High-speed PCB FAQ
PCB common problems
Basic parameters:
Via: inner diameter 8mil/0.2mm, outer diameter is generally twice the inner diameter of 16mil/0.4mm. If it is a 0.5mm pad, use 8~14mil vias, and generally use 10mil in other places.
Line width: 3.5mil/0.09mm
Seam: 3.5mil/0.09mm The gap of the dividing plane is generally 10~20mil
1. The difference between LDO and DC circuit
https://zhuanlan.zhihu.com/p/349162261
LDO：LOW DROPOUT VOLTAGE LDO
(It is the abbreviation of low dropout voltage regulator, rectifier), a low dropout linear regulator, hence the name implies, a linear regulator, which can only be used in step-down applications, that is, the output voltage must be less than the input voltage.
Advantages: good stability, fast load response, small output ripple.
Disadvantages: low efficiency, the voltage difference between input and output cannot be too large. The load can not be too large, the current largest LDO is 5A (but there are many restrictions to ensure the output of 5A)
DC/DC: DC voltage to DC voltage. Strictly speaking, LDO is also a kind of DC/DC, but the current DC/DC multi-finger switching power supply has many topological structures, such as BUCK, BOOST, etc.
Advantages: high efficiency, wide input voltage range.
Disadvantages: The load response is worse than LDO, and the output ripple is larger than LDO.
The LDO circuit can be placed as close to the load as possible, while the DC chip should be placed in the power area as much as possible.
2. The current of the inner circuit is halved
3. The decoupling capacitor should be as close as possible to the chip during layout
4. PLL phase locked loop
http://www.elecfans.com/analog/20171111578043.html
Phase locked loop (phase locked loop), as the name implies, is a phase locked loop. Anyone who has learned the principle of automatic control knows that this is a typical feedback control circuit. It uses an external reference signal to control the frequency and phase of the oscillation signal inside the loop, and realizes the automatic tracking of the output signal frequency to the input signal frequency. Used in closed-loop tracking circuits.
In the layout, the order of the position of the device close to the main chip: decoupling capacitor has priority over the clock circuit before the phase-locked loop circuit
5, difference signal
Effectively suppress common-mode signals, and the electromagnetic interference to the outside is also small.
Differential signal lines must be two lines of equal length, equal width, close proximity and on the same level. (In order to ensure that the assignment changes of the two signals are the same).
An inverter can be used to form two signals with the same amplitude and opposite phases.
The length error of the differential signal is within 5mil
6. Common mode filter for common mode signal and differential mode signal
Common mode signal and differential mode signal (differential signal)
7. Common mode filter
Common-mode filters usually use ferrite cores, double-wired in parallel. The low differential mode noise signal suppresses the interference source and is difficult to deform in the high-speed signal. Good noise suppression countermeasures, high common mode noise suppression and low differential mode noise signal suppression.
8. Analog signal
Operational amplifiers are analog signals, and the wiring of analog signals should be as thick and short as possible
The process of crystal oscillation is generally treated as analog signal processing and needs to be thickened, generally at least 8-10 mils.
9. The power layer of a multilayer PCB
On the power layer, the interval between different voltages is more than 20 mils.
Determine the gap between different power modules on a plane according to the magnitude of the voltage difference. For example, the voltage difference between the 5V power supply and the 1.5V power supply is 3.5V. According to the voltage difference of 1V, increasing the distance of 5mil can get 3.5mil*5mil=17.5mil. Under normal circumstances, it is sufficient to keep 10~15mil
10. Impedance matching
Someone finally understands what is characteristic impedance and what is impedance matching
11. Differential line and single branch line
Differential line routing and differential line equal length processing
The positive signal of the differential line is P, and the reverse signal is labeled N
Differential line, in popular terms, uses two parallel, equal-length traces to transmit the same signal with a phase difference of 180 degrees. To put it bluntly, one wire transmits positive signals and one wire transmits negative signals. The negative signal is subtracted from the positive signal to obtain a useful signal with twice the intensity. The interference signal on the two lines is the same, and the interference signal is gone after the subtraction.
12、BGA
The full name of BGA is Ball Grid Array (PCB with ball grid array structure), which is a packaging method in which integrated circuits use organic carrier boards. Its characteristics are: ①The package area is reduced; ②The function is increased, and the number of pins is increased; ③The PCB board can be self-centered and easy to tin when soldering; ④High reliability; ⑤Good electrical performance, low overall cost. PCB boards with BGA generally have many small holes. Most customers' BGA vias are designed with a finished hole diameter of 8-12mil. The distance between the surface of the BGA and the hole is 31.5mil as an example, generally not less than 10.5mil. BGA vias need to be plugged
Chips with a ball gap of 0.8 mil usually use a 0.4 mil line width.
13 、 FBGA
FBGA is the abbreviation of ***Fine-Pitch Ball Grid Array*** (meaning "Fine-Pitch Ball Grid Array"), which is a fine-pitch ball grid array.
The 0.65 ball gap uses a 4 mil line width and a 4 mil gap, and the fan-out line is generally 8-16 mil vias.
14, fan out
PCB fanout (fanout) is different from the concept in digital systems, it can be said to refer to a process, that is, a certain component pin is taken out of a short line, and then a via is finished (this via is usually connected To the plane layer, of course, it can also be a signal line) this process. The concept of fan-out may not be familiar to some beginners, but in fact, every engineer who draws double-sided and above board layers uses the fan-out function, especially for bypass capacitor components. Usually It is manual fan-out (that is, the meaning of manual hole punching), but for some special packages (such as BGA), when the density is large and the pins are large, the advantages of using automatic fan-out are very obvious. It is fast and tidy. It is "favored" by the majority of senior engineers.
15. Cross-segmentation
The division of the power plane or the ground plane will cause the voltage on the plane to be discontinuous, causing the reference plane of the signal line to cross from one power plane to another power reference plane. This kind of signal line routing across different voltage planes becomes a cross-segmentation. When a crossover occurs, the layout of the power plane should be changed, or the signal line should be routed to an area where the crossover does not appear.
16、DRC
DRC, the Design Rule Check design rule check.
Note 1: Do not cross-split the signal line.
Note 2: The data line and differential signal line should maintain a good ground. Including land on different planes.
Note 3: The data line and differential line should not be too close to strong interference lines such as power or clock signals.
Note 4: The crystal oscillator circuit is covered with ground, and the wires cannot be routed underneath.
Note 5: The high-speed signal line uses the voltage plane as the return path.
Note 6: Make a check list.
17, mark point
The mark point is the position identification point of the PCB used on the automatic placement machine in the circuit board design. The selection of mark points directly affects the placement efficiency of the automatic placement machine.
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Original link: https://blog.csdn.net/qq_44179528/article/details/113865383