PCB manufacturing and PCB design have changed a lot over the past few years. The first board design was very simple, but new technologies have brought new performance requirements to the printed circuit world, and design and manufacturing practices have become more complex. One such approach is controlled impedance in PCB manufacturing, which aims to range from the amount of power in circuit routing to the signal speed.
What is impedance and what does it mean?
The degree of opposition to the energy flow in a circuit or transmission line, expressed in ohms, is called impedance. It is the result of the sum of the resistance of the circuit and the reactance, which is the result of two effects.
Inductance: Induction of voltage in a conductor due to the magnetic field of the current.
Capacitance: Static charge stored due to the voltage between conductors.
At low frequency (DC), the reactance becomes negligible and the impedance is formed only by the resistance, but for high frequency circuits, the reactance and impedance become very important factors.
What is the controlled impedance?
Impedance mismatches in the system can cause failures and degrade the performance level of the system. Ensuring that impedance mismatches are within tolerable limits is a design feature often referred to as controlled impedance. A common example of this feature is the design of a coaxial cable: formed from an inner conductor separated from the shield (outer cylindrical conductor) using an insulator. The electrical characteristics of the insulators, as well as the proportions of all the components, are carefully controlled to regulate the impedance level of the cables.
How does LDI work?
Accordingly, in PCB manufacturing, traces represent conductors, laminates represent insulators, and planes represent shields. Therefore, the impedance of a printed circuit board depends on the size and material used in the manufacturing process.
Why is it important to control impedance?
In a printed circuit board, the purpose of tracing is to transfer the maximum signal power from the source to the load, but this allocation can only be achieved if the impedance value of the source matches the impedance value of the load, otherwise the energy of the signal will be reflected back to the light source, reducing its quality. Taking advantage of today’s fast rise time signals, circuit board designs with controllable impedance have become very popular.
How to make it in PCB production?
Impedance control in PCB fabrication and design is achieved in two ways: well-defined PCB stacking and careful trace design. It is well known that the impedance is proportional to the laminate height and inversely proportional to the wire width and thickness. Therefore, changing the size and spacing of a particular trace will adjust its impedance value.
Finally, the impedance also depends on the layer’s permittivity: a number that quantifies a material’s inherent ability to store electrical energy in an electric field.
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