Guide to PCB Via Design: Best Practices, Tips, and Key Considerations

High-quality PCB design not only requires innovative concepts but also relies on a deep understanding of PCB manufacturing processes. Via design, as one of the critical steps in PCB design, is crucial for both PCB performance and manufacturing efficiency.

Can vias be designed in any size?

To answer this question, let’s first understand how holes on a PCB are created.  

1. Cutting the Material: PCB manufacturers use automatic cutting machines to slice large copper-clad laminates into substrates of specific sizes needed for production. Before cutting, the copper-clad laminate appears as shown below:  

2. Drilling Holes: A CNC drilling machine precisely drills holes at designated locations on the copper-clad laminate. Since drill bits are circular, only round holes can be drilled, and square holes cannot be processed.  

For example, if you place a circular ping-pong ball in a right-angle corner, there will be some gap between the curved part of the ball and the corner. This gap can be understood as the so-called "R angle." Similarly, since drill bits are round, square holes cannot be achieved.  

At JLCPCB , mechanical drilling is used for PCB processing. Drill bit specifications increase or decrease by a minimum of 0.05 mm, with circular drill bits ranging from 0.15 mm to 6.30 mm in diameter. The smallest slot drill size is 0.65 mm (for metallized slots), while the smallest non-metallized slot routing bit is 1.0 mm.  

Therefore, vias cannot be designed in arbitrary sizes when designing a PCB.  

Key considerations for small holes  

What size is considered a small hole? Generally, holes smaller than 0.3 mm are called small holes. The smaller the via, the harder it is to process, especially when approaching the limits of the manufacturing process. This relates to an industry term called "aspect ratio."  

- Aspect Ratio: This is the ratio of board thickness to via diameter. The larger the aspect ratio, the greater the processing difficulty. In other words, smaller vias are harder to manufacture.  

1. Difficulties in Electroplating: As via sizes decrease, the smaller diameter makes it harder for copper to deposit uniformly during electroplating, increasing the likelihood of issues such as non-conductive or defective holes. In summary, smaller vias have a lower success rate for copper deposition.  

To ensure PCB quality, JLC employs four-wire low-resistance testing to detect copper plating conditions on hole walls. Compared to traditional methods, four-wire low-resistance testing provides higher precision, making it easier to identify defective holes and ensuring product reliability and quality.  

2. Low Processing Efficiency: The effective length of small drill bits is shorter, which reduces the number of boards that can be drilled simultaneously, directly impacting production efficiency. To prevent drill breakage, slower feed rates and higher rotational speeds are required when drilling small holes. This process not only increases machining time but also accelerates drill wear.  

Therefore, in PCB design, it is recommended to design via sizes larger than 0.3 mm to ensure processing efficiency and lower costs. Only consider small vias when space is constrained.  

3. Issues with Outer Diameter: When designing through-holes or soldering holes, it is important to consider the requirements for both the inner and outer diameters. These parameters directly affect the PCB manufacturing process.  

At JLCPCB, the minimum inner diameter for double-sided and multilayer boards is 0.15 mm, and the minimum outer diameter is 0.25 mm. JLC can now manufacture high-layer-count PCBs with up to 32 layers. With proprietary ultra-high-layer technology and resin-plugged via technology, JLC can achieve a minimum hole diameter of 0.15 mm, a minimum trace/space of 0.0762 mm, and support hundreds of lamination structures. For boards with more than six layers, resin-plugged via technology with electroplated caps is used, enabling vias to be directly placed on pads for a flatter pad surface and greater routing space.  

Additionally, if the annular ring is too small, misalignment between drilling and film alignment caused by equipment deviations during processing can lead to off-center holes.  

Such misalignment reduces the effective area of the pad, increases manufacturing defects, and may even cause electrical connection failures. Therefore, designing a reasonable annular ring size while accounting for manufacturing tolerances is essential for ensuring PCB quality and reliability.  

Slot Design Considerations  

In addition to small holes, slot design should not be overlooked. Slots are an essential design element for some special packages. When designing slots, pay attention to the following two key points:  

1. Short Slots: Short slots are the most difficult to process in PCB drilling. A short slot is defined as one where the slot length is less than twice its width. Since the slot length is less than twice the width, drilling the starting and ending points can cause the drill bit to be partially on the substrate and partially suspended, leading to uneven forces that cause misaligned or shortened slots. Therefore, the recommended optimal length-to-width ratio for slot design is Length/Width ≧ 2.5.  

2. Long Slots: When designing long slots, it is recommended to choose a solder leveling (HASL) process, especially for slots longer than 5 mm. The slot's annular width should ideally exceed 0.3 mm (with a limit of 0.2 mm) or use an immersion gold process. If the annular width is less than 0.3 mm, insufficient adhesion during solder leveling may result in thermal stress-induced issues such as bursting. Slots with single-sided copper rings are also prone to bursting.  

When slots are densely arranged, the substrate's warp and weft lines may be damaged during drilling, leading to bursting during solder leveling. For such special designs, the immersion gold process is recommended.  

Differences Between Vias and Component Holes  

In a PCB, hole diameters are divided into vias (Via) and component holes (Pad). Their functions and processing requirements differ and should not be mixed.  

- Vias (Via): Primarily used for inter-layer signal connections, typically processed with solder mask.

- Component Holes (Pad): Typically designed as through-holes for installing and soldering components.  

There are two scenarios where vias and component holes might be mixed:  

1. Mistakenly Using Via Attributes for Plated-Through Holes: If vias are mistakenly used as plated-through holes, solder mask coverage might block or clog the holes, preventing proper component installation.  

2. Mistakenly Using Component Holes as Vias: If component holes are incorrectly set as vias and the order specifies solder mask coverage, the exposed copper of the component holes will be covered by solder mask, affecting soldering quality.  

Summary  

1. Vias cannot be arbitrarily sized in PCB design.  

2. Holes smaller than 0.3 mm are considered small holes. Smaller holes are harder to electroplate, reduce processing efficiency, and may cause alignment issues.  

3. Short slots are difficult to process. The recommended length-to-width ratio is ≥ 2.5.  

4. Slot design should ensure annular width exceeds 0.3 mm (limit: 0.2 mm). If space constraints prevent this, use an immersion gold process.  

5. Avoid mixing vias (Via) and component holes (Pad).