What is a Via?

A via is a plated hole that allows electrical signals to pass from one layer of a PCB to another. Vias are essential components in multi-layer PCBs, as they enable the interconnection of traces on different layers, thus saving space and allowing for more complex designs.

Types of Vias

There are several types of vias used in PCB design:

Through-hole Vias

Through-hole vias are the most common type and pass through all layers of the PCB. They are typically used for mounting through-hole components or providing electrical connections between layers.

Blind Vias

Blind vias are visible only on one side of the PCB and connect an outer layer to an inner layer. They are used to save space and reduce the overall size of the PCB.

Buried Vias

Buried vias are hidden within the inner layers of the PCB and do not reach either surface. They are used to create connections between inner layers without consuming space on the outer layers.

Micro Vias

Micro vias are small, laser-drilled vias with diameters less than 150 microns. They are used in high-density interconnect (HDI) PCBs to create fine-pitch connections between layers.

Standard Via sizes

The standard via sizes used in PCB design are based on the finished hole size (FHS) and the pad diameter. The pad diameter is typically larger than the FHS to allow for proper annular ring width and manufacturability.

Imperial Unit Via Sizes

Finished Hole Size (mil)	Pad Diameter (mil)	Drill Bit Size (mil)
8	16	8
10	18	10
12	20	12
16	24	16
20	28	20
24	32	24
28	36	28
32	40	32
36	44	36
40	48	40

Metric Unit Via Sizes

Finished Hole Size (mm)	Pad Diameter (mm)	Drill Bit Size (mm)
0.2	0.4	0.2
0.25	0.45	0.25
0.3	0.5	0.3
0.4	0.6	0.4
0.5	0.7	0.5
0.6	0.8	0.6
0.7	0.9	0.7
0.8	1.0	0.8
0.9	1.1	0.9
1.0	1.2	1.0

Factors Affecting Via Size Selection

Several factors influence the choice of via size in PCB design:

Current Carrying Capacity

The current carrying capacity of a via depends on its cross-sectional area. Larger via sizes can handle higher currents without causing thermal issues or voltage drops.

Manufacturing Constraints

The minimum via size that can be reliably manufactured depends on the PCB fabrication process and the manufacturer’s capabilities. Smaller via sizes may increase manufacturing costs or reduce yields.

PCB Thickness

The aspect ratio, which is the ratio of the via depth to its diameter, should be considered when selecting via sizes. High aspect ratios can make it difficult to achieve reliable plating and may cause issues during manufacturing.

Signal Integrity

Via size can impact signal integrity, especially in high-speed designs. Larger vias have lower inductance and resistance, which can help maintain signal integrity. However, larger vias also consume more space and can cause routing challenges.

Component Placement

The via size should be compatible with the component pad sizes and pitches. Smaller vias allow for tighter component placement and higher routing density.

Micro Via Sizes

Micro vias are increasingly used in high-density PCB designs to create fine-pitch interconnections between layers. The standard micro via sizes are:

Finished Hole Size (microns)	Pad Diameter (microns)	Drill Bit Size (microns)
50	100	50
75	125	75
100	150	100
125	175	125
150	200	150

Micro vias are typically laser-drilled, which allows for smaller sizes and higher precision compared to mechanically drilled vias. However, the use of micro vias may increase manufacturing costs and require special design considerations.

Via Annular Ring

The via annular ring is the copper pad surrounding the via hole. The annular ring provides a connection between the via and the traces on the PCB layer. The minimum annular ring width depends on the manufacturing process and the via size.

A general rule of thumb is to maintain an annular ring width of at least 5 mil (0.127 mm) for mechanical strength and reliability. However, smaller annular ring widths may be possible with advanced manufacturing processes and smaller via sizes.

Via Tenting and Filling

Via tenting and filling are techniques used to cover or fill the via holes on the PCB surface. Tenting involves applying a layer of solder mask over the via, while filling involves plugging the via with a non-conductive material.

Tenting and filling can provide several benefits:
– Preventing solder from entering the via during assembly
– Improving the PCB’s mechanical strength and durability
– Enhancing the PCB’s aesthetic appearance
– Reducing the risk of contamination or short circuits

However, tenting and filling can also increase manufacturing costs and may not be suitable for all applications, such as vias used for thermal management or component mounting.

FAQ

Q1: What is the most common via size used in PCB design?

A1: The most common via size used in PCB design is 20 mil (0.5 mm) finished hole size with a 28 mil (0.7 mm) pad diameter.

Q2: Can I use different via sizes on the same PCB?

A2: Yes, you can use different via sizes on the same PCB. This is often done to accommodate different current carrying requirements, component sizes, or routing constraints.

Q3: What is the minimum via size that can be reliably manufactured?

A3: The minimum via size that can be reliably manufactured depends on the PCB fabrication process and the manufacturer’s capabilities. Generally, finished hole sizes down to 8 mil (0.2 mm) can be achieved with standard manufacturing processes.

Q4: How do I choose the right via size for my design?

A4: When choosing the right via size for your design, consider factors such as current carrying capacity, manufacturing constraints, PCB thickness, signal integrity, and component placement.

Q5: What are the benefits of using micro vias in PCB design?

A5: Micro vias allow for higher routing density, finer pitch connections, and improved signal integrity in high-speed designs. They are particularly useful in HDI PCBs and advanced packaging applications.

Conclusion

Selecting the appropriate via size is a critical aspect of PCB design, as it affects the board’s electrical performance, manufacturability, and cost. By understanding the standard via sizes and the factors that influence their selection, designers can create reliable and efficient PCBs that meet their specific requirements.

As PCB technology continues to advance, the use of smaller via sizes and micro vias is becoming increasingly common, enabling the development of more compact and high-performance electronic devices. However, designers must also consider the manufacturing constraints and costs associated with these advanced via technologies to ensure the feasibility and cost-effectiveness of their designs.