PCB Tech - Discussion on PCB impedance control

With the increasing speed of PCB board signal switching, today's PCB designers need to understand and control the impedance of PCB traces. Corresponding to the shorter signal transmission times and higher clock rates of modern digital circuits, PCB traces are no longer simple connections, but transmission lines.

In practice, it is necessary to control trace impedance when digital marginal speed exceeds 1ns or analog frequency exceeds 300Mhz. One of the key parameters of a PCB trace is its characteristic impedance (the ratio of voltage to current as the wave travels along the signal transmission line). The characteristic impedance of wire on printed circuit board is an important index of circuit board design, especially in PCB design of high frequency circuit, it must be considered whether the characteristic impedance of wire is consistent with the characteristic impedance required by device or signal. This involves two concepts: impedance control and impedance matching. This paper focuses on impedance control and lamination design.

Impedance control

EImpedance Controling, the conductor in the circuit board will have all kinds of signal transmission, in order to improve the transmission rate and must increase its frequency, if the line itself due to etching, stacking thickness, wire width and other different factors, will cause impedance value change, the signal distortion. Therefore, the impedance value of the conductor on the high-speed circuit board should be controlled within a certain range, known as "impedance control".

The impedance of a PCB trace will be determined by its inductive and capacitive inductance, resistance, and conductivity coefficient. The main factors affecting the impedance of PCB wiring are: the width of copper wire, the thickness of copper wire, the dielectric constant of medium, the thickness of medium, the thickness of pad, the path of ground wire, the wiring around the wiring, etc. PCB impedance ranges from 25 to 120 ohm.

In practice, a PCB transmission line usually consists of a trace, one or more reference layers, and insulation materials. Traces and layers form the control impedance. PCBS will often be multi-layered, and the control impedance can be constructed in a variety of ways. However, whatever method is used, the impedance value will be determined by its physical structure and the electrical properties of the insulating material:

Width and thickness of signal trace

The height of the core or prefill material on either side of the trace

Configuration of trace and plate

Insulation constants of core and prefilled materials

PCB transmission lines come in two main forms: Microstrip and Stripline.

Microstrip:

A microstrip line is a strip conductor with a reference plane on only one side, with the top and sides exposed to air (or coated), above the surface of the insulation constant Er circuit board, with the power supply or grounding as a reference. As shown below:

Prepreg/Core concept of insulation layer.

PP (Prepreg) is a kind of dielectric material, composed of glass fiber and epoxy resin. Core is actually a TYPE of PP medium, but its two sides are covered with copper foil, while PP is not. When making multilayer boards, core and PP are usually used together, and PP is used to bond between core and core.

Matters needing attention in PCB lamination design

(1) Warpage problem

The layer design of PCB should be symmetrical, that is, the thickness of medium layer and copper layer of each layer should be symmetrical. Take six layers for example, the thickness of top-GND and bottom-power medium should be consistent with the thickness of copper, AND that of GND-L2 and L3-POWER medium should be consistent with the thickness of copper. This will not warp when laminating.

(2) The signal layer should be tightly coupled with the adjacent reference plane (that is, the medium thickness between the signal layer and the adjacent copper coating layer should be very small); Power copper dressing and ground copper dressing should be tightly coupled.

(3) In the case of very high speed, extra layers can be added to isolate the signal layer, but it is recommended not to isolate multiple power layers, which may cause unnecessary noise interference.

(4) The distribution of typical laminated design layers is shown in the following table.