Impact of pcb through-hole design on signal transmission
Basic concept of PCB board through hole
Via is one of the important components of a multi-layer PCB. The cost of drilling is usually 30% to 40% of the cost of PCB board. Simply put, each hole in the PCB can be called a through hole.
In terms of function, the through holes can be divided into two types: one is for electrical connection between the layers; the other is for fixing or positioning of the device. As seen from the process, these through holes are generally divided into three categories, namely, blind holes (BlindVia), buried holes (BuriedVia) and through holes (ThroughVia). The blind holes are located on the top and bottom surfaces of the PCB and have a depth for the connection of the surface lines to the underlying inner lines. The depth of the holes usually does not exceed a certain ratio (aperture). A buried via is a connection hole located in the inner layer of the PCB that does not extend to the surface of the PCB. The above two types of holes are located in the inner layer of the PCB, and are completed by a through hole forming process before lamination, and several inner layers may be overlapped during the formation of the through holes. The vias pass through the entire PCB and can be used to implement internal interconnects or as mounting holes for components. Since the Tongzi process is easier to implement and lower in cost, most PCBs use it, and less use of the other two types of through holes. The through holes described below are considered as through holes unless otherwise specified.
From the design point of view, a through hole is mainly composed of two parts, one is the center hole (DrillHole), and the other is the boring area around the hole, as shown in Figure 1-9-1. The size of these two parts determines the size of the through hole.
Figure 1-9-1 Structure of the through hole
Obviously, in high-speed, high-density PCB design, the designer always wants the smaller the via, the better, so that more wiring space can be left on the PCB. In addition, the smaller the via hole, the smaller the parasitic capacitance of itself, which is more suitable for high-speed circuits. However, the reduction in the size of the hole also brings about an increase in cost, and the size of the through hole cannot be reduced indefinitely. It is limited by the process techniques such as Drill and Plating: the smaller the hole, the drill The longer the hole takes, the easier it is to deviate from the center position; and when the depth of the hole exceeds 6 times the diameter of the hole, there is no guarantee that the hole wall can be uniformly plated with copper.
For example, if the thickness of a normal 6-layer PCB (through-hole depth) is 50 mils, then the PCB manufacturer can provide a minimum diameter of 8 mils under normal conditions. With the development of laser drilling technology, the size of the drill hole can also be smaller and smaller. Generally, the through hole with a diameter of not more than 6 mil is called a micro hole. Micropores are often used in HDI (High Density Interconnect Structure) designs, which allow the vias to be placed directly on the Via-in-Pad, which greatly improves circuit performance and saves wiring space.
The vias appear as discontinuous breakpoints on the transmission line, causing reflections in the signal. Generally, the equivalent impedance of the through hole is about 12% lower than that of the transmission line. For example, the impedance of the 50Ω transmission line is reduced by 6Ω when passing through the through hole (specifically, the size and thickness of the through hole are also related to the absolute reduction). However, the reflection of the through hole due to the discontinuity of the impedance is actually very small, and its reflection coefficient is only (50-44) / (44 + 50) ≈ 0.06. The problem caused by the via hole is more concentrated on the parasitic capacitance and the inductance. influences.
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