PCB via design rules

In modern electronic design, PCB (Printed Circuit Board) design is a crucial part of the entire product development process. The via design, in particular, directly impacts key performance indicators such as signal integrity, mechanical strength, and manufacturability. An advanced PCB may contain thousands of traces, pads, and vias to conduct signals and deliver power between device pins. The PCB designer’s responsibility is to organize and design these components, connecting them correctly and avoiding contact with other signal or power networks. This requires routing traces in the PCB design and using vias to bridge the gaps between layers.

While using vias may seem like a simple operation, in actual design, many different types of vias may be used. Each type of via may have different usage rules, and if you are a PCB designer, your responsibility is to keep vias organized and apply the correct rules to each type of via in your CAD system. The following will elaborate on several aspects to help users fully understand and master the core points and rules of PCB via design .

What are PCB vias?

A via on a PCB is a hole that allows electrical and power signals to pass between different layers. In complex multilayer PCB designs, vias typically pass through multiple layers to form connections between them, while in single-layer PCB designs, vias are used to better facilitate circuit routing.

PCB via types

Through hole	Through holes are the most common and easiest to manufacture. However, they do have minimum size limitations and take up a lot of space in high-density designs.
Blind holes and buried holes	Blind vias and buried vias either start from the outer layer and only partially traverse the board stack (blind vias) or only traverse between inner layers (buried vias). These vias require more fabrication steps compared to standard vias. However, due to signal integrity requirements, and the ability to create additional routing paths above and/or below them, these vias are shorter and more desirable in designs.
Microvia	Microvias, also known as laser-drilled vias, span only two layers; they are an ideal wiring method for dense, small-pitch, high-pin-count components.

PCB vias

Achieve electrical signal connection	By using vias to pass through multiple layers, different electrical signals can be interconnected to form a complete circuit. This is especially important for transmitting high-frequency and high-speed signals.
Determine the location and positioning of the PCB	Vias allow PCBs to be secured to equipment, enabling precise positioning of the PCB within the device. During the packaging process, vias also facilitate the precise positioning of ICs and other components. This also helps accelerate the production process and improve production efficiency.
Heat dissipation function	High-power devices require vias for heat dissipation. Through vias, heat can be transferred to a heatsink mounted on the bottom of the PCB, thereby reducing component temperature and increasing device reliability and performance.

PCB via rules

Via size setting	Via dimensions should be determined based on the board’s design thickness, number of layers, and the electrical signals to be connected. Generally, the minimum manufacturing requirement is set at 8 mil via diameter and 0.15 mm via width. Via design should ensure sufficient space and the number of in-layer vias to support all necessary connections.
Via position setting	Vias should be kept away from other critical components, especially sensitive sensors. The vias should be placed in the center of the pads, in a location easily accessible in the design, for optimal results.
Via Dimension Confirmation	In layout design, vias should match the preset dimensions. This can reduce manufacturing costs and improve production efficiency. For other external dimensions, attention should be paid to the appropriate aspect ratio of the vias, and the spacing between vias should be carefully considered during actual cutting to ensure production line yield.

Application Locations and Methods of Traditional Through-Hole Designs in PCBs

In current PCB manufacturing standards, standard plated through-holes have become a fundamental element of PCB design. Through-holes can be drilled into circuit boards of any required size, allowing traces or copper traces to connect to vias on any layer of the circuit board. Through-holes have a variety of uses, including:

Conventional cabling	Through-holes are a standard method for transitioning signal traces from one circuit board layer to another.
Escape wiring	Surface mount components typically require pins to be immediately connected to vias for routing on inner layers of multilayer circuit boards. These escape or fan-out vias are usually routed before other traces are laid. The placement pattern of escape vias should be defined to leave as many routing paths as possible in the inner layers of the circuit board below them.
Power cabling	Vias are also used to connect devices to power/ground planes or traces. Most power vias are larger than regular wiring vias to accommodate the larger currents that can pass through them.

Special uses of PCB through-holes in circuit board design

Through holes are also used for special purposes or special situations on circuit boards, including the following:

Heat dissipation holes	For components operating at high temperatures, vias are typically embedded in a large central ground pad to facilitate effective heat dissipation through the board’s ground plane. These vias are crucial to prevent heat buildup from causing malfunctions in other components on the board.
Suture hole	These vias provide multiple connections for high-current circuits conducted by multiple power traces on different layers of the circuit board. Seam vias also help dissipate heat from these circuits and reduce the inductance of the connections.
Shielding hole	Placing multiple grounding vias around sensitive circuits creates a Faraday cage effect. This wiring method helps suppress the impact of potential EMI problems on the circuit.
Grounding transmission hole	In high-speed designs, ground transmission vias are used to preserve signal return paths between multiple ground plane layers when signal routing transitions between layers.
Grounding transmission hole	In high-speed designs, ground transmission vias are used to preserve signal return paths between multiple ground plane layers when signal routing transitions between layers.
Hole in the plate	Vias can be placed within the pads of dense surface-mount components such as ball grid array (BGA) packages. This helps resolve spacing issues, but creating in-pad vias can be challenging for manufacturers using mechanical drills in standard through-holes. Microvias offer a better solution.

PCB through-hole design rules and precautions during production.

Through-holes require a pad on each layer of the circuit board. The pad must be large enough for drilling, yet small enough to avoid taking up too much space. Standard anti-pads must also be provided on the planar layers of the circuit board, with a size appropriate for the drill diameter. The metal ring surrounding the through-hole on the outer layer of the circuit board is called the annulus, and the annulus is one of the main factors determining the reliability class of the circuit board. Class 1 circuit boards allow the annulus to be damaged by drilling, while Class 2 requires the drill to be tangent to the annulus. Class 3 circuit boards have the highest reliability class, and the thickness of the annulus at any point around the drill must not be less than 5 mils.

Another key design rule for through-holes is to select the via size based on the aspect ratio of the drilled hole to the board thickness. Mechanical drills have a limited drilling depth, and the aspect ratio determines the limit of that depth. Typically, board manufacturers want an aspect ratio of no more than 10:1, meaning that on a 62-mil thick board, the smallest reliably drillable via size is 6 mils.

If through-holes are used to connect high-speed signals on the first two layers of a 12-layer circuit board, most of the unused vias can act as antennas, causing signal integrity issues. To address this, many manufacturers back-drill the unused via portions. Back-drilling requires specific instructions and data from designers specifying which vias should be removed. However, back-drilling increases manufacturing costs, and a better option is usually to use microvias, blind vias, or buried vias to improve circuit board performance.

Finally, if necessary, through-holes can be covered or filled, but as with back drilling, this operation also requires the PCB designer to provide specific instructions to the manufacturer:

Through-hole cover oil	Covering vias with solder mask does not actually fill the through-hole, but care must be taken to avoid air bubbles forming in the housing during soldering heating, which could lead to gas leakage. Typically, a small hole is left in the center of the via cap solder mask to allow hot air to escape. Via cap solder mask helps to bring the via closer to the surface mount pad pattern without allowing solder to flow through the hole.
Conductive paste plugging	Through-holes are filled using epoxy resin bonded with metallic materials such as gold, silver, or copper. This via-filling method helps improve the current capacity and heat dissipation performance of the vias, but it is expensive.
Non-conductive slurry plug	This via-plugging method protects the vias without requiring surface treatment, but it does not improve heat dissipation or current capacity. This method typically uses solder mask or other similar materials.

PCB via design rules for blind and buried vias

To create more routing space on circuit boards, manufacturers have devised a method to limit the number of circuit board layers that vias can penetrate. These vias are called blind vias or buried vias, but designers should note that they are more expensive to manufacture than standard through-holes.

What is a blind hole and what are its design rules?

Blind vias begin on the outer layer of a circuit board and penetrate only a portion of the board stack. Because blind vias are machine-drilled, their drill size limitations are the same as through-holes. However, within the circuit board stack, blind vias allow additional routing channels below or above them. This gives blind vias a significant advantage over back-drilled through-holes, as through-holes, due to their tube shape, do not allow routing below or above them. Furthermore, blind vias are manufactured sequentially, meaning that layer pairs are drilled and plated before being bonded together. Because blind vias require additional steps in the manufacturing process, they are not cost-effective, but their use may be necessary for signal integrity or electrical performance considerations. PCB designers should use blind vias with caution, only using them when necessary and after confirming manufacturing details with the PCB manufacturer.

What is a buried borehole and its design rules?

Buried vias, like blind vias, are mechanically drilled, but their start and end points are on the inner layers of the circuit board, not on the surface. Buried vias are very useful for densely wired PCBs, allowing routing channels to be placed above and below them. However, like blind vias, buried vias are also expensive, and their cost-effectiveness is not high unless using them can significantly reduce the number of layers on the circuit board. Besides mechanically drilled blind and buried vias, a better option is microvias drilled using lasers.

design rules in PCB via design

Microvias are holes with a diameter of less than 6 mils, typically drilled using lasers. Due to their tiny size, microvias usually penetrate only one layer but can be used on both external and internal circuit board layers. Because of their small size, microvias are suitable for many situations where through-holes are not feasible, but their manufacturing cost is significantly higher. Other PCB via design rules related to microvias include:

Aspect Ratio	Micropores are small in size, making it difficult to electroplate inside them. The best practice is to make their diameter larger than their depth.
Pad size	Because of the smaller diameter of the microvias, PCB designers can use smaller pad sizes, as small as 12 mils, thus leaving more routing channels.
plug hole	Microvias are typically plugged and electroplated to make them flush with other metal components. This plugging method makes microvias ideal for through-hole applications.

Microvias are widely used in PCB layout and are a key element in high-density designs. Without placing microvias within the small pads of high-density BGAs, it would be impossible to route these pin-count processors and memory devices. Microvias can also be used in combination with other types of vias. Microvias can be stacked not only on top of each other but also with buried or blind vias.

How to use PCB via design ?

Considerations during the design phase	When designing a PCB, engineers need to determine the location, size, and number of vias based on the circuit requirements and board layer structure. This requires engineers to have extensive experience and expertise to ensure the rationality and reliability of the design.
Choose the appropriate via type	Choose the appropriate via type based on the specific application scenario and requirements. For example, blind vias can be chosen for connecting surface and inner layer traces; buried vias can be chosen for connections that need to be completely hidden inside the PCB; and through-hole vias should be chosen for connections that need to penetrate the entire PCB.
Optimize via parameters	To improve electrical performance and mechanical strength, parameters such as via diameter, aperture ratio, and copper foil thickness need to be optimized. Proper selection of these parameters can effectively reduce the adverse effects of parasitic capacitance and inductance, thereby improving signal transmission quality.
Precautions during processing and manufacturing	During the PCB manufacturing process, it is necessary to strictly control key indicators such as via drilling accuracy, hole wall roughness, and copper plating quality. The quality of these indicators directly affects the performance of the vias and the overall quality of the PCB.

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Summarize

In summary, vias on PCBs are a crucial element in circuit design and routing. They not only connect different signals in series but also facilitate board positioning, heat dissipation, and other functions. Via placement is governed by specific specifications and requirements. Therefore, effective via placement can improve thermal efficiency, increase production efficiency, and enhance overall manufacturing quality, thus contributing significantly to the development of the electronics manufacturing industry. If you require large-scale production of custom PCB products, please contact us for assistance.