Analysis of common errors and process limits in multilayer PCB via design

Successful manufacturing of custom PCBs requires not only innovative design concepts but also a deep understanding of various multilayer PCB processes by the manufacturer. Via design, as a crucial element of PCB design, is vital to PCB performance and manufacturing efficiency. Below is our analysis of common errors in PCB via design and the limitations of the manufacturing process.

Basic concepts of vias in multilayer PCBs

In printed circuit board (PCB) design, vias are a crucial element. Simply put, PCB vias are small metallized holes in a multilayer PCB used to connect conductors between different layers. These holes not only provide electrical connections but also serve as mounting or positioning holes for components. Depending on the manufacturing process, vias are mainly classified into three types: blind vias, buried vias, and through-holes.

The function of vias in multilayer PCBs

Electrical connection	In multilayer PCBs, circuits between different layers need to be connected vias. This connection method ensures stable signal transmission and reduces signal attenuation and interference.
Component fixing and positioning	In addition to electrical connections, vias are also used to secure and position components. By drilling holes in the PCB and inserting component leads, components can be firmly mounted.

via design specifications for multilayer printed circuit boards

On PCBs, vias are commonly used to connect circuits at different levels. If you don’t adhere to through-layer specifications during PCB layout and design, vias can become a bottleneck in the manufacturing process. Following specifications can prevent production problems and ensure board quality and reliability.

Avoid blind holes	Blind holes connect only one layer. When it is necessary to pass through several other layers, the internal parts need to be bored or laser drilled. Due to the high manufacturing cost and complexity of the process, blind holes should be avoided as much as possible.
Set the correct mezzanine and crossover	In PCB design, to avoid short circuits between different layers, it is necessary to set proper mezzanine and via rules. Via rules define the minimum spacing of vias, as well as some issues to consider regarding row routing, wiring, and mezzanine spacing.
Select the correct via type	When choosing vias, a balance needs to be struck between cost and reliability for the consumer. For the simplest boards requiring connections at both ends, through-hole vias are suitable. For densely designed boards, through-hole vias, blind vias, and embedded vias can be explored.
Grounding clamp hole	By incorporating grounding solutions such as grounding clamp holes into the layout, electromagnetic compatibility can be effectively improved and static electricity accumulation can be reduced.

multilayer PCBs be designed to any size?

Let’s first look at how the holes on a PCB are made. First, in the material preparation stage, PCB manufacturers use automatic cutting machines to cut large copper-clad laminates into substrates of specific sizes to meet production needs. Then, CNC drilling machines precisely drill holes in designated locations on the copper-clad laminates. Because the drill bit of the drilling machine is round, it can only drill round holes and cannot produce square holes.

Taking Geyuan Electronics as an example, the drill bits used for PCB processing are mechanical drills, with drill bit specifications increasing or decreasing in increments of 0.05mm. Their round hole drill bits range from 0.15mm to 6.30mm, their smallest slot drill bit is 0.65mm (suitable for metallized slots), and their smallest slot router bit is 1.0mm (suitable for non-metallized slots). Therefore, vias cannot be designed to any size when designing a PCB.

Multilayer PCB via design and core manufacturing process limit analysis

Mechanical drilling and laser drilling	The minimum diameter of a mechanical drill bit is typically 0.3 mm; laser drilling can achieve diameters of 0.15 mm or even smaller, but this significantly increases processing costs.
Aspect ratio (depth-to-diameter ratio) limit	When the ratio of board thickness to hole diameter (depth-to-diameter ratio) is too large, the smaller the hole, the more uneven the copper plating on the hole wall. When the hole depth exceeds 6 times the hole diameter or the depth-to-diameter ratio exceeds 10:1, it is very easy to have no copper on the hole wall, broken drill, or plating failure .
Hole spacing and safety clearance	If two holes are too close together, the material will be too thin during drilling, and the drill bit will break due to uneven stress. A safe distance should be maintained between the edge of the hole and the inner layer wiring or adjacent holes (usually, the distance between the loop and the line should be more than 3mil to 6mil).

Common Design Mistakes and Tips

Blindly pursuing extremely small apertures	Non-high-speed circuits or conventional multilayer boards do not require the mandatory use of microvias or high-order buried blind vias, otherwise it will lead to a decrease in production yield and an exponential increase in costs.
Vias and Pad can be used interchangeably (vias can be directly applied to pads).	Placing vias directly onto component pads can cause solder paste to flow away along the vias during reflow soldering, leading to insufficient solder or even causing “tombstone” phenomenon in the components.
Annular ring too small	If the width of the copper ring around the solder pad is insufficient, it can easily cause the hole to break (the copper in the hole breaks) or the solder pad to fall off when the drilling is slightly off.
Multi-layer blind hole alignment deviation	If the positioning accuracy is insufficient or the thermal expansion and contraction is uneven during the lamination and drilling of high-layer boards, it can lead to misalignment of the stacked via centers, sudden impedance changes, or signal disconnection.

Considerations for small hole design in PCB via design

What size hole is considered a small hole? Generally, 0.3mm is used as the dividing line; holes smaller than 0.3mm are considered small holes. The smaller the via, the more difficult it is to machine, especially when the via size is close to the process limit. This involves the industry concept of “aspect ratio.” What is aspect ratio? Simply put, it’s the ratio of the board thickness to the via diameter. The larger the aspect ratio, the higher the machining difficulty. In other words, the smaller the via, the more difficult it is to machine.

Difficult to electroplate	As vias become smaller, the reduced aperture makes it difficult for the copper layer to deposit evenly during electroplating, easily leading to problems such as no copper in the via or bad vias. In short, the smaller the via, the lower the copper plating pass-through rate.
Low processing efficiency	The shorter the effective length of a pinhole drill bit, the fewer boards can be drilled in the same batch, directly impacting production efficiency. To prevent drill bit breakage, the feed rate needs to be reduced and the spindle speed increased when drilling pinholes. This process not only increases processing time but also accelerates drill bit wear. Therefore, in PCB design, via sizes should ideally be greater than 0.3mm to ensure processing efficiency and reduce costs. Pinholes should only be considered when space is limited.
outer diameter issue	When designing vias or solder holes, the dimensional requirements for the inner and outer diameters must be considered. These parameters directly affect the PCB manufacturing process. Taking Geyuan Electronics as an example, the minimum processing parameters for double-sided and multilayer boards are an inner diameter of 0.15mm and an outer diameter of 0.25mm.
Problem of weld ring being too small	If the solder ring is too small, during manufacturing, the PCB may experience misalignment between the drilled hole and the center of the solder pad on the film due to drilling equipment deviations and alignment tolerances, resulting in off-center holes. This misalignment reduces the effective area of the solder pads, increases the risk of manufacturing defects, and may even lead to electrical connection failures. Therefore, properly designing the solder ring size and considering manufacturing tolerances is crucial for ensuring PCB quality and reliability.

Multilayer PCB via design techniques

The following are tips on via design for multilayer PCBs, summarized by the engineering team at Geyuan Electronics .

• When creating standard through-holes using a mechanical drill, it’s important to remember that the minimum acceptable drill bit size depends on the thickness of the circuit board. For example, PCB manufacturers typically require that the aspect ratio of their drill holes not exceed 10:1. This means that for a 62 mil thick circuit board, the minimum mechanical drill bit size you should use is 6 mil or 0.006 inches. If you need holes smaller than this, you should consider using microvias with an aspect ratio of 1:1.

• Although vias are short, they still represent a measurable conductor length, which can cause problems with stringent signal integrity requirements. For example, a via connecting the top two layers of a ten-layer circuit board would have eight layers of unnecessary metal that could introduce interference.

• The size of the via pads is important; you need to ensure that a sufficiently large annular ring is left after drilling. Mechanical drills wobble slightly during drilling, and without a sufficient annular ring, the via may be damaged by drill breakage.

• In densely packed areas of a PCB, designers must be careful not to block routing paths or ground plane return paths with vias. This is why using blind vias and microvias is crucial when planning BGA escape routes, ensuring that every pin can be routed without obstructing important routing paths and planes from underneath the component. Choose staggered vias over stacked vias, as stacked vias require filling and planarization, a time-consuming and costly process.

• Implement controlled depths for blind holes and buried holes.

• Keeping the aspect ratio of high-speed vias to a minimum avoids signal reflection. This provides better electrical performance and signal integrity, lower noise and crosstalk, and reduces EMI/RFI.

• Use smaller vias, especially on HDI boards, to minimize stray capacitance and inductance.

• The vias in the pads must be filled unless they are located within the thermal pads.

• BGA mounting pads can be arranged using blind vias and through-holes, provided these vias are filled and planarized. If they are not planarized, the solder joints may be affected.

• Through-holes are added to the thermal pad beneath the QFN to facilitate solder flow to the conductive plane. These through-holes provide a secure solder joint for the thermal pads, thus preventing solder from floating across the package during assembly. This may affect the solder joint quality of the QFN contacts.

• Assembly shops can compensate for the lack of through-holes in thermal pads by adding a window-shaped opening to the solder paste stencil above the pads, which reduces solder buildup and venting.

• Check the minimum clearance between the traces and vias and the edges of the wiring/scribing.

• In the case of a BGA package, check the location of the vias.

• Each via is separated from its pad using a predefined short trace. Check if a mask gap is provided for the vias under the BGA.

Quality control of vias in multilayer PCBs

To ensure the quality of PCB vias, the following aspects need to be controlled:

Material selection	High-quality plate and copper foil materials are selected to ensure the conductivity and mechanical strength of the vias.
Processing technology	Advanced drilling and copper plating processes are employed to improve the accuracy and reliability of vias.
Detection and Testing	After processing, the vias undergo rigorous inspection and testing, including electrical performance testing and mechanical strength testing, to ensure that they meet design requirements and usage needs.

Geyuan Electronics is a professional multilayer PCB manufacturer.

Geyuan Electronics can now produce high-multilayer PCBs ranging from 6 to 32 layers. Leveraging our self-developed ultra-high layer technology and in-pad via technology, our PCBs can have up to 32 layers, with a minimum hole diameter of 0.15mm and a minimum trace width and spacing of 0.0762mm, supporting hundreds of lamination structures. In particular, our in-pad via technology utilizes resin plugging + electroplating capping for all high-multilayer PCBs with 6 or more layers, allowing vias to be directly placed on the pads, resulting in a smoother pad surface and more routing space. To ensure PCB quality, we use a four-wire low-resistance test to inspect the copper plating on the via walls. Compared to traditional testing methods, the four-wire low-resistance test offers higher accuracy and can better detect bad vias, thus ensuring product reliability and quality. If you need a professional multilayer PCB manufacturer for mass production and customization, please contact us for services.

Summarize

This article not only summarizes the via design specifications for multilayer PCBs but also analyzes common errors and process limits in multilayer PCB via design. Correct via design is not only a prerequisite for manufacturing high-quality multilayer PCBs but also crucial for ensuring PCB reliability. To reduce production costs and prevent problems during PCB manufacturing, it is necessary to establish correct via design specifications and production processes. Choosing the right via method better adapts to different layouts and designs, ensuring PCB performance and quality. If you need mass production of multilayer PCBs, please contact Geyuan Electronics for assistance.