Please share your technical knowledge on via holes and back drills in PCB circuit boards.

1、 In high-speed PCB design, through-hole design often requires the use of multi-layer PCBs, and vias are an important factor in multi-layer PCB design. The vias in PCB are mainly composed of three parts: the holes, the pad area around the holes, and the POWER layer isolation area. 1. The influence of vias in high-speed PCBs. In high-speed PCB multilayer boards, signals transmitted from one layer of interconnects to another layer of interconnects need to be connected through vias. When the frequency is below 1GHz, vias can play a good connection role, and their parasitic capacitance and inductance can be ignored. When the frequency is higher than 1 GHz, the parasitic effect of vias on signal integrity cannot be ignored. At this time, vias exhibit impedance discontinuity breakpoints on the transmission path, which can cause signal integrity problems such as reflection, delay, and attenuation. When a signal is transmitted through a via to another layer, the reference layer of the signal line also serves as the return path for the via signal, and the return current flows between the reference layers through capacitive coupling, causing problems such as ground bounce. 2. Types of Through Holes: Through holes are generally divided into three categories: through holes, blind holes, and buried holes. Blind hole: refers to a hole located on the top and bottom surfaces of a printed circuit board, with a certain depth, used for connecting the surface circuit and the inner layer circuit below. The depth of the hole usually does not exceed a certain ratio to 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 circuit board. Through hole: This type of hole passes through the entire circuit board and can be used for internal interconnection or as a component installation positioning hole. Due to the easier implementation and lower cost of through-hole technology, it is generally used in printed circuit boards. 3. In high-speed PCB design, seemingly simple via designs often have significant negative effects on circuit design. In order to reduce the adverse effects of parasitic effects on via holes, efforts can be made in the design to: (1) choose a reasonable via hole size. For multi-layer general density PCB design, it is better to use 0.25mm/0.51mm/0.91mm (drilling/pad/POWER isolation area) vias; For some high-density PCBs, 0.20mm/0.46mm/0.86mm through holes can also be used, and non via holes can also be tried; For power or ground vias, larger sizes can be considered to reduce impedance; (2) The larger the POWER isolation area, the better. Considering the via density on the PCB, it is generally D1=D2+0.41; (3) The signal routing on the PCB should avoid changing layers as much as possible, which means minimizing the number of vias; (4) Using thinner PCBs is beneficial for reducing two parasitic parameters of vias; (5) The pins of the power and ground should be through the nearest via, and the shorter the lead between the via and the pin, the better, as they can cause an increase in inductance. At the same time, the power and ground leads should be as thick as possible to reduce impedance; (6) Place some grounding vias near 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 vias used for top and bottom layer conduction, the length of the via is equal to the thickness of the PCB. As the number of PCB layers increases, the thickness of the PCB often reaches 5 mm or more. However, in high-speed PCB design, to reduce the problems caused by via holes, the via length is generally controlled within 2.0mm. For vias with a length greater than 2.0 mm, increasing the via diameter can improve the continuity of via impedance to a certain extent. When the through-hole length is 1.0 mm or less, the optimal through-hole diameter is 0.20 mm to 0.30 mm. 2、 Back drilling process in PCB production 1. What PCB back drilling? Back drilling is actually a special type of deep hole drilling. In the production of multi-layer boards, such as 12 layer boards, we need to connect the first layer to the ninth layer. Usually, we drill through holes (once) and then sink copper. In this way, the first layer is directly connected to the 12th layer. In reality, we only need to connect the first layer to the 9th layer. The 10th to 12th layers are like pillars without any wiring connections. This pillar affects the signal pathway and can cause signal integrity issues in communication signals. So drill out this extra pillar (known as STUB in the industry) from the back (secondary drilling). So it's called a back drill, but it's usually not drilled so cleanly because the subsequent process will electrolyze a little copper, and the drill tip itself is also sharp. So PCB manufacturers will leave a small amount, and the length of the STUB left is called the B value, which is generally in the range of 50-150UM. What are the advantages of back drilling? 1) Reduce noise interference; 2) Improve signal integrity; 3) Local plate thickness decreases; 4) Reduce the use of buried blind holes and lower the difficulty of PCB production. What is the purpose of drilling a back hole? The function of a back drill is to drill out through hole segments that do not have any connection or transmission function, avoiding reflection, scattering, delay, etc. in high-speed signal transmission, and causing "distortion" to the signal. Research has shown that the main factors affecting the integrity of signal systems, in addition to design, board materials, transmission lines, connectors, chip packaging, etc., have a significant impact on signal integrity through holes. 4. The working principle of back drilling production relies on the micro current generated when the drill needle contacts the copper foil on the substrate surface during drilling to sense the height position of the surface. Then, the drilling is carried out according to the set drilling depth, and the drilling is stopped when the drilling depth is reached. 5. What is the production process of the back drill? a、 Provide PCB with positioning holes, use the positioning holes to locate and drill holes on the PCB; b、 Electroplating the PCB after drilling a hole, and performing dry film sealing treatment on the positioning holes before electroplating; c、 Create outer layer graphics on the electroplated PCB; d、 Perform pattern electroplating on the PCB after forming the outer layer pattern, and perform dry film sealing treatment on the positioning holes before pattern electroplating; e、 Use the positioning hole used by a drill to perform back drilling positioning, and use a drill knife to back drill the electroplating hole that needs to be back drilled; f、 After drilling the back hole, wash it with water to remove any remaining drill cuttings. What are the technical characteristics of the back drilling plate? 1) Most backboards are hard boards. 2) The number of layers is generally 8 to 50. 3) The board thickness is above 2.5mm? Backplanes are mainly used in communication equipment, large servers, medical electronics, military, aerospace and other fields. Due to the sensitive nature of military and aerospace industries, domestic backboards are usually provided by research institutes, R&D centers, or PCB manufacturers with strong military and aerospace backgrounds in military and aerospace systems; In China, the demand for backboards mainly comes from the communication industry, which is gradually growing in the field of communication equipment manufacturing.