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PCB manufacturing EMC common sense

date:2019-12-16 15:04:35

PCB manufacturing EMC common sense

The ROHS logo of PCB manufacturing is familiar to everyone. As an electronic component, PCB boards also need ROHS certification. The main key is whether it use lead-free tin. PCB electronic products must pass CE, CCC certification, the key to the PCB is EMC.

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The three elements of EMC (1) Interference source refers to the component , equipment or signal that generates the interference. It is described in mathematical language as follows: du / dt,  di / dt where large area is the source of interference. Such as: lightning, relays, thyristors, motors, high-frequency clocks, etc. may become interference sources. (2) Propagation path refers to the path or medium of interference from the source of interference to sensitive devices. Typical interference propagation paths are conducted through wires and space radiation (including differential and common mode radiation). (3) Sensitive devices refer to objects that are easily interfered. Such as: A / D, D / A converter, microcontroller, digital IC, weak signal amplifier, etc. Principle 55: When the clock frequency exceeds 5MHz, or the rise time is less than 5ns, a multilayer board must be used

The signal integrity and power integrity of the PCB.Signal integrity issues are mainly manifested in 5 aspects: crosstalk, reflection, delay, synchronous switching noise (SSN), and electromagnetic compatibility (EMI). Transmission lines on PCBs can be divided into microstrip lines and strip lines by type. Two factors that cause crosstalk are capacitive coupling and inductive coupling. 3W principle (note: the distance between traces is twice the width of the trace), the basic principle of differential signal layout: equidistance, equal length, methods to reduce crosstalk 2 times), B. maintaining the integrity of the loop; C. orthogonal traces of adjacent layers D. reducing parallel trace lengths

PCB signal integrity and power integrity reflection: When the characteristic impedance of the conductor on the PCB (called a transmission line in a high-speed digital system) does not match the load impedance, a portion of the energy will be reflected back along the transmission line after the signal reaches the receiving end. Distortion of the signal waveform, even overshoot and undershoot of the signal, if the signal is reflected back and forth on the transmission line, ringing will occur. Important point: Impedance matching delay: It means that the signal is transmitted at a limited speed on the conductors of the PCB, and the signal is sent from the sender to the receiver, with a transmission delay in between. The delay of the signal will affect the timing of the system. The transmission delay mainly depends on the length of the wire and the dielectric constant of the medium surrounding the wire. Focus: Transmission delay and timing

PCB characteristics: 1mm wide interconnect line (10Z copper thickness) can carry 1 A current, reflow soldering is used for surface mount parts, wave soldering is used for plug-in parts, TG in PCB board parameters means high heat resistance. Factors affecting impedance are: line width, PP thickness, basic parameters of PCB board A. Electric constant (Er) B. Loss factor C. Heat resistance



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