Through hole design of HDI PCB
In high speed PCB design, multi-layer PCB is often used, and through hole is an important factor in multi-layer PCB design. The through hole in PCB is mainly composed of three parts: hole, welding pad area around hole and POWER layer isolation area. Next, we will understand the high speed PCB through the hole problem and design requirements.
Influence of through hole in HDI PCB
In HDI PCB multilayer board, the interconnect between one layer and another layer needs to be connected through holes. When the frequency is less than 1 GHz, the holes can play a good role in connection, and the parasitic capacitance and inductance can be ignored. When the frequency is higher than 1 GHz, the effect of parasitic effect of the over-hole on the signal integrity cannot be ignored. At this point, the over-hole presents a discontinuous impedance breakpoint on the transmission path, which will lead to signal reflection, delay, attenuation and other signal integrity problems.
When the signal is transmitted to another layer through the hole, the reference layer of the signal line also serves as the return path of the signal through the hole, and the return current will flow between the reference layers through capacitive coupling, causing ground bombs and other problems.
Type of Though-Hole, Generally, through hole is divided into three categories: through hole, blind hole and buried hole.
Blind hole: a hole located at the top and bottom surface of a printed circuit board, having a certain depth for connection between the surface line and the underlying inner line. The depth of the hole usually does not exceed a certain ratio of the aperture.
Buried hole: a connection hole in the inner layer of the printed circuit board that does not extend to the surface of the circuit board.
Through hole: this hole passes through the entire circuit board and can be used for internal interconnection or as a mounting locating hole for components. Because the through hole in the process is easier to achieve, the cost is lower, so generally printed circuit board are used
Through hole design in high speed PCB
In high speed PCB design, the seemingly simple VIA hole will often bring great negative effects to the circuit design.In order to reduce the adverse effects caused by the parasitic effect of perforation, we can try our best to:
(1) select a reasonable hole size.For PCB design with multi-layer general density, it is better to choose 0.25mm/0.51mm/0.91mm (drill hole/welding pad/POWER isolation area) through hole.For some high-density PCB can also use 0.20mm/0.46mm/0.86mm through hole, can also try non-through hole;For the power supply or ground wire hole can be considered to use a larger size to reduce the impedance;
(2) the larger the POWER isolation area, the better. Considering the through-hole density on the PCB, it is generally D1=D2+0.41;
(3) try not to change the layer of the signal on the PCB, that is to say, try to reduce the hole;
(4) the use of thin PCB is conducive to reducing the two parasitic parameters through the hole;
(5) the pin of the power supply and the ground should be close to the hole. The shorter the lead between the hole and the pin, the better, because they will lead to the increase of inductance.At the same time, the power supply and ground lead should be as thick as possible to reduce the impedance;
(6) place some grounding passes near the pass holes of the signal exchange layer to provide a short-distance loop for the signal.
In addition, through hole length is also one of the main factors affecting through hole inductance.For top and bottom pass hole, pass hole length is equal to PCB thickness. Due to the increasing number of PCB layers, PCB thickness often reaches more than 5 mm.
However, in high-speed PCB design, in order to reduce the problem caused by the hole, the hole length is generally controlled within 2.0mm.For the hole length greater than 2.0mm, the continuity of the hole impedance can be improved to some extent by increasing the hole diameter.When the through-hole length is 1.0mm and below, the optimal through-hole aperture is 0.20mm ~ 0.30mm.