Characteristic impedance control of PCB

(1) signal line width W;

(2) signal line thickness T;

(3) the thickness of the dielectric layer H;

(4) Dielectric constant εr.

The most influential is the thickness of the medium, followed by the dielectric constant, the width of the wire, and the minimum is the thickness of the wire. After the substrate is selected, the change of εr is small, the change of H is small, T is easier to control, and it is difficult to control the line width W by ±10%, and the line width problem has pinholes, notches, depressions, etc. on the wire. problem. In a sense, the most effective and important way to control Z0 is to control the line width.

1. Characteristic impedance value control reason

1.1. Reasons

When an electronic device (computer, communication device) is operated, the signal from the driver (Driver) will reach the receiving device (Receiver) through the PCB transmission line. When the signal is transmitted in the signal line of the printed board, its characteristic impedance value Z0 must match the "electronic impedance" of the head and tail components, and the "energy" in the signal will be completely transmitted.

1.2. Reasons 2

In the event of poor quality of the printed board, when the Z0 is out of tolerance, the transmitted signal will have problems such as reflection, dissipation, attenuation or delay. In severe cases, the signal will be transmitted and the signal will be crashed.

1.3. Reasons three

Strict selection of plates and control of the production process, Z0 on the multi-layer board can meet the specifications required by customers. The higher the electronic impedance of the component, the faster the transmission speed will be, and the Z0 of the PCB will also increase to meet the requirements of the matching component. Z0 qualified multi-layer boards are considered to be qualified products for high-speed or high-frequency signals.

2. Characteristic impedance ZO and the relationship between the plate and the process

The characteristic impedance Z0 of the microstrip line structure is calculated as: Z0 = 87/r +1.41 ln5.98H / (0.8W+T)

Where: εr - dielectric constant H-media thickness W-wire width T-wire thickness

The lower the εr of the board, the easier it is to increase the Z0 value of the PCB line and match the output impedance value of the high speed component.

2.1. The characteristic impedance Z0 is inversely proportional to the εr of the sheet.

Z0 increases as the thickness of the medium increases. Therefore, for the strict high frequency line of Z0, strict requirements are imposed on the error of the dielectric thickness of the copper clad laminate substrate. Generally, the thickness of the media must not vary by more than 10%.

2.2. Effect of medium thickness on characteristic impedance Z0

As the density of the trace increases, the increase in dielectric thickness causes an increase in electromagnetic interference. Therefore, the signal transmission line of high-frequency lines and high-speed digital lines should reduce the thickness of the medium as the density of the conductors increases, so as to eliminate or reduce the problem of noise or crosstalk caused by electromagnetic interference, or to greatly reduce εr. Εr substrate.

Calculate the formula according to the characteristic impedance Z0 of the microstrip line structure: Z0 = 87/r +1.41 ln5.98H / (0.8W+T)

The thickness (T) of the copper foil is an important factor affecting Z0. The larger the thickness of the wire, the smaller the Z0. But the scope of its change is relatively small.

2.3. Effect of copper foil thickness on characteristic impedance Z0

The thinner the thickness of the copper foil, the higher the Z0 value, but the thickness variation does not contribute much to Z0.

The contribution of thin copper foil to Z0 is better than the fact that thin copper foils contribute to the improvement or control of Z0 by making fine wires.

According to the formula:

Z0 = 87/r +1.41 ln5.98H / (0.8W+T)

The smaller the line width W, the larger Z0; reducing the wire width increases the characteristic impedance.

The change in line width is much more pronounced than the change in line thickness on Z0.

2.4. Influence of wire width on characteristic impedance Z0

Z0 increases rapidly as the line width W becomes narrower. Therefore, to control Z0, the line width must be strictly controlled. At present, the signal transmission line width W of most high-frequency lines and high-speed digital lines is 0.10 or 0.13 mm. Traditionally, the line width control deviation is ±20%. PCB conductors for conventional electronic products that are not transmission lines (wire length < 1/7 of the signal wavelength) can meet the requirements, but for signal transmission lines with Z0 control, the PCB conductor width deviation is ±20%, which is no longer sufficient. Because the Z0 error at this time has exceeded ±10%.

Examples are as follows:

A PCB microstrip line has a width of 100 μm, a line thickness of 20 μm, and a dielectric thickness of 100 μm. Assuming that the finished PCB copper thickness is uniform, the line width varies by ±20%, and can Z0 meet within ±10%?

Solution: according to the formula

Z0 = 87/r +1.41 ln5.98H / (0.8W+T)

Substitution: line width W0 = 100μm, W1 = 80μm, W2 = 120μm, line thickness T = 20μm, medium thickness H = 100μm, then: Z01 / Z02 = 1.20

Therefore, Z0 is just ±10% and cannot reach <±10%. < p="">

To achieve a characteristic impedance Z0 <±10%, the wire width deviation must be further reduced and must be much less than ±20%.

For the same reason, to control Z0 ≤ 5%, the wire width tolerance must be controlled ≤ ± 10%.

Therefore, it is not difficult to understand why the PTFE PCB and some FR-4 PCBs require a line width of ±0.02 mm because the characteristic impedance Z0 value is controlled.

3. Characteristic Impedance Control Printed Board Process Control

3.1. Film production management, inspection

Constant temperature and humidity room (21 ± 2 ° C, 55 ± 5%), dustproof; line width process compensation.

3.2. The design of the puzzle

The edge of the panel should not be too narrow, the plating layer is uniform, the plating is added with a false cathode, and the current is dispersed;

Design the panel edge to test the coupon of Z0.

3.3. Etching

Strict process parameters, reduce side erosion, and conduct the first inspection;

Reduce residual copper, copper slag and copper on the line side;

Check the line width and control within the required range (± 10% or ± 0.02mm).

3.4. AOI inspection

The inner plate must find the wire notch and the notch. For the 2GHZ high-speed signal, even the 0.05mm gap must be scrapped; controlling the inner line width and defects is the key.

3.5. Lamination

Vacuum laminator, reduce the pressure to reduce the flow of glue, try to keep more resin, because the resin affects εr, the resin is stored more, εr will be lower. Control laminate thickness tolerances. Because the plate thickness is not uniform, it indicates that the thickness of the medium changes, which will affect Z0.

3.6. Choose the substrate

The plate type is strictly required according to the customer's requirements. The model is wrong, εr is wrong, the board thickness is wrong, the PCB manufacturing process is all right, and it is also scrapped. Because Z0 is greatly affected by εr.

3.7. Solder mask

The solder mask of the board surface will reduce the Z0 value of the signal line by 1~3Ω. In theory, the thickness of the solder mask should not be too thick, and the effect is not very large. The surface of the copper wire is exposed to air (εr =1), so the Z0 value is measured to be high. However, after the solder mask, the Z0 value will drop by 1 to 3 Ω, because the εr of the solder resist is 4.0, which is much higher than the air.

3.8. Water absorption rate

The finished multi-layer board should avoid water absorption as much as possible, because the water εr = 75, which will bring about a large drop and instability effect on Z0.