PCB Blog - How is PCB board reflow generated

1. The basic concept of backflow In the schematic diagram of a digital PCB board, the propagation of digital signals is from one logic gate to another logic gate. The signal is sent from the output end to the receiving end through wires. It seems to flow in one direction. Many digital engineers therefore think that the loop path is Irrelevant, after all, both the driver and the receiver are specified as voltage-mode devices, so why bother with currents? Actually, basic circuit theory tells us that signals are propagated by currents, specifically, the movement of electrons, electrons One of the characteristics of flow is that electrons never stay anywhere, no matter where the current flows, it must come back, so the current always flows in a loop, and any signal in the circuit exists in the form of a closed loop. For high-frequency signal transmission, it is actually a process of charging the dielectric capacitor sandwiched between the transmission line and the DC layer. 

2. The influence of backflow Digital circuits usually rely on ground and power planes to accomplish reflow. The return paths of high-frequency signals and low-frequency signals are different. The low-frequency signal returns the impedance path, and the high-frequency signal returns the inductive path. When the current starts from the driver of the signal, flows through the signal line, and is injected into the receiving end of the signal, there is always a return current in the opposite direction: starting from the ground pin of the load, passing through the copper-clad plane, flowing to the signal source, and flowing through The current on the signal wire forms a closed loop. The noise frequency caused by the current flowing through the copper plane is equivalent to the signal frequency. The higher the signal frequency, the higher the noise frequency. Instead of responding to the input signal, the logic gate responds to the difference between the input signal and the reference pin. A single-point terminated circuit reacts to the difference between the incoming signal and its logical ground reference plane, so disturbances in the ground reference plane are as important as disturbances in the signal path. The logic gate responds to an input pin and a designated reference pin, and we don't know which is the designated reference pin (usually negative supply for TTL, positive supply for ECL, but not all), With this property, the anti-jamming capability of differential signals can have a good effect on ground bounce noise and power plane slippage. When many digital signals on the PCB are switched synchronously (such as CPU data bus, address bus, etc.), this causes transient load current to flow from the power supply into the circuit or from the circuit into the ground wire. Impedance, synchronous switching noise (SSN) will be generated, and ground plane bounce noise (referred to as ground bounce) will also appear on the ground wire. When the surrounding area of the power lines and grounding lines on the printed board is larger, their radiation energy is larger. Therefore, we analyze the switching state of the digital chip, and take measures to control the return flow method to reduce the surrounding area. area, the purpose of the radiation level. 3. Theoretical knowledge of return path A circuit in a printed board has current passing through the wire. Usually, we only see the wire used to transmit the signal on the surface, from the driving end to the receiving end. In fact, the current can always flow in the loop. , the transmission lines are what we can see, and the paths of current return are usually invisible, they usually flow back through the ground and power planes, because there are no physical lines, the return paths become difficult to estimate, to control them there are certain difficulty. The path of the return current in the printed circuit board is closely related to the frequency of the current. According to the basic knowledge of the circuit, the DC or low-frequency current always flows in the direction of the impedance; and the high-frequency current always flows in the direction of the inductive reactance when the resistance is constant. summary: 1. When there is a continuous, dense and complete copper-clad plane under the signal wiring, the noise interference of the signal return current to the copper-clad plane is local. Therefore, as long as the principle of localization of layout and wiring is followed, that is, the distance between digital signal lines, digital devices and analog signal lines and analog devices is artificially opened to a certain extent, the return current of digital signals to analog circuits can be greatly reduced. interference. 2. The high-frequency transient return current returns to the driver through the plane (ground plane or power plane) immediately adjacent to the signal trace. The termination load for the driver signal traces, which is connected across the signal traces and the plane (ground or power plane) immediately adjacent to the signal traces. 

3. The larger the surrounding area of the power and ground lines on the printed board, the more energy they radiate. Therefore, by controlling the return path, we can make the surrounding area and thus control the degree of radiation. 

4. The solution to the reflow problem: There are usually three aspects to the reflow problem on the PCB board: chip interconnection, copper surface cutting, and via jumping. These factors are analyzed in detail below. 4.1 Reflow problems caused by chip interconnection When a digital circuit is operating, a transition between high and low voltages occurs, which causes transient load currents to flow from the power supply into the circuit or from the circuit into the ground. For digital devices, the input resistance of its pins can be considered as infinite, which is equivalent to an open circuit (i=0 in the figure below). In fact, the loop current is generated by the distributed capacitance and distributed inductance generated by the chip, the power supply and the ground plane. return. The following analysis will take the collector output circuit as an example of the internal circuit of the output signal. 1) The drive terminal changes from low level to high level. When the output signal jumps from a low level to a high level, it is equivalent to the output pin outputting a current to the transmission line. Since the input resistance is infinite, we think that for the chip, no current flows from the input leg. That is, then, this The current must return to the power tube leg of the output chip. 2) Signal traces in close proximity to power planes The drive terminal charges the transmission line formed by the signal trace, the power plane and the terminal load. The current enters the device from the power supply pin of the driver and flows from the driver output terminal to the load terminal; the high-frequency transient return current is in the power supply under the signal trace. The plane returns to the output end of the driver, and the return current directly passes through the power plane and enters the driver from the power supply pin of the driver, forming a current loop. 3) Signal traces are close to the ground plane The driver charges the transmission line formed by the signal traces and the power plane and the terminal load, and the current enters the device from the power supply pin of the driver and flows from the output terminal of the driver to the load terminal. The high-frequency transient return current flows back to the output of the driver on the ground plane under the signal trace. The return current must cross from the ground plane to the power plane and then from the driver with the help of the coupling capacitors between the power plane and the ground plane at the output of the driver. The power supply pins enter the driver, forming a current loop. The drive terminal changes from high level to low level, which is equivalent to the output pin absorbing the current on the transmission line. 1) Signal traces are in close proximity to the power plane The load discharges the transmission line formed by the signal trace, the power plane and the driver output. The current enters the device from the driver's output pin, flows out from the driver's ground pin, enters the ground plane, and passes through the power supply near the driver's ground pin. Plane and ground plane coupling capacitors, across to the power plane and back to the load. The high-frequency transient return current flows back to the load on the power plane below the signal trace, forming a current loop. 2) Signal traces are close to the ground plane The load discharges the transmission line formed by the signal trace, the power plane and the driver output terminal. The current enters the device from the output pin of the driver, flows out from the ground pin of the driver, enters the ground plane, and returns to the load terminal; high-frequency transient return current Return to the load terminal on the ground plane under the signal trace to form a current loop. In the vicinity of the output pins and ground pins of the driver, the coupling capacitors of the power plane and the ground plane should be arranged to provide a return path for the return current. It makes the return path difficult to predict and control, thus causing crosstalk to other traces) on PCB board.