PCB News - Detailed explanation of PCB dielectric constant

1. Our commonly used PCB boardmedium is FR4 material, and the relative dielectric constant of air is 4.2-4.7. This dielectric constant will change with temperature, and the maximum change range can reach 20% in the temperature range of 0-70 degrees. The change of the dielectric constant will cause a 10% change in the line delay. The higher the temperature, the greater the delay. The dielectric constant also changes with the frequency of the signal, the higher the frequency, the smaller the dielectric constant. Below 100M, 4.5 can be used to calculate the inter-board capacitance and delay.

2. The transmission speed of the inner layer signal in the general FR4 material PCB board is 180ps/inch (1inch=1000mil=2.54cm). The surface layer generally depends on the situation, generally between 140 and 170.

3. The actual capacitor can be simply equivalent to L, R, and C in series. The capacitor has a resonance point, which will be inductive at high frequencies (beyond this resonance point). The resonance point is different if the capacitance value and process of the capacitor are different. And the products produced by different manufacturers will be very different. This resonance point mainly depends on the equivalent series inductance. Now, for example, the equivalent series inductance of a 100nF chip capacitor is about 0.5nH, and the ESR (equivalent series resistance) value is 0.1 ohms. Then the filtering effect is best at about 24M, and the AC impedance is 0.1 ohms. The equivalent inductance of a 1nF chip capacitor is also 0.5nH (the difference between different capacitance values is not too large), and the ESR is 0.01 ohms, which will have the best filtering effect around 200M. In order to achieve a better filtering effect, we use a combination of capacitors with different capacitance values. However, due to the equivalent series inductance and capacitance, there will be a resonance point between 24M and 200M, at which there is a maximum impedance, which is greater than the impedance of a single capacitor. This is an undesirable result. (In the section from 24M to 200M, small capacitors are capacitive, and large capacitors are already inductive. Two capacitors in parallel are equivalent to LC in parallel. The sum of the ESR values of the two capacitors is the series resistance of the LC loop. If LC is connected in parallel, If the series resistance is 0, then there will be an infinite impedance at the resonance point, which has the worst filtering effect. This series resistance will inhibit this parallel resonance phenomenon, thereby reducing the impedance of the LC resonator at the resonance point ). In order to reduce this effect, a capacitor with a larger ESR can be used as appropriate. ESR is equivalent to the series resistance in the resonant network, which can reduce the Q value, thereby making the frequency characteristics flatter. Increasing ESR will make the overall impedance converge. In the frequency bands below 24M and above 200M, the impedance will increase, while in the 24M and 200M frequency bands, the impedance will decrease. Therefore, the frequency band of board switching noise must be considered comprehensively. Some foreign boards are designed with a small capacitor (680pF) connected in series with a few ohm resistors when the large and small capacitors are connected in parallel. This is probably because of this consideration. (From the above parameters, the Q value of a 1nF capacitor is 10 times the Q value of a 100nF capacitor. Since there is no specific equivalent string inductance and ESR value from the manufacturer on hand, the parameters in the above example are based on the data seen in the past. Yes. But the deviation should not be too large. In the past, the materials seen in many places are that the resonant frequencies of 1nF and 100nF ceramic capacitors are 100M and 10M, respectively. Considering that the L of the chip capacitor is much smaller, but it is not found The reliable value is calculated as 0.5nH for convenience. If you have a specific and reliable value, I hope to post it.

The dielectric constant (Dk, ε, Er) determines the speed at which the electric signal propagates in the medium. The speed of electrical signal propagation is inversely proportional to the square root of the dielectric constant. The lower the dielectric constant, the faster the signal transmission speed. Let's make a vivid analogy, just like you are running on the beach. The depth of the water floods your ankles. The viscosity of water is the dielectric constant. The more viscous the water, the higher the dielectric constant, and the slower you run.

The dielectric constant is not very easy to measure or define. It is not only related to the characteristics of the medium, but also related to the test method, test frequency, and the state of the material before and during the test. The dielectric constant also changes with temperature. Some special materials take into account the temperature factor in the development. Humidity is also an important factor affecting the dielectric constant, because the dielectric constant of water is 70, and there is very little moisture., Will cause significant changes.

The following are the dielectric constants of some typical materials (under 1Mhz):

It can be seen that for high-speed and high-frequency applications, the most ideal material is an air medium wrapped in copper foil, with a thickness tolerance of +/-0.00001". As a material development, everyone is working hard in this direction, such as Arlon The patent-developed Foamclad is very suitable for the application of base station antennas. But not all designs have a smaller dielectric constant, the better. It is often based on some actual designs. Some circuits that require a small volume often require a high dielectric constant. Materials such as Arlon AR1000 are used in miniaturized circuit design. Some designs, such as power amplifiers, usually have a dielectric constant of 2.55 (such as Arlon Diclad527, AD255, etc.), or a dielectric constant of 3.5 (such as AD350, 25N/FR, etc.). Some designs are also used. 4.5 dielectric constant, (such as AD450) is mainly changed from FR-4 design to high-frequency application, and hope to continue to use the previous design.

In addition to directly affecting the transmission speed of the signal, the dielectric constant also determines the characteristic impedance to a large extent. In different parts, the characteristic impedance matching is particularly important in microwave communication. If the phenomenon of impedance mismatch occurs, the impedance mismatch is also called It is VSWR (Standing Wave Ratio).

MAX2242: FR4 or G-10 should be selected for printed circuit board materials. This type of material is a good choice for most low-cost wireless applications with operating frequencies below 3 GHz. The MAX2242 evaluation board uses 4-layer FR4 with a dielectric constant of 4.5, an insulating layer thickness of 6 mil, and 1oz of copper.

When designing a low-impedance circuit like MAX2242 with an output impedance of only about (8 + j5) at 2.45GHz, a 0.5nH inductor can produce an inductive reactance of 8, which is equivalent to a dielectric constant of 4.5 and a thickness of 6mil The impedance generated by the 60mil x 10mil microstrip line on the FR4 printed circuit board.

Resonance frequency calculation: f=1/(2*3.14159*SQRT(L*C))

The above is an introduction to the detailed explanation of PCB dielectric constant. Ipcb is also provided to PCB manufacturers and PCB manufacturing technology