How to generate PCB reflow
BasIC concept of backflow PCB board, digital signal propagation is from one logic gate to another The signal is sent from the output terminal to the receiving terminal through a wire It seems to flow in one direction In retrOSPect, many digital engineers believe that the loop path is irrelevant. After all, the driver and receiver are both designated as voltage mode devices. Why bother with the current? In fact, the basic circuit theory tells us that the signal is transmitted by the current. Specifically, one of the characteristics of the movement of electrons and the flow of electrons is that electrons never stay anywhere. No matter where the current flows, it must come back. Therefore, the current always flows in a loop, and any signal in the circuit exists in the form of a closed loop Used for high frequency signal transmission, it is actually the process of charging the dielectric capacitor sandwiched between the transmission line and the DC layer
2. Impact of backflow
3. The larger the area around the power line and ground wire on the printed board, the greater the energy they radiate. This, by controlling the return path, we can control the radiation level by making the surrounding area.
4... Solution to the reflow problem: PCB board: wafer interconnection, copper surface cutting, through jumping These factors are analyzed in detail below 4.1 Backflow problem caused by chip interconnection, conversion between high voltage and low voltage, resulting in transient load current flowing from the power supply to the circuit or from the circuit to the ground For a digital device, the input resistance of its pin can be regarded 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 inductance generated by the chip, the power supply and the ground plane Come back The following analysis will take the collector output circuit as an example to explain the internal circuit of the output signal 1) The drive terminal changes from low level to high level. When the output signal jumps from low level to high level, it is equivalent to the output pin outputting current to the transmission line Since the input resistance is infinite, we think it is a chip, and there is no current flowing through the input branch That is, the current must then be returned to the power pin 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 current enters the device from the power supply pin of the driver and flows to the load terminal from the driver output terminal; High frequency transient return current is under the signal track of power supply When the aircraft returns to the output terminal of the driver, the return current directly passes through the power panel and enters the driver from the power pin of the driver to form 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 The output of the driver on the ground plane under the signal path of high-frequency transient return current flow With the help of the coupling capacitor between the power plane and the ground plane of the driver output, the return must pass through the power plane from the ground plane and then through the driver The power supply pin enters the driver to form 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 current enters the device from the driver output pin, flows out from the driver ground pin, enters the ground plane, and passes through the power supply near the driver ground pin Plane and ground plate coupling capacitors pass through the power board and return the load High frequency transient return current flows back to the load on the power panel below the signal track, 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 current enters the device from the output pin of the driver, flows out from the grounding pin of the driver, enters the ground plane, and returns to the loading terminal; High frequency transient return current returns to the load terminal on the ground plane below the signal path, forming a current loop The coupling capacitor of the power board and the grounding plate near the driver output pin and the grounding pin shall be arranged to provide a return path for the return This makes it difficult to predict and control the return path
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