How Cost and Quality Standards Affect Design with Buried Vias
Buried via technology allows designers to provide more functionality in less space. Avoiding designs where blind and buried vias overlap in the layer buildup is strongly suggested.
Anyone who wants to increase the density of their design and reduce the form factor will be familiar with using buried vias. The vias open up a world of opportunity for designers by freeing up valuable real estate on the board that would be otherwise taken up a thru hole on the layer HDI PCB where it isn’t connected.
Although the technology has existed for years, many designers don't know how to work with buried vias. However, once you start using buried vias in your design, you may never want to go back. Buried vias are more expensive and require designs to plan ahead before using them.
Before selecting a PCB board, ensure that the manufacturer’s processes comply with industry standards, including IPC, UL, and ISO standards. It is not advised to use buried vias in PCBs if their design does not accommodate the technology.
Manufacturers often place limits on the size of the annual rings in buried vias, the number of thermal press steps and lamination used in a process that can affect the various design aspects.
Laser drilling vs Mechanical drilling
Laser drilling is recommended for placing buried vias in HDI PCBs. However, the number of thermal and lamination press cycles is often constrained because of strict quality control standards, especially those dictated by UL. In such a case, laser drilling between each PCB may not be recommended because of the limits on the number of thermal press cycles.
In this case, it is recommended to use mechanical drills and use plated through hole buried vias to reach the internet layers. It may be possible to access the outer layer with the help of buried and blind vias.
These limitations will require designers to become creative with their stackup and define drill pairs between layers. This way, they won't exceed the minimum number of lamination cycles and thermal press cycles.
The Cost of Buried and Blind Vias
Buried and blind vias are relatively more expensive compared to through hole vias. Buried vias cost more to manufacturers per via because the board requires extra machining steps. However, the tradeoff is that they facilitate denser routing on the layers and allow designs to lower the number of layers.
This may reduce the long-term cost of manufacturing the board but it will require designers to be fairly experienced and be creative with their routing.
The Parasitic Capacitance of Vias
Buried vias are known to have parasitic capacitance to the ground. Parasitic capacitance is measured by the following formula:
C =1.41ε TD1/(D2-D1),
Where D1 is the diameter of the via pad, D2 is the diameters of the isolation hole on the ground layer, T is the thickness of the PCB, and ε is the dielectric constant of the substrate.,
The parasitic capacitance of buried vias can impact circuit performance by t reducing speed and increasing the rise time of the signal. It is important to lower the capacitance value to limit its impact on the circuit.
The Parasitic Inductance of Buried Vias
Buried vias exhibit parasitic inductance that can affect the overall circuit design and is more disruptive than parasitic capacitance.
Parasitic inductance can weaken the entire power supply system of the circuit. It is measured by the following formula:
Where D is the diameter, and h is the length. The formula shows that the diameter of the buried via does not have a major influence on the inductance. However, the length of the buried via can have a major influence on the circuit.
Greatly Reducing the Size of PCB using Non-Thru Via Technology
It is possible to greatly reduce the size of the PCB and layer count with the help of non-thru buried vias. When used correctly, non-thru buried vias can lower costs, improve electromagnetic compatibility, and increase the efficiency of the design.
For starters, quality standards of traditional thru-holes leave a lot to be desired. For starters, they take up too much space. Secondly, most thru holes make it very difficult for designers to route the inner layers of multilayered circuits.
Thru holes also take up wiring space and can adversely affect the quality standards of the board by disrupting impedance characteristics. This can make the power supply ineffective. Moreover, mechanical drilling of thru holes is much more labor intensive than non-thru via technology.
Although the size of buried vias has been greatly reduced in PCBs, the aspect ratio of the via will increase if the board’s thickness is reduced proportionally.
The advancement of laser drilling technology has made it possible to make smaller buried holes. In case the diameter of these non-through buried vias is less than 0.3 mm, their parasitic parameters will be one-tenth that of the conventional holes, which can greatly improve the quality standards of the PCB.
Thanks to non-through via technology, designers now have more spacing for PCB routing. The additional real estate can be used for shielding and improving the board’s overall EMI and RFI performance. The extra spacing can be used to shield the device and improve electrical performance.
Using non through vias make it easy to fan out component pins, route high density devices like BGA packages, reduce the length of wires, and create high speed circuits by meeting their requirements.
The only disadvantage of using non-through vias is that they are more costly. The cost and quality standards of the PCB will restrict drilling and electroplating technology. Moreover, it takes longer to drill smaller holes, and there is a high risk of deviating from the central position.
As you can see, the use of buried and micro via PCB in boards can greatly affect design. We recommend using CAD tools to automate the routing process and visualize the layout in both 2D and 3D.
For more information, please get in touch with PCB experts at Hemeixin PCB Manufacturer.