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What is Microvia Aspect Ratio in Printed Circuit Board & How to Choose the Right?

The main aim of technology is to simplify projects or simply make them possible to finish in a better way. If the technology brings both benefits to you, then it’s a cherry on top. For years, people have had to go through the troubles of switching lights in the plug during Christmas or other holidays and then plugging them out at night. This causes a real hassle. However, technology makes the process simple. Ultimately, upgrading those plug-ins to smart plugins with a timer on them. Most of them not only schedule the on and off time but also take voice commands. This makes the process of turning on and off the lights a lot easier. 

Similarly, engineers and manufacturers are on a constant hunt for new and advanced technologies for the PCB. Advanced tech is needed to accomplish the work as, with time, PCBs become denser and smaller. Microvia falls in this category which requires keen understanding. Laser vias have a small size and distinct aspect ratio as compared to regular vias. With their compact and small size, they make trace routing simpler and allow routing more traces in a tight space. This article helps in understanding different aspect ratios of micro vias, how to use them, and how they can help in the Design of PCB.

PCB Vias and Microvia Aspect Ratio In Review


Firstly, let’s dive into some general information about vias and their usage in PCBs. Vias serve as a hole drilled into a PCB. They serve the main purpose of transferring signals electrically from one panel to another. Traces conduct electrical signals in the horizontal form, while vias conduct them in a vertical form. Vias come in a variety of sizes, from big to small. Ground and power nets use larger vias and also use to join the mechanical structure of a PCB. Standard vias go through manual drilling and have three types of categories. 

  1. Through-hole vias: This represents a type of hole that drills in a PCB from the upper layer to the lower layer.
  2. Blind Via: This refers to a type of hole that drills into an exterior layer to an interior layer without passing through from top to bottom like a plated through-hole.
  3. Buried Via: This type of hole only drills inside the internal layer without touching the outer layers. It does not affect the exterior layer.

Microvias differ from standard vias in many ways. Designers use laser drills to create micro vias, which offer them to drill a hole smaller than 0.15 mm based on the width of a PCB. The size of micro vias can also get smaller than this. Another major difference between micro vias and standard via is that micro via can only stay up to two-layer. This happens because of the difficulty of plating copper into these tiny holes. For making a direct connection from top to bottom, you can stack the micro vias over each other from one layer to another and create a connection easily. 

Surface layers micro vias do not require any filling, whereas buried micro vias must need filling. This filling depends upon the usage and application. Stacked micro vias mostly fill with electroplated copper as it provides a better connection between the vias. Another practical method to connect layer stack micro vias is staggering them and then using short traces to create a connection. 

Microvia Size

The designer must have to design the stack up of PCBs before choosing the depth and diameter of micro vias. This implies that the designer must measure the number of layers that are going to be used in a PCB. It supports the fanout of BGA and dense components routing, specially clear-pitch BGAs. However, to access fine-pitch electrical components and balance the routing, the sizing of micro vias comprises only two parameters. These parameters can only be accessible after determining the stack-up. 

  1. Pad diameter and Microvia diameter can be determined through the IPC standard protocols depending on the class of your product.
  2. Micro via depth can measure through layer stack up. It also determines through the thickness of the dielectric that supports the routing of HDI.

The typical method to determine the thin external dielectric numbers in HDI circuit boards that require a stack-up is to see the ball out over the biggest BGA component in a PCB. The number of columns and rows can determine the complete layer count that will be needed for the stack-up of the HDI board. Once you determine the layer count, then compare it with the thickness and select dielectric and core materials that need to support the routing of HDI. All these thin and compact dielectrics give full support to the controlled impedance. This will allow them to be used in high-speed and dense devices. 

What are the Microvia Aspect Ratio and its importance?


The aspect ratio represents the proportion between the hole diameter and hole depth. For example, the value of the aspect ratio for a standard PCB with a thickness of 0.062 inches along a through-hole of 0.020 inches in it is three by 1. The ratio serves as a follow-up guide that ensures that the manufacturer does not surpass equipment capabilities while drilling holes in it. The aspect ratio for standard holes must not exceed 10:1. This allows 0.062 inches thickness of board with 0.006 inches drilled vias in it. 

However, the aspect ratio varies in micro vias because of its variant depth and size. Smaller hole plating is difficult. Manufacturers go through a lot of trouble in plating small holes in a PCB with ten layers. The plating gets easy if the holes have to pass through only two layers. Moreover, in the past, IPC defined micro vias according to their sizes. The sizes were smaller or equal to the size of 0.006 inches. With the passage of time, the same size became the most popular. This made IPC change its definition of micro vias to avoid the hassle of constantly modifying its specification with growing technology. Today, IPC describes micro vias as a hole with an aspect ratio of 1:1 as far as the depth of the hole remains the same and does not exceed 0.25mm or 0.010 inches.

Microvia Aspect Ratio for Trace Routing On Your Printed Circuit Board

Mostly, manufacturers aim to get as much trace routing on small areas of PCB with increasing density. However, this leads them to use buried, blind, and embedded vias into the mount pads of the surface. Moreover, buried and blind vias need extra steps of drilling, which makes this process difficult to manufacture. This drilling process also leaves a residue of materials inside the holes, which results in defects in manufacturing. At the same time, the regular vias do not come in line for HDI devices because it takes up larger space for incorporation. That is why micro vias come forward to help in this situation. 

Microvias fabricate more easily than buried and blind vias. They easily fit in compact and small areas mount pads which makes them ideal for use in high-pitch devices such as BGA. Moreover, due to their compact and small size, micro vias allow for extra routing of traces. They also help in reducing the EMI and enhancing the integrity of signals due to their small size. 

Microvias fall in advanced technology of PCB manufacturing. But if you don’t need them, then you can go with standard vias, which also reduces the overall cost. Although, if the PCB has a dense design, then you may need extra space, so instead of looking for other options, use micro vias. 

Precision Use Of Microvia Aspect Ratio Depends On PCB Design Tools

After setting up with the manufacturer, look for PCB layout tools that work well with micro vias. You have to do a lot of things in order to operate the design tools efficiently for micro vias design. These things include the shapes of vias and the rule of design that goes well. As micro via need stacking which cannot be done with standard vias. Therefore, ensure that the tool you use must comply with the stacking technology. 

Moreover, Cadence provides that very specific design tool for PCB micro vias that work perfectly for your PCB. Allegro falls appropriate for creating and designing micro vias designs along with library simulators, design tools, and schematics.

How Can RayPCB Help?


RayPCB gives HDI PCB layout and design services simply to try to follow the IPC standards, however, following the reliability warning of IPC while making the stack up of HDI. This does not guarantee the reliability of your device or application. Therefore, it is advisable for engineers to ensure the process of design by going through test practices. So that it gives a perfect and reliable end-product; moreover, if your design house gives you information about the fabrication process first, then be at ease as you will get a reliable assembled and fabricated product in the end.


All the above-mentioned guidelines and information are only suitable for the Circuit boards and devices that comprise HDI principles. However, you can also discuss this with your manufacturer and get recommendations from them about staggering and stacking the buried or blind via for distinct aspect ratios. Ensure that you fully convey your specification and tell them your requirements. This may include the pitch BGA, your ideal count layer, and the type of thickness of your PCB. Moreover, ensure that they must support basic stack-ups, which support their process of fabrication and provide high-quality and reliable end-products.




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