Impedance Control PCB Fabrication

Controlled Impedance PCB Meaning

Impedance control has been one of the essential concerns and severe problems in high-speed PCB design. Impedance is the sum of the resistance and reactance of an electrical circuit. The resistance is the opposition to current flow present in all materials. In high-frequency applications, controlled impedance helps us ensure that signals are not degraded as they route around a PCB. Resistance and reactance of an electrical circuit have a significant impact on functionality, as specific processes must be completed before others to ensure proper operation.

Why need impedance control?

Because of complex processors, USB devices and antennas are printed directly on the circuit board surface. The speed of signal switching on the circuit board increases, and the electrical characteristics of the tracks that transmit signals between devices become more and more critical. Therefore, more and more PCB designs need impedance control and testing.

Impedance control technologies are quite important in high-speed digital circuit design in which effective methods must be adopted to ensure the excellent performance of high-speed PCB. The impedance of a PCB is largely determined by some factors, such as trace width, copper thickness, dielectric thickness, dielectric constant. Impedance, generally measured in Ohms. We must control the resistance and reactance of an electrical circuit to ensure the quality of the signal in the sophisticated design—for example, single-ended impedance 50 ohm ±10%, differential impedance 100ohm ±10%.

Impedance Control PCB

What Determines Controlled Impedance?

The characteristic impedance of a PCB trace is typically determined by its inductive and capacitive reactance, resistance, and conductance. These factors are a function of the physical dimensions of the trace, the dielectric constant of the PCB substrate material, and dielectric thickness. Typically PCB trace impedance can range from 25 to 125 ohms. The following factors will determine the impedance value generated from the PCB structure:

  • Width and thickness of the copper signal trace (top and bottom)
  • The thickness of the core or prepreg material on either side of the copper trace
  • The dielectric constant of the core and prepreg material
  • Distance from other copper features

When to Use Controlled Impedance ?

When a signal must have a particular impedance to function appropriately, controlled impedance should be used. In high-frequency applications matching the impedance of PCB traces is essential in maintaining data integrity and signal clarity. If the impedance of the PCB trace connecting two components does not match the components` characteristic impedance, there may be increased switching times within the device or the circuit. There may also be random errors.

Please note that we also need special instructions here: In DC circuits, there is no reactance, and the resistance of copper conductors is typically insignificant. However, in high-speed AC circuits (those with sharp changes in voltage and/or current), the reactance and thus the impedance can become very significant. This can become critical to a design’s functionality because of the effects that changes in the impedance along the signals path from transmitter to receiver will have on the efficiency of power transfer as well as signal integrity. While a circuit’s speed is often expressed as the frequency of the waveform: the critical concern is the speed at which the voltage and/or current is required to change.

Controlled Impedance PCB Circuit Board

Applications of Controlled Impedance

Impedance Control PCB Board

Controlled Impedance should be considered for PCBs used in fast digital applications such as:

  • Telecommunications
  • Computing 100MHz and above
  • High-Quality Analog Video
  • Signal Processing
  • RF Communication

Impedance Control In PCB Design

Electrical Impedance: A measure of opposition to time-varying electric current in an electronic circuit.

The Problem: “A,” “B,” and “C” signals all reach the component at the same time.

The Solution: Apply Impedance to Circuit “C” to slow the signal enough for the component to first calculate (“A”+B”).

Example of Printed Circuit Board with Controlled Impedance Design

Similar to a cable, the signal encounters a change of impedance arising from a change in material or geometry. Part of the message will be reflected and partly transmitted. These reflections are likely to cause aberrations on the signal, which may degrade circuit performance (e.g., low gain, noise, and random errors). In practice, board designers will specify impedance values and tolerances for board traces and rely on the PCB Manufacturer to conform to the specification.

When rise times continue to reduce, inevitability, the number of traces requiring impedance control will continue to increase. Where impedance control is needed, it is essential to control it accurately, calculating it with the most representative cross-section we can create.

Impedance Control PCB Manufacturing

To meet the growing demand for impedance control PCB, PCB manufacturers have invested a lot of money and workforce and material resources in production technology to meet the production requirements of customers.

Due to the difficulty of impedance control PCB production, PCB manufacturers generally require PCB designers or purchasers to provide more detailed information and requirements, such as the type of material needed, copper weight and thickness of the board, the number of layers, Gerber file, so that the manufacturer can prepare the bill of materials (BOM). Impedance information should be marked in the Gerber file. If the necessary information is lacking, even a little critical information will make it difficult or impossible for the manufacturer’s engineers to know exactly what the customer needs.

Impedance-controlled PCBs are also divided into three different situations:

  1. No Impedance Control: This is a situation where you do not need any extra design elements to ensure correct impedance because you have very loose impedance tolerance. Naturally, this will result in a faster-completed, less expensive board because the manufacturer does not have to include any special measures.
  2. Impedance Watching: What is impedance watching? This is a situation where the designer will outline the impedance control trace, and the PCB provider adjusts the trace width and dielectric height accordingly. Once the manufacturer approves these specifications, they can begin to manufacture the board. You can request a Time Domain Reflectometry (TDR) test to confirm the impedance for a fee.
  3. Impedance Control: Actual impedance control is something you will typically only request when your design has tight impedance tolerances that could be tough to hit the first time around. When the capability limits of the manufacturer get close to the dimension requirements, it can be tough to ensure target impedance on the initial attempt.

In the case of impedance control, the manufacturer makes the circuit board achieve the target impedance as far as possible. Then they tested TDR to see if they succeeded. If not, they adjust accordingly and try again until they reach the required impedance.

Impedance Control PCB Fabricator

Impedance Control PCB is too complicated? No worry! JHYPCB engineers can help you at any time as we have three shifts. Free stack-up and impedance calculations can be offered upon request. We will work with your engineering team at the conceptual level of the PCB design to assist you in getting better results in controlling impedance by choosing the suitable laminate and layer stack up.

Why can the Impedance Control PCB be shipped timely all the time? As common materials are stored well in JHYPCB warehouse regularly. We have a heavy copper thickness, 2oz, 3oz, or more. Odd copper thickness H/1 oz, H/2 oz, 1/2 oz. Foil: 1/4 oz, H oz, 1 oz, 2 oz, 3 oz.

If you need order Impedance Control PCB, just make notes in Readme file, and send it together with Gerber, our engineer will take care of it. HAVE A TRY!

FAQ

Controlled impedance is the characteristic impedance of a transmission line formed by PCB conductors. It is relevant when high-frequency signals propagate on the PCB transmission lines. Controlled impedance is important for signal integrity: it is the propagation of signals without distortion.

Impedance matching. Impedance matching means that when energy is transmitted, the load impedance must be equal to the characteristic impedance of the transmission line. In high-speed PCB design, the matching of impedance is related to the quality of the signal.

The characteristic impedance of a PCB trace is typically determined by its inductive and capacitive reactance, resistance, and conductance. These factors are a function of the physical dimensions of the trace, the dielectric constant of the PCB substrate material, and dielectric thickness.

Impedance intuition is readily acquired through two simple equations. Trace impedance “Z” is directly related to trace inductance (L) and capacitance (C), or Z=sqrt(L/C).

At the compromise value of 50 ohms, the power has improved a little. So 50 ohm cables are intended to be used to carry power and voltage, like the output of a transmitter.

Why is impedance important? It is used for determining the interrupting capacity of a circuit breaker or fuse employed to protect the primary of a transformer.

Antenna impedance relates the voltage to the current at the input to the antenna. Let’s say an antenna has an impedance of 50 ohms. This means that if a sinusoidal voltage is applied at the antenna terminals with an amplitude of 1 Volt, then the current will have an amplitude of 1/50 = 0.02 Amps.

In electronics, a signal trace on a printed circuit board (PCB) is the equivalent of a wire for conducting signals. Each trace consists of a flat, narrow part of the copper foil that remains after etching.

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