As PCB signal switching speeds continue to increase, today's PCB planners need to understand and manipulate 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 desirable to manipulate the trace impedance when the digital edge speed is above 1 ns or when the simulated frequency exceeds 300 Mhz. One of the key parameters of a PCB trace is its characteristic impedance (ie, the ratio of voltage to current as the wave travels along the signal transmission line). The characteristic impedance of the conductor on the printed circuit board is an important indicator of board layout. Especially in the PCB planning of high-frequency circuits, it is necessary to consider whether the characteristic impedance of the conductor and the characteristic impedance required by the equipment or signal are common and match. . This involves two concepts: impedance steering and impedance matching. This article points to the issues of impedance steering and stack planning.

Impedance control

Impedance control (eImpedance Controling), there are various signals transmitted in the conductors in the circuit board. It is necessary to improve the frequency to improve the transmission rate. If the line itself is etched, laminated thickness, wire width and other different elements, The impedance is worth changing and the signal is distorted. Therefore, the conductor on the high-speed circuit board, its impedance value should be controlled within a certain range, called "impedance control."

The impedance of the PCB trace will be confirmed by its inductive and capacitive inductance, resistance and conductance. The main factors affecting the impedance of the PCB trace are: the width of the copper wire, the thickness of the copper wire, the dielectric constant of the dielectric, the thickness of the dielectric, the thickness of the pad, the way of the ground wire, and the traces around the trace. The PCB impedance ranges from 25 to 120 ohms.

In practice, PCB transmission lines typically consist of a wire trace, one or more reference layers, and insulating materials. Traces and slabs form the steering impedance. PCBs will often be multi-layered and the steering impedance can be built in a variety of ways. However, regardless of the method used, the impedance value will be determined by its physical structure and the electrical properties of the insulating material:

Signal trace width and thickness

Height of the core or prefilled material on either side of the trace

Trace and board configuration

Insulation constant of the core and prefilled material

There are two main forms of PCB transmission lines: Microstrip and Stripline.

Microstrip:

The microstrip line is a strip conductor, which refers to a transmission line with a reference plane on one side, and the top and sides are exposed to the air (also coated with a coating layer), which is placed on the surface of the insulation constant Er circuit board to The power or ground plane is referred to. As shown below:

Note: In the practice of PCB manufacturing, the board factory generally applies a layer of green oil on the surface of the PCB board. Therefore, in the practical impedance calculation, the surface microstrip line is generally calculated using the model shown in the following figure:

Stripline:

The strip line is a strip conductor placed between two reference planes, as shown in the following figure, the dielectric constants of the dielectrics represented by H1 and H2 can be different.

The above two examples are only a typical example of microstrip lines and strip lines. There are many kinds of microstrip lines and strip lines, such as laminated microstrip lines, which are related to the laminated structure of a specific PCB.

The mathematical calculation for calculating the equivalent of the characteristic impedance is usually based on the field solution method, which includes the analysis of the gap element. Therefore, using the special impedance accounting software SI9000, what we need to do is to manipulate the characteristic impedance parameters:

The dielectric constant Er of the insulating layer, the trace widths W1, W2 (trapezoidal), the trace thickness T, and the thickness H of the insulating layer.

Description of W1, W2:

It is necessary to calculate the value in the red box. Other conditions analogy.

The following uses SI9000 accounting to meet the requirements of impedance control:

First calculate the single-ended impedance control of the DDR data line:

TOP layer: The copper thickness is 0.5OZ, the trace width is 5 MIL, the spacing from the reference plane is 3.8 MIL, and the dielectric constant is 4.2. Pick the model, substitute the parameters, and select the lossless calculation, as shown:

Coating indicates the coating. If there is no coating, fill in the thickness with 0, and the dielectric constant is filled with 1 (air).

The substrate indicates that the substrate layer, that is, the dielectric layer, is generally selected from FR-4, and the thickness is calculated by impedance calculation software, and the dielectric constant is 4.2 (when the frequency is less than 1 GHz).

Click on the Weight(oz) item to set the copper thickness of the copper. The thickness of the copper determines the thickness of the trace.

9. The concept of Prepreg/Core for insulation:

PP (prepreg) is a kind of dielectric material, composed of glass fiber and epoxy resin. Core is also a PP type medium, but his two sides are covered with copper foil, but PP is not. When making multi-layer board, CORE and C are generally PP cooperation, CORE and CORE are bonded with PP.

10. Precautions in PCB stacking planning:

(1), warpage problem

The lamination planning of the PCB should be symmetrical, that is, the dielectric layer thickness and the copper plating thickness of each layer are symmetrical. When the six-layer board is used, the dielectric thickness and copper thickness of TOP-GND and BOTTOM-POWER are common, GND-L2 Common with the thickness and copper thickness of L3-POWER. This does not cause warpage at the time of lamination.

(2) The signal layer should be tightly coupled to the nearby reference plane (ie, the thickness of the medium between the signal layer and the nearby copper layer should be small); the power supply copper and ground copper should be tightly coupled.

(3) In a very high speed situation, it is possible to participate in the excess formation to block the signal layer, but it is not recommended to block multiple power layers, which may form unnecessary noise interference.

(4) The typical stack layout layer distribution is shown in the following table:

(5), the general guidelines for the layout of layers:

The underside of the component surface (the second layer) is the ground plane, supplying the shielding layer of the equipment and supplying the reference plane for the top layer wiring;

All signal layers may be adjacent to the ground plane;

Try to prevent the two signal layers from directly adjacent;

The main power source may be adjacent to it correspondingly;

Consider the symmetry of the laminated structure.

Regarding the layer layout of the mother board, the existing motherboard is difficult to control the parallel long interval wiring, and the board operating frequency is above 50 MHz.

(For conditions below 50MHZ, refer to the appropriate relaxation), the recommended layout guidelines:

The component surface and the welding surface are complete ground planes (shield);

No adjacent parallel wiring layers;

All signal layers may be adjacent to the ground plane;

The key signal is adjacent to the stratum, not across the partition