With the rapid development and advancement of electronic products such as miniaturization, digitization, high frequency and multi-function, as the electrical interconnection in electronic products - the role of the wires in the PCB, not only the problem of current circulation or not, Moreover, it acts as a "transmission line". That is to say, for the electrical test of the PCB for the transmission of high-frequency signals or high-speed digital signals, it is necessary to test whether the "on", "off", "short circuit", etc. of the line are satisfactory, and also the "characteristic impedance value" "If it is satisfactory, only these two aspects are "qualified", and the PCB is in compliance with the acceptance.
1. Proposal of signal transmission line
1.1 Definition of signal transmission lines
This is the name given to distinguish between conventional wires. According to the definition of 3.4.4 of IPC-2141: "When the signal is transmitted in the PCB wire, if the length of the wire is close to 1/7 of the signal wavelength, the wire becomes the signal transmission line at this time." Some literatures believe that when the length of the wire is close to 1/10 of the wavelength, it should be processed according to the signal transmission line. Obviously, the latter is more rigorous (it seems 'excessive'), but most people identify it as the former.
As we all know, when a current passes through a conductor, it is subject to a "resistance", which is a resistance in direct current, in accordance with Ohm's law. That is: R=V/I “resistance†in AC is the combined result of “resistanceâ€, “inductance†and “capacitive reactanceâ€, ie: Z=[R2+(XL-XC)2]1/21.2 signal Transmission line judgment
The component has a very high frequency signal transmission, but after the wire is transmitted, the frequency decreases (time delay). The longer the wire, the longer the time is extended. When the length of the wire is close to the wavelength, or the signal speed (frequency) is increased to some In the range, the transmitted signal will show significant "distortion".
(1) Transmission of high frequency signals.
Assume: (1) The signal transmission frequency of the component is f=10MHZ, and the wire L=50cm, then C=f*λλ= C/fλ/L= C/f*L=60 is a conventional wire. (2) The signal transmission frequency of the component is f=1GHZ, and the length of the wire is L=10cm, then λ/L=C/f*L=3 is not a conventional wire, and the transmission line with characteristic impedance value control should be performed.
(2) Transmission of pulse signals. The rise time tr from "0" to "1" in the digital circuit is very short. However, the frequency fmax can be calculated by the following formula: fmax = 0.35 / tr Assumption: the rise time tr of the component is = 2 ns, then fmax = 0.35 /tr=175 MHZ L= C/ fmax*7=24.5 cm When the wire length is ≥24.5 cm, it should be treated as a signal transmission line. At present: tr of TTL (transister-transister logic) is 4 ns → 1 ns → 0.5 ns → Erl (emitter-coupled logic) tr is 3 ns → 1 ns → 0.5 ns →
(3) The signal transmission line must be controlled for characteristic impedance values. If the characteristic impedance value control is not performed, the signal "reflected" generated in the line will "offset" the signal being transmitted. The smaller the λ/L ratio, the more serious the "reflection", the following problems occur: 1 signal (or energy) transmission efficiency is significantly reduced; 2 due to repeated interference (cancellation) signal transmission, will be severe with increasing frequency; Part of the "energy" is radiated by electromagnetic waves, creating EMI between internal wires or networks. 1.3 difference between signal ordinary line and signal transmission line
There are three main differences between the signal common line and the signal transmission line:
(1) The signal normal line means that the second signal is transmitted after the first signal transmission is accepted, so the "reflection" signal during the first signal transmission does not cancel the second signal. The signal transmission line is characterized in that the first signal is not yet accepted, and the second signal is transmitted, so that the "reflected" signal generated during the first signal transmission cancels the second signal and weakens the second signal. The faster the frequency is transmitted, the more "distortion" is, and even the signal disappears.
(2) Signal ordinary line, because the signal transmission speed is slow, the "reflected" signal will not cancel the signal transmitted later. Therefore, the thickness, defects (notches, pinholes), etc. of the wires are allowed to exist to some extent. In the signal transmission line, these thicknesses, defects, etc. are subject to very strict requirements.
(3) Signal ordinary line, no characteristic impedance value control is required, and only electrical tests of "on", "off", and "short circuit" are required. The signal transmission line requires characteristic impedance value control, that is, in addition to electrical tests requiring "on", "off", and "short circuit", it is also necessary to have a characteristic impedance value control test.
2. Design of characteristic impedance value Z0 in PCB
2.1 Structure type and calculation method of Z0
There are two main types: microstrip lines and strip lines and their various structures. How to choose them depends on components and electronic products.
Microstrip line (suitable for large Z0 occasions). Z0 = {87 / (εr + 1.41) 1/2 } ln {5.98H / (0.8W + T)} strip line (suitable for occasions where Z0 is small).
Z0 = 60ln{4D/[0.67Ï€(0.8W+T) ]} The D in the formula is the thickness of the dielectric layer. 2.2 Structure and calculation method of microstrip line Various structures and their calculation methods can be formed according to the different positions of the signal transmission line (see Chapter 14 of Modern Printed Circuit Basics). 2.3 General design rules for characteristic impedance value Z0 (1) Select the appropriate substrate (CCL) material and PCB structure, determine the length of the signal transmission line, etc. to determine the PCB size. (2) Reasonable layout and wiring, so that the characteristic impedance value Z0 of each group (network) wire matches the characteristic impedance value of the element (group). (3) Measures and methods for remediation and correction of the characteristic impedance value Z0 deviation in the PCB should be considered in consideration of the unstable fluctuation of the substrate material quality, the deviation of the PCB manufacturing process, and the control and PCB design factors.
3. Layout of signal transmission lines
3.1 The length of the signal transmission line is as short as possible
According to the definition of the signal "transmission line", the signal line is laid out very short, so that its length is less than 1/7 of the transmission signal wavelength, which can eliminate the problem that the transmission signal is "reflected" by the signal. In other words, if the signal line is laid out and its length is as short as 1/7 of the wavelength of the transmission signal, the laid wire can be processed as a normal line. How to make the signal line shorter? In addition to the high-frequency components, the interconnection structure should be worked on the PCB board, such as buried/blind holes, hole in pad, stacked holes and HDI/BUM to shorten the trace. 3.2 High-density wiring, the thinner the dielectric layer, the smaller the crosstalk
The thicker the dielectric layer, the stronger the electromagnetic cross induction, and the more serious the crosstalk! To make the dielectric layer thin, you must choose a low εr material. 3.3 Using non-parallel traces
Dense parallel traces will result in greater inductance and capacitance, resulting in greater crosstalk and one of the causes of noise. It should be used: (1) The adjacent conductor layers are arranged at right angles to each other; (2) Stepped oblique (45 degree) is used on the same layer; (3) Stranded through the via. 3.4 Using differential transmission lines
The use of differential transmission lines can significantly reduce transmission line interference, which is very important in high frequency and high speed digital signal transmission. (1) Differential transmission lines can significantly reduce the interference of signals in the transmission line and improve the integrity of the transmission signal, which is familiar to PCB designers. However, the degree to which different differential transmission lines reduce interfering signals is different. In order to reduce the "common mode" interference to the transmitted signal, the differential transmission line used should mainly be as follows: (1) The shape and length are the same, so that the "common mode" corner is achieved, that is, the shape and length are not different. And cause "common mode" interference; (2) from a right angle to a 45 degree angle, experiments show that its "common mode" interference can be reduced by 50%; (c) the use of compensation capacitors, such as a short line in the corner plus a suitable capacitor , can reduce interference; (4) form a twisted pair differential transmission line. (2) Twisted pair differential transmission line. The use of vias to form a twisted pair differential transmission line between different layers is currently the most effective way to reduce interference signals. 1 There is a bias (shift) twisted pair differential transmission line. Also known as a conventional twisted pair differential transmission line. 2 There is no offset (shift) twisted pair differential transmission line. Better signal interference can be obtained.
4, the characteristic impedance value Z0 requirements for the substrate (CCL) material
From Z0 ={87/(εr+1.41)1/2 }ln{5.98H/(0.8W+T)}, we can see that the main factors affecting the characteristic impedance value Z0 are: (1) dielectric constant εr; (2) the thickness of the dielectric layer H; (3) the width W of the signal transmission line; (4) the thickness of the signal transmission line. These indicate that the characteristic impedance value Z0 is closely related to the substrate material. Experiments have also shown that the influence characteristic impedance value Z0 is sequentially arranged from 9 (2), (3), (1), and (4). 4.1 Influence of dielectric constant εr on characteristic impedance value Z0
(1) The dielectric constant εr affects the transmission speed of the signal. The transmission speed of the signal decreases as the dielectric constant εr increases. According to the Maxwell's formula in the theory of electromagnetic waves, namely: Vs=c/(εr) 1/2 Table 1 (2) The magnitude of the dielectric constant εr is the "weighted sum" of the composite. That is to say, the magnitude of the dielectric constant εr is related to the composition and structure (composite composition and structure) of the dielectric layer. For example, in the FR-4 material, since the structure of the E-glass cloth (such as 7628, 2116, 1080, 106, etc.) is different, the resin content is different, and therefore, the dielectric constant εr value is different. For the strict control of the characteristic impedance value Z0, PCB design and manufacturing should be understood and calculated in order to obtain more precise control and results.
(3) The magnitude of the variation of εr value is greater than other factors, ranking third. The effect of the dielectric constant εr on the characteristic impedance value Z0 can be seen from the formula of Z0: Z0 = {87 / (εr + 1.41) 1/2 } ln {5.98H / (0.8W + T)} Obviously, dielectric The smaller the value of the constant εr is, the larger the Z0 value is, and the magnitude of the variation of the εr value is large, which should be carefully controlled. 4.2 Influence of dielectric thickness on characteristic impedance value Z0
(1) As can be seen from the formula of Z0, the value of Z0 is proportional to the natural logarithm of the thickness H of the medium.
(2) At the same thickness, the microstrip line has a large Z0 value.
(3) The influence of the thickness deviation on the Z0 value is in the first place, so the thickness of the dielectric layer must be well controlled. However, since the thickness deviation is mainly controlled by the CCL manufacturer, followed by the PCB manufacturer (multi-laminate), the general deviation can be controlled within a small range. 4.3 Influence of wire thickness on characteristic impedance value Z0
(1) As can be seen from the formula of Z0, the value of Z0 increases as the thickness T of the wire decreases. (2) At the same thickness, the microstrip line has a large Z0 value. (3) The influence of the thickness deviation on the Z0 value is the smallest. 4.4 Effect of wire width on characteristic impedance value Z0
(1) As can be seen from the formula of Z0, the value of Z0 increases as the width W of the wire decreases.
1 Calculations and experiments show that the influence of the wire width W on the characteristic impedance value Z0 is the largest.
2 wire width W is the most difficult to control PCB production, but also the most need to control.
(2) The meaning of wire width deviation control. The significance of the wire width deviation control is to some extent controlled the range of the characteristic impedance value Z0 of the PCB (OEM design). After selecting the CCL material and completing the PCB design, this means: 1 dielectric constant εr value, dielectric thickness H value and wire thickness T value are basically unchanged, or little change; 2 wire width deviation is the largest, and the most difficult Control, because the manufacturing process is long and has a lot of impact. The longer the 3 wires are used to transmit signals, and the wire width deviation is the biggest factor affecting the characteristic impedance value Z0. Therefore, the control of the wire width deviation value is a key technology of today's HDI/BUM board.
(3) Control of wire width deviation. 1 The wire width dimension is rapidly reduced, and the control is more difficult, belonging to the control of "fine" pitch. 2 Conventional graphics transfer technology is increasingly unable to meet the requirements of fine wires. 3 laser direct imaging technology is currently the best choice for manufacturing fine wires.
5, the test of the characteristic impedance value Z0
5.1 Test sample for characteristic impedance The test sample for characteristic impedance can be carried out in accordance with IEC 61188-1-2. IPC-D-275 (four circuit board transmission lines), IPC-D-317 (type of transmission line in high-speed circuit board design specifications) and IPC-TM-650 are also specified. 5.2 The characteristic impedance tester is currently a characteristic impedance tester manufactured by Polar Company of the United Kingdom. It consists of a time domain reflectometer (TDR), a desktop computer, and a special 1 meter long cable test probe and a sample (or interconnect) to be tested. The characteristic impedance test principle is that a signal voltage (high-frequency signal or high-speed pulse signal voltage) is emitted from the time domain reflectometer (TDR) to the printed board, and the reflected voltage change is measured, and then calculated and outputted by the PC. The impedance value is Z0. Calculation formula: Z0 = Z-parameter V-line / (V--V-line) 5.3AOI control of characteristic impedance value 5.4 Due to the importance of wire manufacturing integrity (size deviation) in the control of characteristic impedance values, more and more Towards refinement. The use of "visual inspection" is no longer sufficient, and with the continuous improvement and improvement of AOI, it has become a reality to use AOI technology to control fine wires. Although it can not completely replace the characteristic impedance test, it can improve PCB productivity (qualification rate). Further achieve the purpose of controlling the characteristic impedance value.