Изготовление и сборка импедансной платы - комплексное обслуживание
A good laminated structure can control the characteristic impedance of the printed circuit board, and its routing can form an easily controllable and predictable transmission line structure called an impedance board. The decision condition of the impedance control requirement: When transmitting signals in the printed circuit board leads, if the length of the wire is close to 1/7 of the signal wavelength. The wires at this time become the signal transmission line, and the general signal transmission line needs to be impedance controlled. When making printed circuit boards, it is up to the customer to decide whether or not to control the impedance. If the customer requires impedance control for certain line width, the impedance of the line width should be controlled during production.
The characteristic impedance of the conductor on the printed circuit board is an important indicator of the board design. Especially in the PCB design of the high-frequency circuit, it must be considered whether the characteristic impedance of the conductor and the characteristic impedance required by the device or signal are consistent and match. This involves two concepts: impedance control and impedance matching.
Impedance control: The conductors in the circuit board will transmit various signals. In order to increase the transmission rate, the frequency must be increased. If the line itself is etched, laminated thickness, and wire width, the impedance value will change. Distort the signal. Therefore, the conductor value on the high-speed circuit board should be controlled within a certain range to become “impedance control”.
Factors affecting the impedance
1. Trace width/trace spacing: The trace width is inversely proportional to the impedance. The thinner the line width, the higher the Impedance. The trace spacing is proportional to the impedance value. The smaller the trace spacing, the lower the impedance value.
2. Copper thickness: The copper thickness is inversely proportional to the impedance value. The thicker the copper thickness, the lower the impedance value.
3. Dielectric layer thickness: The thickness of the dielectric layer between the circuit layer and the bottom layer is proportional to the impedance value. The thicker the dielectric layer, the higher the impedance value.
4. Dielectric layer constant: The dielectric constant is inversely proportional to the impedance value. The higher the dielectric layer constant, the lower the impedance value.
5. Solder Mask Thickness: The thickness of the solder mask is inversely proportional to the impedance value. The thicker the solder mask, the lower the impedance value.
Measurement of impedance values: The impedance of any transmission line is related to time. Usually TDR (time-domain reflectometer) for measurement.
Impedance board flow test control
1. After the platen is completed, the medium thickness of the plate is measured by a large slice, and the control for estimating the upper and lower limits of the etched line is further calculated (the etching change of the impedance line is controlled within 10% of the line width).
2. The first test of the characteristic impedance is placed after the etching QC and given the impedance reference value; if the impedance strip is designed on the board outside the shipping board, the final impedance value is measured in a process before the forming process.