In high-speed PCB design, the hole design is an important factor, it is composed of holes, the pad area around the hole and the POWER layer isolation area, usually divided into blind holes, buried holes and through holes three categories. In the process of PCB design, the parasitic capacitance and parasitic inductance of the through hole are analyzed, and some points for attention in the design of high-speed PCB through hole are summarized.
At present, the design of high-speed PCB is widely used in the fields of communication, computer, graphics and image processing, and all high-tech value-added electronic products are designed in the pursuit of low power consumption, low electromagnetic radiation, high reliability, miniaturization, lightweight and other characteristics, in order to achieve the above goals, in high-speed PCB design, through the hole design is an important factor.
1. through the hole
Through hole is an important factor in multi-layer PCB design, a through hole is mainly composed of three parts, one is the hole; Second, the pad area around the hole; The third is the POWER layer isolation zone. The process of passing the hole is to plate a layer of metal on the cylindrical surface of the hole wall by chemical deposition to connect the copper foil that needs to be connected in the middle of each layer, and the upper and lower sides of the hole are made into ordinary pad shape, which can be directly connected with the lines of the upper and lower sides, or not. The hole can play the role of electrical connection, fixing or positioning devices.
Blind holes, which are located on the top and bottom surfaces of the printed circuit board, have a certain depth for the connection of the surface line and the inner line below, and the depth of the hole is usually not more than a certain ratio of the aperture.
Buried hole refers to the connection hole located in the inner layer of the printed circuit board, which does not extend to the surface of the board.
Both blind holes and buried holes are located in the inner layer of the circuit board, which is completed by the through hole molding process before lamination, and several inner layers may be overlapped during the formation of the through holes.
Through holes, which pass through the entire circuit board and can be used for internal interconnection or as mounting positioning holes for components. Because the through hole is easier to achieve in the process and the cost is lower, the general printed circuit board uses the through hole.
2. Parasitic capacitance through the hole
There is a parasitic capacitance to the ground through the hole itself. If the diameter of the isolation hole on the pavement is D2, the diameter of the welding pad through the hole is D1, the thickness of the PCB is T, and the dielectric constant of the substrate is ε, the parasitic capacitance through the hole is approximately: C =1.41εTD1/(D2-d1).
The main effect of the parasitic capacitance through the hole on the circuit is to extend the rise time of the signal and reduce the speed of the circuit, and the smaller the capacitance value, the smaller the impact.
3. Parasitic inductance through the hole
In the design of high-speed digital circuits, the harm caused by the parasitic inductance is often greater than that of the parasitic capacitance. The parasitic series inductance through the hole will weaken the function of the bypass capacitor and weaken the filtering effect of the whole power system. If L refers to the inductance of the hole, h is the length of the hole, d is the diameter of the central hole,
The parasitic inductance through the hole is approximately: L=5.08h[ln(4h/d) 1]
It can be seen from the formula that the diameter of the hole has little influence on the inductance, while the length of the hole has the greatest influence on the inductance.
4. non-perforated guide hole technology, non-perforated guide holes include blind holes and buried holes.
In the non-perforate pilot hole technology, the application of blind holes and buried holes can greatly reduce the size and quality of the PCB, reduce the number of layers, improve electromagnetic compatibility, increase the characteristics of electronic products, reduce costs, and also make the design work easier and faster. In traditional PCB design and processing, through holes can bring many problems. First of all, they occupy a large amount of effective space, followed by a large number of through holes dense place also cause huge obstacles to the multi-layer PCB inner layer wiring, these through holes occupy the space required for wiring, they densely pass through the surface of the power and ground layer, but also destroy the impedance characteristics of the power ground layer, so that the power ground layer failure. And the conventional mechanical drilling method will be 20 times the workload of non-perforated pilot hole technology.
In the PCB design, although the size of the pad and through holes has been gradually reduced, if the thickness of the plate is not decreased proportionally, it will lead to an increase in the aspect ratio of the through hole, and an increase in the aspect ratio of the through hole will reduce reliability. With the maturity of advanced laser drilling technology and plasma dry corrosion technology, it is possible to apply non-penetrating small blind holes and small buried holes. If the hole diameter of these non-penetrating guide holes is 0.3mm, the parasitic parameters brought by them are about 1/10 of the original conventional holes, improving the reliability of the PCB.
Due to the use of non-through-hole technology, there will be few large through-holes on the PCB, which can provide more space for the wiring. The remaining space can be used for large area shielding purposes to improve EMI/RFI performance. At the same time, more remaining space can also be used in the inner layer to partially shield the device and key network cable, so that it has the best electrical performance. The use of non-perforated pilot holes makes it easier to fan out the device pins, making high-density pin devices (such as BGA packaged devices) easy to route, shorten the line length, and meet the high-speed circuit timing requirements.
5. ordinary PCB hole selection
In the ordinary PCB design, the parasitic capacitance and parasitic inductance of the hole have less impact on the PCB design, and the 0.36mm/0.61mm/1.02mm(drilling/pad /POWER isolation area) hole is generally selected for 1-4 layer PCB design. Some signal lines with special requirements (such as power cables, ground cables, clock lines, etc.) can use 0.41mm/0.81mm/1.32mm holes, and other sizes can also be selected according to the actual hole.
6. high-speed PCB hole design
Through the above analysis of the parasitic characteristics of the hole, we can see that in high-speed PCB design, seemingly simple holes often bring great negative effects to the design of the circuit. In order to reduce the adverse effects caused by the parasitic effect of the hole, the design can be as far as possible:
6.1. choose a reasonable hole size. For multi-layer general density PCB design, it is better to choose 0.25mm/0.51mm/0.91mm(drilling/pad/POWER isolation area) through the hole; For some high-density PCB can also use 0.20mm/0.46mm/0.86mm pass hole, you can also try non-pass hole; For the through-hole of the power supply or ground wire, a larger size can be considered to reduce the impedance;
6.2. the larger the POWER isolation area, the better, considering the through hole density on the PCB, generally D1=D2 0.41;
6.3. the signal wiring on the PCB should not be changed as far as possible, that is to say, the hole should be reduced as much as possible;
6.4. the use of a thinner PCB is conducive to reducing the two parasitic parameters of the hole;
6.5. the pin of the power supply and the ground should be made to the nearest hole, the shorter the lead between the hole and the pin, the better, because they will lead to an increase in inductance. At the same time, the power and ground leads should be as thick as possible to reduce impedance;
6.6. place some ground holes near the holes of the signal layer change in order to provide a short-distance loop for the signal.
Of course, specific problems need to be analyzed in the design. From the comprehensive consideration of cost and signal quality, in high-speed PCB design, the designer always hopes that the smaller the hole, the better, the more wiring space can be left on the template, in addition, the smaller the hole, its own parasitic capacitance is smaller, more suitable for high-speed circuits. In high-density PCB design, the use of non-through the pilot hole and the reduction of the size of the hole at the same time brings about an increase in cost, and the size of the hole can not be reduced without limitation, it is limited by PCB manufacturers drilling and electroplating and other process technology, in the high-speed PCB hole design should be given balanced consideration.
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