Embedded~PCB Album 23

1. Detailed explanation of 20 analog circuits

bridge rectifier circuit

Unidirectional conductivity of the diode: When a forward voltage is applied to the PN junction of the diode, it is in a conducting state; when a reverse voltage is applied, it is in a cut-off state. Its volt-ampere characteristic curve is as shown below.

Ideal switch model and constant voltage drop model: The ideal model means that when the diode is forward biased, its tube voltage drop is 0, and when it is reverse biased, its resistance is considered to be infinite and the current is zero, that is Deadline. The constant voltage drop model means that when the diode is turned on, its tube voltage drop is a constant value, 0.7V for silicon tubes and 0.5V for germanium tubes.

Bridge rectifier current flow process: when u2 is the positive half cycle, the diodes Vd1 and Vd2 are turned on; while the diodes Vd3 and Vd4 are turned off, the current of the load RL flows through the load from top to bottom, and the load is the same as the positive half cycle of u2. voltage. In the negative half cycle of u2, the actual polarity of u2 is positive below and negative above. The diodes Vd3 and Vd4 are turned on and Vd1 and Vd2 are turned off. The current on the load RL still flows through the load from top to bottom, and the load is positive and positive with u2. Same voltage for half cycle.

Power filter

Process analysis of power supply filtering: Power supply filtering is to connect a larger-capacity capacitor in parallel at both ends of the load RL. Since the voltage across the capacitor cannot change suddenly, the voltage across the load will not change suddenly either, so that the output voltage can be smoothed to achieve the purpose of filtering.

waveform formation process

The output terminal is connected to the load RL. When the power supply is powered, it provides current to the load and also charges the capacitor C. The charging time constant: τ=(Ri∥RL·C)≈Ri·C

Generally, Ri is much smaller than RL. Ignoring the influence of Ri voltage drop, the voltage on the capacitor will rise rapidly with u2.

When ωt=ωt1, there is u2=u0. After that, u2 is lower than u0, and all diodes are cut off. At this time, the capacitor C discharges through RL, the discharge time constant is RLC, the discharge time is slow, and u0 changes gently.
When ωt=ωt2, u2=u0. After ωt2, u2 changes to be larger than u0, and the charging process starts again, and u0 rises rapidly.
When ωt=ωt3, u2=u0, after ωt3, the capacitor is discharged through RL.

Repeatedly, periodic charging and discharging. Due to the energy storage effect of capacitor C, the voltage fluctuation on RL is greatly reduced. Capacitor filtering is suitable for situations where the current changes little. LC filter circuit is suitable for situations where the current is large and the voltage ripple is small.

Selection of filter capacitor capacity and withstand voltage value

The output voltage Uo of the capacitor filter rectifier circuit is between √2·U2~0.9·U2, and the average value of the output voltage depends on the discharge time constant.

Capacitance RLC≧(3~5)·T/2, where T is the cycle of the AC power supply voltage. In practice, it is often further approximated as Uo≈1.2·U2 The maximum reverse peak voltage URM=√2·U2 of the rectifier, and the average current of each diode is half of the load current.

signal filter

The function of the signal filter: attenuate the unwanted signal components in the input signal to a small enough level, but at the same time, the useful signal must pass through smoothly.

Differences and similarities with power filters

Difference: The signal filter is used to filter the signal, and its passband is a certain frequency range, while the power filter is used to filter out the AC component, so that the DC passes through, so as to keep the output voltage stable; the AC power supply only allows a specific frequency passes.

The same point: They all use the amplitude-frequency characteristics of the circuit to work.

Impedance calculation for LC series and parallel circuits

When connected in series, the circuit impedance is:

Z=R+j(XL-XC)=R+j(ωL-1/ωC)

When connected in parallel, the circuit impedance is:

The amplitude-frequency relationship and phase-frequency relationship curves are as follows:

Differential & Integral Circuits

Differential and integral circuits, as shown below.

Differential circuits can convert rectangular waves into sharp pulse waves, and are mainly used in pulse circuits, analog computers and measuring instruments to obtain information contained in the leading and trailing edges of pulses, such as extracting time-based standard signals, etc.

The integrating circuit converts the input square wave into a triangular wave or ramp wave, which is mainly used in occasions such as waveform conversion, elimination of offset voltage of amplifying circuit, and integral compensation in feedback control. Its main uses are:

Used for delays in electronic switches;
waveform transformation;
In A/D conversion, the voltage quantity is converted into a time quantity;
Phase shift.

common emitter amplifier circuit

The common emitter amplifier circuit is shown below.

The structure of the common-emitter amplifier circuit is simple, with large voltage amplification and current amplification, moderate input and output resistance, but unstable operating point. It is generally used when the temperature changes are small and the technical requirements are not high.

Features:

The input signal and the output signal are inverted.
There is greater current and voltage gain.
Generally used as the intermediate stage of amplifier circuit.
Between the collector of the common emitter amplifier and the zero potential point is the output terminal, which is connected to the load resistor.

Voltage-biased common-emitter amplifier circuit

Voltage-biased common-emitter amplifier circuit, as shown below.

The voltage-dividing bias common-emitter amplifier circuit, that is, the base voltage-dividing emitter bias circuit, is one of the three configurations of the BJT amplifier circuit. The three configurations are: co-ejective, co-set, and co-base.

Among them, the common set configuration has the function of current amplification. The input resistance is the highest and the output resistance is the smallest. The common base configuration has a voltage amplification effect, with the smallest input resistance and larger output resistance. The common-emitter configuration has both voltage amplification and current amplification. The input resistance is centered and the output resistance is large.

Therefore, the common set configuration is mostly used in the input stage or output stage or buffer stage of multi-stage amplifier circuits. Common base configuration is often used in high-frequency or wide-band low input impedance applications. The common-emitter configuration is often used in the intermediate stage of amplifier circuits.

common collector amplifier circuit

Common collector amplifier circuit (emitter follower), as shown in the figure below.

The common-collector amplifier circuit outputs signals from the emitter, and the signal waveform and phase are basically the same as the input, so it is also called emitter follower or emitter follower, referred to as emitter follower, and is often used as a buffer.

The common collector amplifier circuit is often used as a current amplifier. It is characterized by high input impedance and large current gain, but the voltage output has almost no amplification. That is, the output voltage is close to the input voltage, and the output impedance is low due to high input impedance. characteristics, it is also often used as an impedance converter.

Circuit feedback block diagram

The circuit feedback block diagram is as follows.

Feedback is the process of bringing part or all of the output of the amplifier circuit back to the input loop of the amplifier circuit in a certain way through the feedback network to affect the input signal of the circuit.

The static operating point of the amplifier circuit will fluctuate up and down with changes in temperature, and its amplification factor is unstable. In order to stabilize the static operating point of the amplifier circuit, a voltage-dividing operating point stabilizing circuit can be used to introduce a DC current negative feedback into the circuit.

In order to increase the input resistance and reduce the output resistance, an emitter output device can be used, and voltage series negative feedback can be introduced in the emitter output device circuit.

Diode voltage stabilizing circuit

Diode voltage stabilizing circuit, as shown below.

Zener diode refers to a diode that stabilizes voltage by utilizing the reverse breakdown state of pn junction, the phenomenon that the current can change within a wide range while the voltage remains basically unchanged.

The forward characteristics of the volt-ampere characteristic curve of the Zener diode are similar to those of ordinary diodes. The reverse characteristics are that when the reverse voltage is lower than the reverse breakdown voltage, the reverse resistance is very large and the reverse leakage current is extremely small. However, when the reverse voltage approaches the critical value of the reverse voltage, the reverse current suddenly increases, which is called breakdown. At this critical breakdown point, the reverse resistance suddenly drops to a very small value. Although the current changes within a wide range, the voltage across the diode is basically stable near the breakdown voltage, thereby achieving the voltage stabilizing function of the diode.

Series voltage stabilizing circuit

Series voltage stabilizing circuit, as shown below.

In addition to voltage transformation, rectification and filtering, the series voltage regulator circuit generally has four links in the voltage regulator part: adjustment link, reference voltage, comparison amplifier and sampling circuit.

When the grid voltage or load variation causes the output voltage V0 to change, the sampling circuit feeds a part of the output voltage V0 back to the comparison amplifier for comparison with the reference voltage.

The error voltage generated by it is amplified to control the base current of the adjustment tube, automatically change the voltage between the adjustment tube collector and the emitter, and compensate for the change of V0, so as to maintain the output voltage basically unchanged.

Differential amplifier circuit

Differential amplifier circuit, as shown below.

The differential amplifier circuit has the characteristics of circuit symmetry, which can stabilize the operating point, and is widely used in the input stage of direct coupling circuits and measurement circuits.

The differential amplifier circuit has two basic input signals, differential mode and common mode. Due to the symmetry of the circuit, when the signals connected to the two input terminals are equal in magnitude and opposite in polarity, it is called a differential mode input signal; when the signals connected to the two input terminals are equal in magnitude and opposite in polarity, it is called a differential mode input signal; When the signals are equal in size and polarity, they are called common mode signals. Usually we input the signal to be amplified as a differential mode signal, and the impact of environmental factors such as temperature on the circuit is input as a common mode signal. Therefore, our ultimate goal is to amplify the differential mode signal and suppress the common mode signal.

The differential amplifier circuit is the basic component of the direct coupling amplifier circuit. This circuit has different effects on different input signals. It has a strong inhibitory effect on common-mode signals and amplifies differential-mode signals, and the circuit's The amplification capability is related to the output mode.

field effect transistor amplifier circuit

Field effect transistor amplifier circuit, as shown below.

Like transistors, field effect transistors also have an amplifying effect, but contrary to ordinary transistors, which are current-controlled devices, field-effect transistors are voltage-controlled devices. It has the characteristics of high input impedance and low noise.

The three electrodes of the field effect transistor, namely the gate, source and drain, are equivalent to the base, emitter and collector of the transistor respectively.

MOS tubes can work in the amplification area and are very common. For mirror current sources, operational amplifiers, feedback control, etc., MOS tubes are used to work in the amplification area. Due to the characteristics of the MOS tube, when the channel is in a state of on-off, the gate voltage directly affects the conductivity of the channel, showing a certain linear relationship. Since the gate is isolated from the source and drain, its input impedance can be regarded as infinite. Of course, as the frequency increases, the impedance becomes smaller and smaller. At a certain frequency, it becomes non-negligible. This high impedance feature is widely used in op amps. The two important principles of virtual connection and virtual disconnection in op amp analysis are based on this feature. This is incomparable to triodes.

Frequency selective (bandpass) amplifier circuit

Frequency-selective (bandpass) amplifier circuit, as shown below.

The frequency-selective amplifier circuit is usually located at the front end of the receiving system, and the amplified signal has a small amplitude and a high frequency. It is also called a high-frequency small-signal resonant amplifier or a band-pass amplifier.

Operational amplifier circuit

The operational amplifier circuit is shown below.

The operational amplifier in the circuit has a non-inverting input terminal and an inverting input terminal. If the polarity of the input terminal is the same as the output terminal, it is a non-inverting amplifier, while if the polarity of the input terminal is opposite to the polarity of the output terminal, it is called an inverting amplifier. .

The input impedance of the non-inverting input is high and the input impedance of the inverting input is low. The input impedance of the non-inverting input is basically determined by the bias resistor connected in parallel at the non-inverting end. This resistor can be used very large. When inverting the input, since there is a feedback resistor connected in parallel between the inverting end and the output end, this feedback resistor is impossible. It is used very large, so the input impedance of the inverting input is relatively low.

Differential input operational amplifier circuit

Differential input operational amplifier circuit, as shown below.

The output voltage is proportional to the input voltage difference across the op amp, enabling subtraction operations. Commonly used as subtraction operations and measurement amplifiers.

voltage comparator

The voltage comparator is a circuit that identifies and compares input signals. It is the basic unit circuit that constitutes a non-sinusoidal wave generating circuit. Commonly used voltage comparators include single-limit comparators, hysteresis comparators, window comparators, three-state voltage comparators, etc.

A voltage comparator can be used as an interface between an analog circuit and a digital circuit, and can also be used as a waveform generation and conversion circuit, etc. A simple voltage comparator can be used to convert a sine wave into a square or rectangular wave of the same frequency.

RC oscillator circuit

The oscillation circuit composed of RC frequency selection network is called RC oscillation circuit. It is suitable for low-frequency oscillation and is generally used to generate low-frequency signals of 1Hz~1MHz. The circuit consists of four parts: amplification circuit, frequency selection network, positive feedback network, and amplitude stabilization link. The main advantages are simple structure, economy and convenience. According to the different forms of the RC frequency selection network, the RC oscillation circuit can be divided into an RC lead (or lag) phase shift oscillation circuit and a Wien circuit oscillation circuit.

LC oscillation circuit

An LC circuit, also known as a resonant circuit, tank circuit or tuned circuit, is a circuit containing an inductor (indicated by the letter L) and a capacitor (indicated by the letter C) connected together. The circuit can be used as an electrical resonator (an electrical analogue of a tuning fork), storing the energy of the oscillations when the circuit resonates.

LC circuits are used both to generate signals of specific frequencies and to separate signals of specific frequencies from more complex signals. They are key components in many electronic devices, especially radio equipment, and are used in oscillator, filter, tuner and mixer circuits.

Quartz crystal oscillator circuit

Quartz crystal is the abbreviation of quartz crystal resonator. The silicon dioxide crystal is cut into very thin wafers in a certain direction, and then the two corresponding surfaces of the wafer are polished and coated with silver layers, and used as two pole leads. After being packaged, a quartz crystal resonator is formed. It has a very stable natural frequency.

Quartz crystal is in the shape of a hexagonal cylinder and needs to be cut into appropriate sizes before use. In order to obtain quartz crystals with different oscillation frequencies, different cutting methods need to be used during processing. A cut quartz crystal is sandwiched between a pair of metal sheets to form a quartz crystal oscillator. It has a piezoelectric effect, that is, when an external voltage is applied to the two poles of the crystal, the crystal oscillator will deform: conversely, if an external force deforms the crystal, the metal sheets on the two poles will Voltage will be generated. If an appropriate alternating voltage is applied, the quartz crystal will resonate. When the frequency of the applied alternating voltage is exactly the natural resonant frequency of the quartz crystal, its amplitude is the largest.

Power amplifier circuit

A power amplifier circuit is an amplifier circuit designed to output larger power. It generally drives the load directly and has a strong load capacity. Power amplifier circuits usually serve as the output stage of multi-stage amplifier circuits.

2. A few animated pictures to understand the transistor

The current amplification effect of the transistor should be regarded as a difficult part of the analog circuit. I would like to use these animations to briefly explain why the small current Ib can control the size of the large current Ic, and the principle of the amplification circuit.

The triode here is also called a bipolar transistor, and is used in analog amplifier circuits and simple digital logic circuits. There are collector c, base b, emitter e, and two PN junctions: collector junction and emitter junction. The collector area is relatively large, the base thickness is thin and the carrier concentration is relatively low. The picture below is an NPN type transistor: when the emitter junction is forward biased, the charge distribution will change and the width of the emitter junction will become narrower; it is equivalent to opening a door from e to b for electrons. When the collector junction is reverse biased, the charge distribution Will also change, the collector junction width will become wider. It is equivalent to opening the door that prevents electrons from escaping from the C-level, as shown in the animation below:

The b stage will be connected to a large resistor RB to limit the size of the current Ib. The excess electrons that run to the b pole have to pass through the collector junction to form the current Ic, as shown in the animation below:

If the base voltage is doubled, the charge distribution will continue to change, the emitter junction width will become narrower, the gate will become wider, and more electrons will run to the b level. As shown in the animation below:

Since RB is a large resistor, Ib is still very small even if it is doubled, so more electrons will pass through the collector junction, so that Ic is also doubled. As shown in the animation below:

Two DC power supplies can be combined together, plus a small signal ui and two capacitors, the amplification circuit is obtained, as shown in the figure below:

If the resistor size is appropriate, this amplifier circuit can amplify the small signal ui into a large signal uCE with opposite phase, as shown in the animation below:

Red is the input terminal, and changes in ui will affect UBE. Think of the emitter junction as a small resistor, and the red Q point will move along the black line, and then draw the image of iB; according to iC=βiB, draw the image of iC, The ordinate has changed from μA to mA; and the output terminal has UCE=UCC-ICRC. When UCC and RC remain unchanged, UCE and IC are inverted.

Finally, let’s talk about the shortcomings of these animations:

The trumpet-like triode is not my original creation. There is also this link. However, the metaphor of the water tank can easily cause people to misunderstand that IC is the largest. In fact, IE is the largest current. whaosoft aiot http://143ai.com
The animation completely ignores the thermal velocity of electrons, which is much greater than the drift velocity of electrons under the action of voltage.
The animation does not reflect the energy level, energy band, Fermi distribution and other content.

3. How to reduce ground bounce in PCB design

#1. What is Ground Bounce?

Ground bounce is a type of noise that occurs in transistor switching devices when the PCB ground and chip package ground are at different voltages.

To better understand ground bounce, consider the push-pull circuit below, which can provide a logic low or logic high output.

push-pull circuit

This circuit consists of 2 MOS tubes: the source of the upper P-channel MOS tube is connected to Vss, and the drain is connected to the output pin. The drain of the lower N-channel MOS is connected to the output pin and the source is connected to ground.

These two MOS tube types have opposite responses to the MOS tube gate voltage. An input logic low signal at the gate of the MOS tube will cause the P-channel MOS tube to connect Vss to the output, and cause the N-channel MOS tube to disconnect the output from GND.

An input logic high signal at the gate of the MOS tube will cause the P-channel MOS tube to disconnect its Vss from the output, and cause the N-channel MOS tube to connect the output to GND.

Connecting the pads on the IC chip to the pins of the IC package are tiny bond wires, these necessities have a small amount of inductance, modeled by the simplified circuit above. Of course, there is also a certain amount of resistance and capacitance in the circuit, which is not modeled and does not necessarily need to be understood.

push-pull circuit

There are 3 inductors shown in the equivalent circuit of a full bridge switch, the inductor symbol represents the package inductance (inherent to the IC package design), and the circuit output is connected to some components.

Imagine encountering this circuit after the input has remained at a logic level for a long time. This state causes the upper transistor to connect the output of the circuit to Vss through the upper MOS tube. After a reasonably long time, there will be a stable magnetic field at LO and LA and the potential difference between ΔV O, ΔV A and ΔV B will be 0 volts and a small amount of charge will be stored in the traces.

Once the input logic switches to low level, the upper MOS transistor will disconnect Vss from the output, and the lower gate will trigger the lower MOS transistor to connect the output of the circuit to GND.

This is when the input logic changes and the results move throughout the system.

#2. Causes of ground bounce

Potential difference between output and ground Current moves from output down to ground through the lower MOS. The inductor uses the energy in the stored magnetic field to create a potential difference between ΔV O and ΔV B in an attempt to resist changes in the magnetic field.

Even with an electrical connection, the potential difference between the output and ground is not immediately at 0V. Remember, the output is at Vss, and the source of MOS tube B is at 0V potential. When the output line discharges, the previous potential difference will cause current to flow.

At the same time that current begins to flow from the output to ground, the inductive characteristics of the package create a potential difference between ΔV and ΔV in an attempt to maintain the previously established magnetic field.

Inductors LB and LO change the source and drain potential of the MOS tube. This is a problem because the MOS tube gate voltage is referenced to the ground on the chip package. When a circuit oscillates near the gate firing threshold, the input voltage may no longer be sufficient to keep the gate open or cause it to open multiple times.

When the circuit switches again, a similar set of circumstances will cause a potential to build up on ΔVA, thus reducing the source voltage of MOSFET A below the triggering threshold.

#3. Why is ground bounce bad?

When the input changes state, the output and MOS tube are no longer in the defined state, in between. The result may be a false or double switch. Also, any other sectors on the IC chip that share the same GND and Vss connections will be affected by switching times.

But the effects of ground bounce are not limited to IC chips. Just as ΔVB forces the source potential of the MOS transistor to be higher than 0V, it also forces the GND potential of the circuit to be lower than 0V. A lot of the images you see depicting ground bounce show external influences.

If multiple gates are switched simultaneously, the effect is more complex and can completely break the circuit. You can see the bounce in the example below. The image below shows significant GND and Vss bounce connected and activated.

Here, about 1V of noise is generated on the 3.3V line during switching, which continues to resonate significantly in the signal line before eventually falling into the background line noise.

Noise is not limited to doors being opened and closed. The switch gates are connected to the IC power pins, while the PCB usually shares common power and ground rails. Meaning that noise can easily be transferred to other places in the circuit through coupling from direct electrical connections of Vss and ground on the chip to traces on the PCB. In the image above, channel 2 (cyan) shows ground and Vss bounce in the undamped signal line, which is so severe that it gets carried over to another signal line on channel 1 (yellow line)

#4. Methods to reduce ground bounce

1. Use decoupling capacitor 1 to locate ground bounce

The preferred solution to reduce ground bounce is to install SMD decoupling capacitors between each power rail and ground, as close as possible to the IC. Distant decoupling capacitors have long traces that increase inductance, so they do themselves no favors by mounting them far away from the IC. When a transistor on an IC chip switches states, it changes the potential of the transistor on the chip and the local power rail.

Decoupling capacitors provide the IC with a temporary, low-impedance, stable potential and limit the effects of bounce, preventing it from spreading to the rest of the circuit. By keeping the capacitor close to the IC, you can minimize the inductive loop area in the PCB trace and reduce interference.

Mixed-signal ICs usually have separate analog and digital power supply pins, and you can install decoupling capacitors on each power input pin. The capacitor should be located between the IC and multiple vias connected to the relevant power plane on the PCB. Decoupling capacitors should be connected to the power plane via vias

Multiple vias are preferred, but are often not possible due to PCB size requirements. If possible, use a copper pour or teardrop to connect the vias, if the drill bit is slightly off center the extra copper will help connect the vias to the traces. Copper pads for IC (U1) and four capacitors (C1, C2, C3, C4)

C1 and C2 are decoupling capacitors for high frequency interference. Add C3 and C4 to the circuit as recommended by the datasheet. Via placement is not ideal due to constraints on other planes.

Sometimes, it is physically impossible to place decoupling capacitors close to the IC. However, if it is placed far away from the IC, it will create an inductive loop, causing the ground bounce problem to be worse.

If so, the decoupling capacitors can be placed on the other side of the PCB below the IC. If that doesn't work, you can make your own capacitors within the board using copper on adjacent layers. Such capacitors are called embedded planar capacitors. Since the PCB consists of parallel copper pours separated by very small dielectric layers. One of the added benefits of this type of capacitor is that the only cost is time.

2. Use resistors to limit current

Use a series current-limiting resistor to prevent excess current from flowing into and out of the IC. This not only helps reduce power consumption and prevents the device from overheating, but also limits the current flowing from the output line through the MOSFET to Vss and GND, thereby reducing ground bounce.

3. Use wiring to reduce inductance

If possible, keep the return path on adjacent traces and adjacent layers, the distance between layer 1 and layer 3 on the board is usually between layer 1 and layer 2 due to the presence of thick core material several times the distance. Any unnecessary separation between the signal and return paths will increase the inductance of that signal line and subsequent ground bounce.

You can see the PCB layout in the picture below. Analog and digital grounds are highlighted in white and yellow respectively

The board has separate analog and digital ground return pins, and the layout of the PCB counteracts the effect of keeping them separate, with no clear and direct path between the IC's digital ground pins and the ground pins on the header strip.

The signal will travel through the circuitous path of the IC to the header pin and return to the circuitous path through the ground pin.

4. Reduce ground bounce through programming and design considerations

As the number of switches increases, so does the ground bounce interference. If possible, switch doors with a short delay offset.

For example: Your design might blink various LEDs at different intervals (1 second, 2 seconds, 3 seconds, etc.) to indicate the status of the design. Ground bounce affects the circuit the most when all 3 LEDs are switching at the same time.

In this example, you can mitigate the effects of ground bounce by offsetting the LEDs slightly so they're not completely in sync. Introducing a 1 millisecond delay between LEDs will not be noticeable to the user, but will reduce the ground bounce effect by about 3 times.

5. Other PCB layout design principles

Where possible, use via-in-pad vias where the design allows.
Reduce signal return path distance. The reduction in distance will reduce parasitic capacitance. To achieve this, it is best to place the component directly above its ground point
Do not use sockets or cord strips
Do not share ground vias or traces for ground connections. It is recommended to use separate vias and traces to connect to the ground plane.

Do not connect capacitors directly to the output.
Implements Low Voltage Differential Signaling (LVDS) as the /0 standard, which provides high bandwidth and high noise immunity.
Choose a package with short leads to reduce series inductance, and use of a BGA is also recommended.
Use a solid ground plane to reduce IR losses and inductance and avoid ground split planes
If the design allows, try using lower switching elements

4. Causes of PCB welding defects

Solderability of circuit board holes affects welding quality

Poor solderability of circuit board holes will produce virtual soldering defects, which will affect the parameters of components in the circuit, lead to unstable conduction between multi-layer board components and inner layer lines, and cause functional failure of the entire circuit.

The so-called solderability is the property of the metal surface being wetted by molten solder, that is, the metal surface where the solder is located forms a relatively uniform, continuous and smooth adhesion film. The main factors affecting the solderability of printed circuit boards are:

The composition of the solder and the properties of the solder being soldered

Solder is an important part of the welding chemical treatment process. It consists of chemical materials containing flux. Commonly used low melting point eutectic metals are Sn-Pb or Sn-Pb-Ag. The impurity content must be controlled at a certain ratio to prevent the oxides produced by the impurities from being dissolved by the flux. The function of flux is to help the solder moisten the circuit surface of the soldered board by transferring heat and removing rust. Galbanum and isopropyl alcohol solvents are generally used.

Welding temperature and metal plate surface cleanliness

Welding temperature and sheet metal surface cleanliness also affect solderability. If the temperature is too high, the diffusion rate of the solder will be accelerated. At this time, it is highly active, which will rapidly oxidize the circuit board and the molten solder surface, resulting in welding defects. Contamination of the circuit board surface will also affect the solderability and cause defects. These defects Including solder beads, solder balls, open circuits, poor gloss, etc.

Welding defects caused by warpage

Circuit boards and components warp during the welding process, and defects such as virtual soldering and short circuits occur due to stress deformation. Warping is often caused by a temperature imbalance between the upper and lower parts of the board. For large PCBs, warping will also occur due to the weight of the board falling.

Ordinary PBGA devices are about 0.5mm away from the printed circuit board. If the device on the circuit board is large, as the circuit board cools down and returns to normal shape, the solder joints will be under stress for a long time. If the device is raised by 0.1mm, it is enough to cause Welding is open.

Circuit board design affects welding quality

In terms of layout, when the size of the circuit board is too large, although the welding is easier to control, the printed lines are long, the impedance increases, the anti-noise ability decreases, and the cost increases; if it is too small, the heat dissipation decreases, the welding is not easy to control, and adjacent lines are easy to appear Mutual interference, such as electromagnetic interference from circuit boards. Therefore, PCB board design must be optimized:

Shorten the connection between high-frequency components and reduce EMI interference.
Components with heavy weight (such as more than 20g) should be fixed with brackets and then welded.
Heat dissipation issues should be considered for heating components to prevent defects and rework due to large ΔT on the component surface. Thermal sensitive components should be kept away from heat sources.
The arrangement of components is as parallel as possible, which is not only beautiful but also easy to weld, and is suitable for mass production. The best circuit board design is a 4:3 rectangle. Do not have sudden changes in wire width to avoid wiring discontinuities. When the circuit board is heated for a long time, the copper foil is prone to expansion and falling off. Therefore, the use of large areas of copper foil should be avoided.

Based on the above, in order to ensure the overall quality of the PCB board, during the production process, it is necessary to use excellent solder, improve the solderability of the PCB board, and prevent warpage and defects.