“Friends who do hardware know that the design of PCB vias is actually very particular. Today, I will share with you the technical knowledge of vias and back drilling in PCBs.
Friends who do hardware know that the design of PCB vias is actually very particular. Today, I will share with you the technical knowledge of vias and back drilling in PCBs.
1. Via design in high-speed PCB
In high-speed PCB design, multi-layer PCBs are often required, and vias are an important factor in multi-layer PCB design.
The vias in the PCB are mainly composed of three parts: the hole, the pad area around the hole, and the POWER layer isolation area.
1. The impact of vias in high-speed PCBs
In high-speed PCB multi-layer boards, the signal transmission from one layer of interconnect lines to another layer of interconnect lines needs to be connected through vias. When the frequency is lower than 1GHz, vias can play a good connection role. Its parasitic capacitance and inductance can be ignored.
When the frequency is higher than 1 GHz, the influence of the parasitic effect of the via on the signal integrity cannot be ignored. At this time, the via appears as a discontinuous impedance breakpoint on the transmission path, which will cause signal reflection, delay and attenuation. and other signal integrity issues.
When the signal is transmitted to another layer through the via, the reference layer of the signal line also serves as the return path of the via signal, and the return current will flow between the reference layers through capacitive coupling, causing problems such as ground bounce.
2. Types of vias
Vias are generally divided into three categories: through holes, blind holes and buried holes.
Blind hole: refers to the surface of the top and bottom layers of the printed circuit board, with a certain depth, used for the connection of the surface circuit and the underlying inner circuit, the depth of the hole and the diameter of the hole usually do not exceed a certain ratio.
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 circuit board.
Through Hole: This type of hole runs through the entire circuit board and can be used for internal interconnection or as a mounting hole for components. Because the through hole is easier to realize in the process and the cost is lower, it is generally used in the printed circuit board.
3. Via Design in High Speed PCB
In high-speed PCB design, seemingly simple vias often bring great negative effects to circuit design. In order to reduce the adverse effects caused by the parasitic effects of vias, you can try to do as much as possible in the design:
(1) Select a reasonable via size. For multi-layer PCB design with general density, it is better to use 0.25mm/0.51mm/0.91mm (drilling/pad/POWER isolation area) vias; for some high-density PCBs, 0.20mm/0.46 mm/0.86mm vias, you can also try non-through vias; for power or ground vias, you can consider using a larger size to reduce impedance; (2) The larger the POWER isolation area, the better, consider the PCB The density of the via holes is generally D1=D2+0.41; (3) The signal traces on the PCB should not be changed as much as possible, that is to say, the via holes should be minimized; (4) The use of a thinner PCB is conducive to reducing the size of the via holes. Two kinds of parasitic parameters; (5) The pins of power and ground should be close to vias, and the leads between vias and pins should be as short as possible, because they will lead to an increase in inductance. At the same time, the leads of the power supply and the ground should be as thick as possible to reduce the impedance; (6) Some ground vias should be placed near the vias of the signal switching layer to provide a short-distance loop for the signal.
In addition, via length is also one of the main factors affecting via inductance. For the vias used for top and bottom conduction, the length of the vias is equal to the thickness of the PCB. Due to the continuous increase of the number of PCB layers, the thickness of the PCB often reaches more than 5 mm. However, when designing high-speed PCBs, in order to reduce the problems caused by vias, the length of vias is generally controlled within 2.0mm. For vias with a via length greater than 2.0 mm, the continuity of via impedance can be improved to a certain extent by increasing the via diameter. When the via length is 1.0 mm and below, the optimal via diameter is 0.20 mm – 0.30 mm.
2. Back-drilling process in PCB production
1. What PCB back drill?
Back drilling is actually a special kind of deep hole drilling. In the production of multi-layer boards, such as the production of 12-layer boards, we need to connect the 1st layer to the 9th layer. Usually, we drill through holes (one drill). , and then copper sinking. In this way, the first layer is directly connected to the 12th layer. In fact, we only need the first layer to be connected to the 9th layer, and the 10th to 12th layers are connected like a pillar because there are no lines.
This pillar affects the path of the signal and can cause signal integrity problems in the communication signal. So drill this extra column (called STUB in the industry) from the reverse side (secondary drilling). So it is called a back drill, but it is generally not so clean, because the subsequent process will electrolyze a little copper, and the drill tip itself is sharp. Therefore, the PCB manufacturer will leave a small point. The length of the STUB left is called the B value, which is generally in the range of 50-150UM.
2. What are the advantages of back drilling?
1) Reduce noise interference;
2) Improve signal integrity;
3) The local plate thickness becomes smaller;
4) Reduce the use of buried blind vias and reduce the difficulty of PCB fabrication.
3. What is the function of back drilling?
The function of back-drilling is to drill out the through-hole section that does not play any connection or transmission function, so as to avoid reflection, scattering, delay, etc. of high-speed signal transmission, which will bring “distortion” to the signal. In addition to the design, board material, transmission line, connector, chip packaging and other factors, the main factor is that the via hole has a great influence on the signal integrity.
4. Working principle of back drilling production
When the drill pin is used to drill down, the micro-current generated when the drill pin touches the copper foil on the substrate surface senses the height position of the board surface, and then drills down according to the set drill-down depth, and stops when the drill-down depth is reached. Figure 2:
5. What is the production process of back drilling?
Provide a PCB with positioning holes on the PCB, and use the positioning holes to drill and position the PCB and perform a drilling; electroplating the PCB after the drilling, and dry film sealing the positioning holes before electroplating Hole processing; making outer layer graphics on the plated PCB;
Perform pattern electroplating on the PCB after the outer layer pattern is formed, and perform dry film sealing on the positioning holes before pattern plating; use the positioning holes used for a drill to perform back-drilling positioning, and use a drill to back-drill the required holes. The plated holes are backdrilled; after backdrilling, the backdrilling holes are washed with water to remove the remaining drilling chips in the backdrilling holes.
6. What are the technical characteristics of the back drilling plate?
1) Most backplanes are hard boards
2) The number of layers is generally 8 to 50 layers
3) Plate thickness: more than 2.5mm
4) Thickness and diameter are relatively large
5) The board size is larger
6) Generally, the minimum hole diameter of the first drill is >=0.3mm
7) There are fewer outer layers, and most of them are designed for a square array of crimping holes.
8) Back drilling is usually 0.2MM larger than the hole that needs to be drilled
9) Back drilling depth tolerance: +/-0.05MM
10) If the back drilling requires drilling to the M layer, the minimum dielectric thickness from the M layer to the M-1 (the next layer of the M layer) layer is 0.17MM7. What field is the back drilling plate mainly used for?
Backplanes are mainly used in communication equipment, large servers, medical electronics, military, aerospace and other fields. Since the military and aerospace are sensitive industries, domestic backplanes are usually provided by military and aerospace systems research institutes, R&D centers, or PCB manufacturers with strong military and aerospace backgrounds; in China, the demand for backplanes mainly comes from the communications industry. Develop and expand the field of communication equipment manufacturing.