Printed Circuit Board Via Diameter Design Specifications
In modern electronic design, PCB (Printed Circuit Board) design is a crucial part of the entire product development process. The hole design, in particular, directly impacts key performance indicators such as signal integrity, mechanical strength, and manufacturability. The following will elaborate on several aspects to help users fully understand and master the core elements of PCB hole design.
A brief introduction to PCB vias
Printed circuit boards (PCBs) are crucial components or basic units in circuit construction, upon which various circuit elements are connected. However, the most critical factor in PCB design and component mounting is the fabrication of the various holes. These holes are manufactured using different techniques and processes. Each type of hole has its unique manufacturing process and performance characteristics. The primary functions of holes are to facilitate component mounting on the board, provide robust and reliable electrical connections, and enhance the structural strength of the PCB.
PCB via definition
In double-sided and multilayer boards, a via is a common hole drilled at the intersection of printed conductors that need to be connected between layers to connect the printed conductors. In terms of PCB manufacturing process, a layer of metal is chemically deposited onto the cylindrical surface of the via wall to connect the copper foils of the intermediate layers, while the top and bottom surfaces of the via are made into circular solder pads. The main parameters of a via include the outer diameter of the hole, the solder ring, and the drill hole size.
Advantages and disadvantages of PCB vias
| advantage | The signal connection replaces the previous jumper connection process, making the process simpler and improving production efficiency. |
| shortcoming | Vias possess both parasitic capacitance and parasitic inductance. In the design of high-speed digital circuits, the harmful effects of parasitic inductance from vias often outweigh the effects of parasitic capacitance. In transmission lines, this manifests as impedance discontinuities and delays in high-speed signals. In power supply systems, parasitic series inductance weakens the contribution of bypass capacitors, thus reducing the overall filtering effect of the power supply system. |
Types of PCB vias
| Through-hole (PTH) | Metallized holes that penetrate the entire board typically have a diameter of ≥0.2mm. |
| Blind Via | For local vias extending from the surface to the inner layer, the depth-to-diameter ratio is recommended to be ≤1:1. |
| Buried Via | Interlayer vias require multi-layer lamination process. |
PCB Via Design Rules : Size Limits, Spacing Requirements, and Manufacturing Specifications
Regardless of the type of PCB, there are corresponding specifications and requirements for its design. Below is a summary of the relevant rules and specifications for PCB via design.
Core design specifications require
| Aperture to plate thickness ratio (depth-to-diameter ratio) | For ordinary through holes, the depth-to-diameter ratio is recommended to be controlled within 1:10 or 1:12 to prevent poor electroplating. |
| Pad and aperture relationship | The outer ring width is typically ≥ 5mil, and the inner ring width is typically ≥ 8mil, to ensure mechanical strength and electrical connection. |
| Hole spacing requirements | The center distance or edge distance is recommended to be ≥ 0.5mm. If it is less than 0.3mm, it is easy to cause solder bridging or breakage during processing. |
| Process optimization suggestions | For vias with a diameter ≤ 0.5mm, it is recommended to apply via plugging and sealing oil treatment to prevent solder from flowing into the via. |
Common Sizes and Specifications
| Standard through hole | Common inner diameter ≥ 0.2mm, outer diameter ≥ 0.4mm (in extreme cases, the outer diameter can be as small as 0.35mm). |
| Micro-holes/Blind holes | The inner diameter of a first-order blind hole is typically 0.1 mm, and the outer diameter is 0.25 mm. |
| Power/Ground Vias | It is recommended to use a slightly larger size (outer diameter 0.55mm – 0.7mm or larger) to carry a larger current . |
| Signal via | Commonly used diameters are 0.4mm – 0.55mm. |
Spacing and Layout Rules
| Hole spacing | A center distance of ≥ 0.5mm is ideal; a distance less than 0.3mm is considered a violation. |
| Pads and vias | Avoid placing vias directly on tiny pads (such as ≤ 0402 RC pads). It is recommended to maintain a spacing of 4-8 mil to prevent tombstoning or solder leakage during soldering. |
| Plug requirements | Vias with a diameter ≤ 0.5mm usually require via plugging and encapsulation. Vias under BGAs must be centered on the pad and plugged. |
Key design parameters
| Aperture Specification | Mechanical drilling limits: 0.15-6.5mm (standard) / 0.1-0.15mm (high precision) |
| Laser drilling: capable of creating microholes of 0.05-0.1mm. | |
| Aspect ratio limitation: ≤10:1 (standard design), recommended ≤8:1 | |
| Hole spacing requirements | The distance from the hole edge to the wire is ≥0.15mm. |
| The spacing between vias in the same network is ≥0.1mm. | |
| The spacing between vias in different network types is ≥0.2mm. | |
| Pad Design | Standard pad diameter = hole diameter + 0.2mm (0.1mm ring width on each side) |
| The HDI board ring width can be reduced to 0.075mm. | |
| NSMD pads are recommended for BGA areas. |
Signal integrity requirements
| Via impedance control | Increase the size of the anti-solder pad (≥2 times the aperture). |
| Back drilling technology is adopted (residual pile length ≤ 0.15mm). | |
| High-speed signal vias ≤ 2 per transmission path | |
| Heat dissipation design | Power/ground via array spacing 2-3mm |
| The diameter of the heat dissipation holes is ≥0.3mm. | |
| Dark ink treatment (30% increase in emissivity) |
Manufacturing process specifications
| Solder resistance requirements | Plug depth ≤ 0.8 times the pore diameter |
| Ink thickness 8-15μm | |
| The diameter of the window opening on one side should be ≥0.05mm. | |
| Drilling compensation | Mechanical drilling compensation = Hole diameter × 3% + 0.05mm |
| Laser drilling compensation ±0.01mm | |
| Testing standards | The copper thickness of the hole wall is ≥18μm (1oz base copper). |
| Hole position accuracy ±0.05mm (LDI process) | |
| Through-hole copper ductility ≥15% (meets IPC6012B) |
Special design requirements
| Stepped hole design | Step difference ≤ 0.3mm |
| Transition angle ≥ 120° | |
| Spacing between adjacent layers ≥ 3 × plate thickness | |
| Hole plugging treatment | Conductive adhesive plugs for holes: suitable for diameters ≤ 0.3mm |
| Resin plugging: enables planarization. | |
| Maximum plug diameter ratio: 1:1.5 | |
| It is recommended to consider the specific design process. | Verify via impedance using an impedance calculation tool (such as Polar SI9000). |
| Perform DFM analysis (using tools such as Valor and NPI). |
Optimal ratio of PCB vias
When space permits, a via size of 0.3mm-0.5mm is generally considered optimal. The optimal ratio is for the outer diameter of the via to be 12mil larger than its inner diameter; for example, if the via diameter is 12mil, the outer diameter would be 24mil, resulting in the optimal ratio. The outer diameter size has a certain impact on the difficulty of alignment during production and the reliability of the product. Each production process has tolerances. If the outer diameter is too small, and the drilling and alignment processes deviate significantly, the final PCB may have a risk of bursting holes (what PCB manufacturers call “broken pads”). Broken pads can also lead to insufficient current carrying capacity in the product.
PCB via design considerations
1. Consider both cost and signal quality when selecting a suitable via size. For example, for 6-10 layer memory module PCB designs, a 10/20 mil (drill hole/pad) specification is recommended; for high-density small circuit boards, an 8/18 mil via diameter can also be tried. Under current technological conditions, it is difficult to use smaller via sizes. For power or ground vias, larger sizes can be considered to reduce impedance.
2. Using a thinner PCB helps reduce two parasitic parameters of vias.
3. Avoid changing the layer of signal traces on the PCB as much as possible, that is, minimize the use of unnecessary vias.
4. The power and ground pins should be drilled close to each other, and the lead length between the via and the pin should be as short as possible, as this will increase the inductance. At the same time, the power and ground wires should be made as thick as possible to reduce impedance.
5. Placing ground vias near signal layer vias provides the closest ground loop for the signal. Even a large number of redundant ground vias can be placed on the PCB, provided the design is flexible. The aforementioned via model applies to scenarios where pads are present on all layers. Sometimes, pads on some layers can be reduced or even removed, especially when via density is extremely high, as this can lead to broken trenches forming broken loops in the copper layer. To solve this problem, besides adjusting the via positions, one can also consider placing the vias directly on the copper layer.
Equivalent Model of PCB Vias in High-Speed Circuits
Vias inevitably introduce parasitic capacitance and inductance. While this may not be noticeable in low-speed or high-frequency transmission lines, it can cause delays or waveform distortions in high-speed lines, becoming significant and potentially impacting the success or failure of the system design. Vias on transmission lines act as discontinuities in impedance, causing signal reflection. Generally, the equivalent impedance of a via is about 12% lower than that of the transmission line. For example, a 50-ohm transmission line will have its impedance reduced by 6 ohms when passing through a via (the exact reduction depends on the via size and board thickness, and is not absolute). However, the reflection caused by the impedance discontinuity of a via is negligible, with a reflection coefficient of only (44-50)/(44+50) = 0.06. The problems caused by vias are primarily due to the effects of parasitic capacitance and inductance.
Choose Geyuan Electronics as your high-volume PCB manufacturing partner
Geyuan Electronics is an AI-driven one-stop intelligent infrastructure platform for electronic products. By integrating industrial software and engineer ecosystems, original equipment manufacturer (OEM) supply chain ecosystems, and flexible manufacturing ecosystems, we have built a closed-loop ecosystem in the electronic supply chain field, committed to making hardware innovation simpler. Since its inception, we have developed the “Geyuan Solution,” which boasts superior IoT solution design capabilities; AI Agents and electronic design automation tools; internal manufacturability design analysis tools; one-stop electronic component procurement services; a highly reliable multilayer PCB R&D and intelligent manufacturing system; SMT assembly; PCBA; and one-stop electronic product manufacturing services. We provide customers with a one-stop intelligent solution that connects design, production, procurement, and delivery, reshaping the traditional fragmented and information-asymmetric model of electronic product manufacturing, improving the efficiency of the entire chain from design to mass production, and creating value for users’ creative ideas.
Summarize
PCB holes are a critical component in PCB design and the proper functioning of electronic devices and projects. These holes are essential for establishing electrical connections between different circuit board layers and providing mechanical strength. Therefore, when designing a PCB, it is crucial to adhere to design rules and specifications to avoid various quality issues during mass production, which could delay the project schedule.
This article summarizes the rules and requirements for PCB via design, compiled by the engineering team at Geyuan Electronics. These design requirements need to be adjusted according to the specific application scenario (consumer electronics/industrial/automotive, etc.) and signal frequency (digital/high-frequency RF). For example, automotive electronic boards require an additional safety margin of 20% or more, while high-frequency PCBs may require stricter impedance tolerances (±5%). If you have needs for high-volume PCB production, please contact us for assistance.