The importance of through holes cannot be ignored by designers of multi-layered PCBs. This article describes the different types of through holes and how they help to properly design PCBs.
It is a well-known fact that a PCB is the heart of an electronic system. Hardware engineers have many expectations from PCB designers regarding signal integrity, electrical integrity, electromagnetic compatibility (EMC) as well as thermal analysis. Depending on the type of circuit, all these parameters must be managed in a PCB design in order for an assembly to be successful for its intended use.
In this digital world the signal speed ranges from a few MHz to a few GHz. With the increased need for functions per assembly / chip and miniaturization of electronic systems, PCBs become more and more complex. The growing need for multiple functions on a chip results in complex surface mounted device (SMD) components, with many pins placed close together or even leadless packages. Multi-pin components such as ball grid arrays (BGAs) and thin quad flat packages (TQFPs) are forcing PCB designers to go for multilayer PCBs.
Through holes play a major role while designing multilayer PCBs. A via hole is a plated through hole (PTH), which is used to transfer a signal from one layer of a PCB to another layer, without the dimensions of mounting any component in these holes.
Most engineers use through holes just to transfer signals. But there are other parts also connected to it through holes. These through holes can be used in different ways. A through hole can have a positive effect when it comes to the route of a signal (interconnection) and a negative effect when it comes to the route of a sensitive signal.
Different types of through holes
Through the hole through, where one can see through a PCB. Such a through hole starts from the top layer of a PCB and ends at the bottom layer. This should be defined in each and every electrical layer of a PCB.
Blind through, whose starting point is visible, but the end point is invisible. Such through holes are used in multilayer PCBs. The blind via starts from the outer layer and fades to the inner layer.
Buried by, which is invisible to all. It is used to transfer a signal from one inner layer to another inner layer. The buried via is used to save space for signal routing on the outer or inner layers.
Micro via, with a relatively small drill diameter. Due to the reduction of electronic assemblies and routing of high-density-interconnection PCBs, by size (diameter) is decreasing. As per IPC, any via hole with a drill diameter less than or equal to 0.15mm is a micro via. Such through holes are drilled by laser.
Through the pad is usually used where it is difficult to place a via hole between pads/land areas due to the limitation of available space. In this case, a through hole is drilled into an area of land, and we can say that the area of land has become a pad. Such a diameter of the via-hole should be small (according to the manufacturer’s limits) because there is a possibility of stealing the solder during reflow soldering. Components such as bill grid array (BGA) require via pads. (The ground area is a copper area without a hole, where an SMD component is soldered. The pad is a copper area with a hole in the middle.)
By sewing also used to connect large areas of copper in different layers. This helps to reduce the return path of signals and thus maintain low impedance. A through shield is made by placing one or more rows of holes in the path of the signal path, which helps reduce electromagnetic interference (EMI) and crosstalk.
Thermal through used to improve the heat dissipation of the surface mount components rather than transferring a signal. In the structure, a penetrating hole is opened in the board. Thermal resistance depends on the area and thickness of the copper foil on the PCB that serves to dissipate heat, as well as the thickness and material of the board.
To effectively use thermal vias, it is important that the thermal vias are located near the heating elements, such as directly below the components. As shown in Fig. 4, the effect of thermal equilibrium is used, and therefore it should be seen that connecting areas with large temperature differences is a good plan.
Through holes and signal integrity
In the case of low frequency, vias do not affect the signal transmission. As the frequency increases (above 1GHz) and the signal gain can be steep (1ns at most), however, the vias cannot be considered as a function of the electrical connection, and the influence of the vias on the signal integrity must also be considered. consider. Vias behave as breakpoints with constant impedance in the transmission line, causing signal reflections.
However, the issues brought about by vias mainly concentrate on parasitic capacitance and parasitic inductance. The influence of the parasitic capacitance in a circuit is mainly to extend the rise time of the signals and reduce the running speed of the circuit. Parasitic inductance, however, weakens the contribution of the bypass circuit and reduces the filtering function of the entire power system.
The bigger the via diameter is, the more the stop through the reason for the high-frequency and high-speed PCB design. This is a temporary information, because it also depends on the position of the electric planes, by the diameter of the hole, the thickness of the plating, etc. It should be noted that a via will add 1-3nH to the inductance of the signal. Further study is required when a designer can face such a situation. This is known as the Black Magic of a PCB when compared to circuit simulation results.
Current carrying capacity of a via
Generally, I never recommend moving a power rail between layers when designing a power PCB. However, one can use component-mounting pads for the same. If necessary, then use more holes.
According to the researchers, the calculation of the current carrying capacity of a via hole is a function of many factors such as, length of the via, its drill diameter, placement of power planes, amount of power that transfer between layers, glass transition temperature (Tg) of the PCB base material, copper plating thickness, etc. As a rule of thumb the finished diameter of a through hole should be at least as big in track width divided by three. For a digital circuit, using via holes for VCC or Ground is obvious because the power is very low.
Planning through the holes in diameter for your design is a big concern regarding the capabilities of a manufacturer. You can plan different through the hole diameter for a PCB according to your requirement.
The typical drill diameter of a via hole is 0.5mm (20mils), which can be drilled by most PCB manufacturers. If you are planning anything less than this, you should check the drilling and plating capabilities of the manufacturer.
With the reduction of the drill diameter, the spindle speed of a CNC PCB drilling machine must be high. That is why the micro vias are laser drilled rather than mechanical drilling.
Shavinder Singla (CID-IPC, USA), Senior Technical Officer, Center for Development of Advanced Computing, Mohali (Punjab)