How to Design High-Density PCBs With Buried and Blind Vias
It is common for designers to use blind and buried vias to connect the layers of high-density PCBs. Vias are vertical conductive holes that establish connections between the layers of PCBs.
These barrel-shaped vertical structures serve useful but distinct purposes. Buried vias form connections between two or more inner layers but are not compatible with outer layers - hence the name. Blind vias create connections between the inner layers and the outer layers, but they don’t go through the entire HDI PCB.
Together, these vias connect the various layers of PCB boards with each other. They are especially useful when there is limited space on the board.
Signal Integrity
Most components on high density PCBs have specific signal integrity requirements that may be affected by placing a thru hole via barrel. These vias emit EMI and disrupt the signal return path of critical nets. Buried and blind vias can be used to eliminate signal integrity problems that may be caused by standard through hole vias.
Routing Channels
Buried and blind vias do not go all the way through the PCB stackup, which opens up additional routing channels into the design. Moreover, the strategic placement of these vias allows designers to route parts with hundreds of pins. In normal circumstances, components with this many pins that only use thru hole vias would block all the inner layer routing channels.
Intersecting Buried and Blind Vias
Buried and blind vias are said to intersect under the following conditions:
- If two or more vias pass through a minimum of 2 layers in the PCB
- Both vias are contacting the same layer
So, for instance, a PCB with four layers with buried and blind vias can intersect at layers 2, 3, and 4. Intersecting vias are used to provide a shorter ground path from load devices to the power source. This maintains a healthy ground return path and creates a low resistance in the ground plane.
Soldering Thru-hole Components into Blind Vias
In some cases, it may be desirable to solder thru-hole parts into blind vias. This design strategy is particularly useful in high density PCBs where real estate may be hard to come by. It is important, however, to be careful when soldering thru-hole components into blind vias so as to not affect the efficiency of the PCB design.
Common Design Parameters for Buried Vias
Drill file
The holes used for each connection must be treated as a separate drill file. The ratio of the drill diameters to the hole depth should be 1:1: This ratio may be adjusted depending on the board's requirements.
Distance
The distance of the circuit board should be considered when placing buried vias. A great way of doing this is to use the value of the smallest via to calculate the depth. This would make it easier to determine the distance between the outer layers and inner layers on the high-density PCB.
Common Design Parameters for Blind Vias
Hole Connection
An important parameter for blind vias is the size of the hole. Holes for each connection level on high-density boards should be treated as a distinct drill file. The aspect ratio of the drill diameter to the hole depth should be 1:12. Designers may adjust the aspect ratio depending on the requirements for the PCB.
Depth
The depth of the PCB may only be determined by the smallest hole. This will influence the depth of the high-density PCB.
Pro tip: Use annular rings to improve the electrical connection's stability.
Layout Rules for Buried and Blind Vias
It is easy to configure buried and blind vias within the constraint manager in any workspace. Via placement is subject to standard clearance rules.
It is bad practice to place vias directly between the pads of a surface mount component because the solder flux may get trapped and lead to corrosion problems. Moreover, the presence of solder flux under the SMD can make it difficult to inspect and diagnose problems between the pad and via once the PCB is assembled.
Vias are Used to Save Spaces on a Crowded PCB
As mentioned earlier, a key reason for using buried and blind vias is to connect layers of circuits where space is at a premium. Experienced designers can reduce a board with 10 layers to 8 layers with the help of via placement.
Moreover, the vias are used to free up space from the circuit to mount components such as SMTs and other high pin count boards. Blind vias, in particular, can save up to 50% of the space on the board compared to thru-hole vias.
They only require you to drill one side of the circuit allow placement of SMT components on the blind via. This gives you more space for placing SMT components at a later stage of board assembly.
Reduce EMI on High Density Boards with Vias
Both buried and blind vias can be used to reduce EMI on the circuit. The problem with thru-hole vias is that their stubs act as transmission lines that reflect all signals through the structure into the PCB.
The advantage of using buried and blind vias is that they don’t have stubs that could cause a high degree of reflection. This isn't to say that buried and blind vias don’t cause EMI issues. Designers are recommended to follow quality standards for creepage and electrical clearance when placing buried and blind vias.
Drill Holes for Buried and Blind Vias
The typical diameter for drill holes ranges from 6 mils to 24 mils. This can be achieved with the help of advanced mechanical drilling techniques. However, laser drilling techniques can allow designers to create vias as small as 3 mils in diameters.
Wrapping Up
Buried and blind vias play an important role in high density circuits and provide much-needed real estate for component placement. Make sure to talk to your manufacturer before placing new vias on your HDI PCB. The manufacturer will provide you with instructions related to via placement. Doing so will save you time, money and frustration.
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