PCB Technology
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Figure 2.2. Cross section of a circuit board showing a copper plane on the top and bottom layers, with a signal layer between them. Source is Bogatin Enterprises.
But there is more to a circuit board than just the copper layers and the dielectrics between them.
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With signals, power, and ground on multiple layers, electrical connections between layers have to be made with vias. There are three types of commonly used vias in a board, defined by how they are manufactured and what they connect. These three types of vias are illustrated in Figure 2.3.
Figure 2.3. Examples of the three types of vias in a circuit board: (1) blind via, (2) buried via, and (3) through-hole via. Courtesy of Altium.
The most common type of via is a through-hole or plated through- hole (PTH) via. If you do not want metal on a specific layer to connect to the PTH via, add a clearance hole on that layer where the drill passes through the layer. This is done automatically in most routing tools based on net names. The clearance hole in the plane layer is sometimes referred to as an antipad.
To connect two inner layers, such as two ground planes, with a through-hole via, you would make sure there is no trace on the top or bottom layers where the drill passes through the board. The resulting through-hole via would only connect the inner layers.
The residual piece of plated through-hole barrel that sticks up or down from the last connected layer is called a stub. In very high frequency boards, with signals operating above 10 Gbps, the via stubs can be a serious signal integrity problem. However, for all boards operating at 5 Gbps and below, the via stubs are rarely a performance problem and can be ignored.
An example of a 4-layer board constructed with through-hole vias is illustrated in Figure 2.4.
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Figure 2.4 Example of a 4-layer board with through holes. Courtesy of Altium.
In this example, all the PTH vias drilled in the board go all the way through the board.
All the features and dimensions of the PTH are considered part of the via pad stack design. Unless you have a strong compelling reason otherwise, the features should be:
✓ Drill diameter 13 mils (this makes the finished, plated hole diameter 10.2 mils)
✓ Capture pad diameter around the via on any layer with a connection, 25 mils (6 mil copper annulus around the 13 mil drill diameter)
✓ Clearance annulus on a copper plane with no connection of 6 mils.
Part of the process of plating the via holes requires there be capture pads on the top and bottom layers of the via hole. The capture pad is to provide a larger target for the drill to hit to make up for
registration tolerances. On layers on which there are no connections to be made by the via, there is no need for a capture pad. It is a good habit to remove these non-functional capture pads.
The annulus of the capture cannot be narrower than 6 mils for the lowest cost. This makes the outer diameter of the capture pad 13 mils + 6 mils + 6 mils = 25 mils.
With 6 mils as the narrowest clearance between copper features, the closest spacing between vias is 31 mils. A robust via-to-via pitch to
use is 35 mils. This leaves 10 mils as the spacing between the edges of the via pads.
A buried via typically connects only between two adjacent layers. It is drilled and plated at the time when the 2-layer core is patterned.
This extra drilling and plating step will add cost to the board.
The advantage of the buried via is that it does not block routing channels above and below it. This can be of value in very dense boards and may be worth paying the extra price. Most low-cost fab shops do not offer buried vias. For example, this type of via is NOT an option at JLCpcb.com.
A blind via is drilled from the top of the board to a controlled depth at the same time the through-hole vias are drilled. This makes a small, blind hole, from the top or bottom of the board, stopping on a lower layer.
After drilling, the inside of the hole has to be plated. This is difficult in a blind hole if the aspect ratio is larger than 1 to 1. It requires special chemistry to remove the air bubble in the hole and to get plating fluid to fill the hole and make contact to the exposed layers.
The value of a blind via is that a via can be placed inside a pad to make a low inductance contact to the layer below and it will not block routing channels on other layers.
If you were to add a through-hole via in a pad on which a component is soldered, the quality of the solder joint can be degraded. If there is a plated through-hole via inside a component pad, when the solder paste melts and reflows, the solder may wick down the plated hole by capillary action. This will steal or thieve some of the solder from the pad. This means there is the chance there may not be enough solder left on the pad to make a good solder joint to the component.
This is why a through-hole via should never be placed in a pad if automated assembly will be used.
Blind vias are often used for via-in-pad applications for decoupling capacitors where low loop inductance connections are of high value.
A blind via will not wick solder from a pad and will not cause an
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assembly problem. This is why blind vias are ok to use as via in pad connections.
Via in pad is not an option for many low-cost fab shops since it requires specialized plating chemistry and drilling.
Through-hole vias are the lowest cost and most common type of via. Unless you have a strong compelling reason otherwise, ALWAYS use through-hole vias and never any
other type of via.
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