provide a dedicated return loop that bridges the amplifier’s power ground and the headphone jack’s ground return. Note that these dedicated loops do not cross digital signal lines, as they will block digital return current.

Maximize the effect of bypass capacitors. Almost all devices require a bypass capacitor to provide transient current that the power supply cannot provide. These capacitors need to be placed as close as possible to the power pins to reduce parasitic inductance between the capacitor and the device pins, which reduces the effectiveness of the bypass capacitor.

5 Conclusion

The above well-designed PCB is a time-consuming and extremely challenging task, but the investment is indeed worth it. A good PCB layout can help reduce system noise, improve RF signal suppression capabilities, and reduce signal distortion. Good PCB design will also improve EMI performance, potentially requiring less shielding. If the PCB is unreasonable, avoidable problems will occur during the testing phase. If you take measures at this time, it may be too late, and it will be difficult to solve the problems you are facing. It will require more time and effort, and sometimes additional components will be added, increasing system cost and complexity.

In recent years, with the continuous development of science and technology, the demand for electronic products has become more and more diversified. As one of the core components of electronic products, PCB (Printed Circuit Board) rigid-flexible board has received widespread attention due to its unique advantages.

First, let us understand the definition of PCB rigid-flexible board. PCB rigid-flexible board is a circuit board that combines rigid and flexible characteristics. By stacking rigid boards and flexible circuit boards together, it achieves both rigid and flexible characteristics on one circuit board. This design can not only meet the requirements for high-density wiring and three-dimensional space layout, but also adapt to the needs of products for bending and folding.

So, what are the advantages of PCB rigid-flexible boards compared to traditional rigid boards?

PCB rigid-flexible board has high reliability and stability. Due to the existence of flexible circuit boards, PCB rigid-flexible boards can better absorb external shock and vibration, reducing the risk of circuit board damage and breakage. At the same time, the use of rigid boards also ensures the stability and reliability of the circuit board, allowing it to work normally in various harsh environments.
Secondly, rigid-flexible boards have a higher degree of design freedom. Traditional rigid board wiring is limited by plane layout, while rigid-flexible boards can arrange more components and lines in three-dimensional space through the bending and folding of flexible circuit boards, improving the wiring density and functional integration of circuit boards. , providing greater space for innovative product design.
Rigid-flex boards also have lower costs and fast production cycles. Compared with the traditional method of separate design and manufacturing of rigid boards and flexible circuit boards, the integrated design and manufacturing process of PCB rigid-flexible boards can greatly shorten the production cycle and reduce labor and material costs in the manufacturing process, thereby reducing the overall cost.
So, why choose PCB rigid-flex board?
Rigid-flexible boards can meet more application needs. Whether in the fields of consumer electronics, medical equipment, aerospace equipment or smart wearable devices, rigid-flexible boards can provide better performance and reliability and meet the special needs of different products for circuit boards.
Secondly, rigid-flexible boards have higher adaptability and scalability. Due to the existence of flexible circuit boards, rigid-flexible boards can better adapt to the shape, size and functional requirements of the product, and can be easily expanded and upgraded later, providing greater space for product development and improvement.

PCB rigid-flex board has become an important choice in the field of electronic product design and manufacturing due to its advantages of high reliability, design freedom, low cost and fast production cycle. Whether you are pursuing innovative design, improving product performance or reducing manufacturing costs, choosing PCB rigid-flexible board is a wise decision. It is believed that with the continuous advancement of technology, PCB rigid-flexible boards will play an important role in more fields and bring more convenience and possibilities to our lives.

In PCBA processing and production, we often encounter situations where there are a lot of post-soldering materials. At this time, wave soldering is needed for post-soldering processing. So what should we pay attention to during wave soldering operations? Today I will talk to you about wave soldering:

We all know that the wave surface is covered by a layer of oxide scale, which remains almost static along the entire length of the solder wave. During the wave soldering process, the PCB contacts the front surface of the tin wave, and the oxide scale breaks. The tin wave is pushed forward without wrinkles? This shows that the entire oxide scale and PCB move at the same speed.

Generally, in order to avoid poor wave soldering, the following methods can be used: use components/PCBs with good solderability, increase the activity of the soldering flux, increase the preheating temperature of the PCB, increase the wetting performance of the pad, and increase the temperature of the solder. , Remove harmful impurities and reduce the cohesion of the solder to facilitate the separation of the solder between the two solder joints.

Common preheating methods in wave soldering machines: air convection heating, infrared heater heating, heating by a combination of hot air and radiation; wave soldering process curve analysis: the wetting time refers to the time when the wetting starts after the solder joint comes into contact with the solder Time and residence time refer to the time from when a certain solder joint on the PCB contacts the wave crest surface to leaving the wave crest surface. Preheating temperature refers to the temperature reached before the PCB contacts the wave crest surface. Welding temperature refers to the soldering temperature. Important welding parameters. Usually 50°C ~60°C higher than the melting point of the solder (183°C). In most cases, it refers to the temperature of the soldering furnace. The temperature of the PCB solder joints soldered during actual operation is lower than the furnace temperature. This is because the PCB absorbs heat.

The so-called copper pouring is to use the idle space on the circuit board as the reference plane and then fill it with solid copper. These copper areas are also called copper filling.

The significance of copper coating is to: reduce the impedance of the ground wire and improve the anti-interference ability; reduce the voltage drop and improve the power efficiency; and connect to the ground wire to reduce the loop area.

Also for the purpose of keeping the PCB from deforming as much as possible during welding, most PCB manufacturers will also require PCB designers to fill the open areas of the PCB with copper or grid-like ground wires. If the copper is not handled properly, it will The gain outweighs the loss. Does copper coating have “more pros than cons” or “does more cons than pros”?

Everyone knows that at high frequencies, the distributed capacitance of the wiring on the printed circuit board will work. When the length is greater than 1/20 of the corresponding wavelength of the noise frequency, the antenna effect will occur, and the noise will be emitted outward through the wiring. If there is poorly grounded copper in the PCB, the copper becomes a tool for transmitting noise.

Therefore, in high-frequency circuits, do not think that the ground wire is connected to the ground somewhere. This is the “ground wire”. You must drill holes in the wiring with a spacing of less than λ/20, and connect it to the ground. The ground plane of the shelf is “well grounded”. If the copper coating is handled properly, the copper coating not only increases the current, but also plays the dual role of shielding interference.

There are generally two basic methods of copper pouring, namely large-area copper pouring and grid copper pouring. People often ask whether it is better to pour copper pouring over a large area or with grid copper pouring. It is difficult to generalize.

why? Covering a large area with copper has the dual functions of increasing current and shielding. However, if a large area of copper is covered with wave soldering, the board may warp or even blister. Therefore, when covering a large area with copper, several grooves are usually opened to alleviate blistering of the copper foil.

Simple grid copper coating mainly has a shielding effect, and the effect of increasing current is reduced. From the perspective of heat dissipation, the grid is beneficial (it reduces the heating surface of copper) and plays a certain role in electromagnetic shielding.

It should be pointed out that the grid is composed of traces in staggered directions. We know that for circuits, the width of the traces has its corresponding “electrical length” for the operating frequency of the circuit board (the actual size divided by The digital frequency corresponding to the working frequency is available, please refer to relevant books for details).

When the operating frequency is not very high, perhaps the role of the grid lines is not very obvious. Once the electrical length matches the operating frequency, it is very bad. You will find that the circuit cannot work properly at all, and interference is being emitted everywhere. signal of.

The suggestion is to choose based on the working conditions of the designed circuit board, and don’t stick to one thing. Therefore, high-frequency circuits have multi-purpose grids that require high interference resistance, and low-frequency circuits have high-current circuits and other circuits that commonly use complete copper laying.