Overview Of PCB Overhole Technology
- Jan 27, 2018-
Via is one of the important parts of multi-layer PCB, and the cost of drilling is usually 30% to 40% of PCB board cost.Simply put, each hole on the PCB can be called a hole.
From the point of view, the hole can be divided into two types:
One is to use as an electrical connection between layers;The second is used as a fixed or positioning device.In terms of process, these holes are generally divided into three categories: blind via, buried via, and through via.Blind hole located at the top of the printed circuit board and the underlying surface, has the certain depth, used for surface line and below the inner line connection, the depth of the hole is usually no more than a certain ratio (aperture).Buried hole is the connection hole in the inner layer of printed circuit board. It does not extend to the surface of the circuit board.The above two types of holes are located in the inner layer of the circuit board, which is completed by the pass hole forming process before laminating, and may overlap several inner layers in the process of overhole forming.The third is called a through-hole, which passes through the circuit board and can be used to achieve internal interconnection or to install a positioning hole as a component.Because the hole is easier to implement in the process, the cost is lower, so most printed circuit boards use it instead of two other holes.There is no special explanation for the holes mentioned below, all of which are considered as through-holes.From the point of view of design, a hole is mainly composed of two parts, one is the drill hole in the middle, and the other is the welding disk area around the hole, as shown in the figure below.The size of these two parts determines the size of the hole.Obviously, at a high speed, high density PCB design, designers always want a hole as small as possible, and this model can leave more space for wiring, in addition, a hole is smaller, also its parasitic capacitance of the smaller, more suitable for high speed circuit.But the hole size decrease with the increase of the cost, and a hole the size of the impossible unlimited decreases, and it is drilling (drill) and the limitation of the plating (plating), and other technology: the smaller the hole, the longer drilling cost, also the easier it is off center location;And when the hole depth is more than 6 times the diameter of the borehole, there is no guarantee that the wall can be evenly coated with copper.For example, the thickness of a normal 6-layer PCB plate is about 50Mil, so the diameter of the borehole provided by PCB manufacturer can only reach 8Mil.
2.Via the parasitic capacitance of the via itself there is a parasitic capacitance to the ground, if known to the isolation of hole diameter on the hole in the shop, the formation of D2, the diameter of the hole welding plate for D1, the thickness of the PCB for T, board base material dielectric constant is epsilon, a hole the size of the parasitic capacitance approximation in: C = 1.41 epsilon TD1 / (- D1, D2) via the parasitic capacitance will give the main impact of the circuit is to extend the signal rise time, reduced the speed of the circuit., for example, for a piece of PCB thickness of 50 mil, if you use the inner diameter of 10 mil, bonding pad for 20 mil hole diameter, welding plate and the distance of 32 mil laying copper area, then we can through the hole on the approximate formula to calculate parasitic capacitance is roughly: C = 1.41 x4.4 x0.050 x0.020 / (0.032 0.020) = 0.032 pF, the rise time variation caused by partial capacitance as: T10-90 = 2.2 C (Z0/2) = 2.2 x0.517 x (55/2) = 31.28 ps.Can be seen from these values, even though a single via rising caused by the parasitic capacitance of slow effect is not obvious, but if it used many times in the hole to switch between layers, designers still want to reconsider.
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