Microvias Design Challenges

Designers can pack an astounding amount of functionality into a small device, and they have the HDI design technique to be thankful for this. HDI design techniques, including microvias design, are useful in generating compact printed circuit designs. They pack very small structures and traces in multilayer PCBs to create a high interconnect density and a high layer count. If a size study determines that 6 mil smaller traces are necessary to fit all the components on a printed circuit board, the design is dense enough to need microvias that will support routing between layers. In this article, Rush PCB Inc. discusses how they form these structures and the microvias design challenges.

What is a Microvia?

As the name suggests, a microvia is a miniature version of a typical via. However, rather than a straight barrel of a regular via, a microvia has a conical frustum shape. The walls of a microvia slope inwards while it makes a layer transition, terminating in a pad in the layer just beneath it.

In an ideal case, microvias will span only a single layer providing maximum reliability. For forming connections across multiple layers, designers must use stacked microvias. This requires building up blind and buried microvias to form a stack for reaching across multiple layers. As stacking microvias often leads to reliability problems, designers prefer to use staggered microvias.

CAD Software and Microvias

Designers begin defining microvias in the PCB stackup editor. Here, they define layer pairs and select the material. For this, designers must choose an appropriate laminate that can support their intended fabrication process. After they have built up their proposed stackup, they should send it to their fabricator for review. It is important to get the fabricator’s input up front, so there will not be any manufacturability issues with microvias.

In the PCB editor of the CAD software, the designer must pay attention to the designation of the layer pair on the pad during the PCB layout, as IPC considers six mils as the upper limit for microvias.

To calculate the aspect ratio of microvias the designer places in the PCB layout, they must consider the hole diameter together with the dielectric layer thickness. When sizing microvias, the aspect ratio is an important factor as it is also related to reliability.

Microvia Fabrication

Designers consider typical microvias as a via with a diameter of less than 6 mils or 150 µm. While bigger vias can be drilled mechanically and plated, forming microvias requires a high-power laser. High volume PCB manufacturers prefer using lasers for forming microvias, as this process has a high throughput. They are constantly improving the laser process, and newer laser drilling techniques can form microvias with diameters as small as 0.6 mil or 15 µm.

The microvia hole must be plated after it has been drilled and cleaned. Many plating processes are in use, including electrolytic deposition, sputtering, and the electroless copper plating process. The plating process prevents the formation of dimples, voids, bumps, and or other structural defects in the filled via. Voids can be detrimental to the reliability of the board, as an application of stress on the via structure can concentrate the stress around the edge of the void where the copper is the thinnest.

Compared to mechanically-drilled vias, laser-drilled microvias have a lower tendency to manufacture defects. This is because drill vibration increases as the drill bit wears out, leading to more defects in mechanically drilled microvias. It is good that mechanical microvia drilling is only useful for creating 6-8 mil diameter holes, as these are more prone to failure during filling, plating, and assembly.

Types of Microvias

Although there are a few different types of microvias, all of them have two common characteristics:

Low Aspect Ratio

In comparison to the through-hole vias in regular PCBs, microvias typically have smaller aspect ratios of around 0.75:1. Although it is possible to fabricate microvias with larger aspect ratios of 1:1 or 2:1, but they have reliability issues. Because of their low aspect ratio, microvias typically span only a single layer. As per the IPC definition, a via with an aspect ratio of more than 1, is not a microvia.

Susceptibility to Fracture

A through-hole via with a high aspect ratio of around 10:1 tends to be susceptible to fracture, specifically near the middle of its barrel. The plating method for microvias makes copper curve inwards into the barrel region, concentrating the stress build-up there. This leads to microvias being more susceptible to fracture in the neck when subjected to repeated thermal cycling, mechanical shock, or strong vibration.

Apart from the above two qualities, microvias differ only in their diameters and their placement in the circuit board:

In-Pad Microvia

The in-pad microvia configuration is commonly found in boards carrying fine-pitch BGA components. Here, the small distance between solder balls does not allow dog-bone type fanout.

It is also possible to place microvias in a pad on the surface layer, but they require filling with electro-deposited copper, or conductive epoxy and plated over with copper. The advantage of the in-pad configuration is the pad is solderable. However, it also brings reliability concerns because of the internal fill and the copper wrap plating.

Blind Microvias

Just like a blind via, a blind microvia never shows up on both the surface layers of a PCB. They typically start in one of the surface layers and terminate in the layer immediately below it. If the aspect ratio is kept low, they could be made to terminate two layers below the surface layer. But it is advisable to rather use stacked or staggered microvias, as these are more reliable. Blind microvias can be filled or unfilled.

Buried Microvias

With the same structure as blind vias, buried microvias also span two adjacent layers, while not showing up on any of the two surface layers of the PCB. As with blind microvias, it is best to keep the aspect ratio low and allow them to span a single layer, thereby easing fabrication and ensuring higher reliability.

Manufacturers typically fill buried microvias with copper, using either an epoxy+copper resin or pure copper, to ensure a strong connection across the head of the microvia. Essentially, the plating process must result in a void-free structure for maximum reliability.

Stacked and Staggered Microvias

Using microvias in a PCB design with multiple layers can result in numerous blind and buried vias on a board. However, the layer-wise process of forming microvias of low aspect ratio, makes them useful for stacking. Stacked microvias are just buried vias or blind vias stacked one on top of others. This helps to span between multiple layers in an HDI PCB.

The manufacturer must fill the internally buried microvias in the stack with conductive paste before plating them over. This ensures the via makes strong contact with the next via in the stack as it is deposited and plated. Rather than stacking one atop the other, it is also possible to use staggered microvias, where the microvia on the next layer is offset from the via on the current layer.

The aspect ratio enforced in the design decides the use of individual buried and blind microvias, and whether they should be stacked or staggered. The designer must also be aware of the potential failure of stacked vias at their interface.

Filled/Unfilled Microvias

It is possible to either fill microvias with copper or leave them unfilled. For buried microvias, it is necessary to fill the via hole with copper, especially if they are to be stacked. Any voids present in the interior of the via barrel can lead to premature fracture during reflow. Although it is possible to leave blind microvias unfilled, in-pad blind microvias must always be filled.

Following the filling, the manufacturer will plate the microvia. They typically use epoxy and copper resin or pure copper for the process. Beginning with a conformal coating, the manufacturer typically uses pulse plating to fill in the body of the microvia with solid copper, thereby eliminating voids. Additives in the filler material are necessary during the plating process, as their absence may result in void formation. Another reason for using additives is to prevent the copper from being concentrated on the walls and the top surface of the microvia. Uneven deposition of copper along the via body may also be conformal plating, and this may lead to the formation of voids.

Reliability of Microvias

It is necessary to discuss the reliability of blind, buried, and stacked microvias. The points of failure are mainly at the interface between microvias in a stack and the butt joint at any copper wrap plating. Actual failure depends on several factors, including:

• The aspect ratio of the microvias in the stack
• The number of microvias stacked
• Size of voids formed in the filling material during plating
• Copper plating thickness at the corner of the top pad and via hole

The concerns about reliability are specifically due to thermal cycling. When the PCB is subjected to thermal cycling, such as during assembly, the substrate expands and exerts high stress on the thin copper plating on the microvia. This may cause failure at the plated interfaces, such as that between two stacked microvias, or that between a microvia and its pad.

Another common failure location is at the knee, where the copper plating slopes into the via at the top. This location is particularly vulnerable, if part of the plating is missing, and the remaining copper is very thin. Although it is difficult to pinpoint the specific criteria that will lead to failure, the following points are a cause for concern:

Aspect Ratio

Smaller aspect ratios are more reliable. For instance, testing between different aspect ratios shows 0.7 aspect ratio microvias surviving accelerated life tests, while microvias with aspect ratios of 1 fail within a few thermal cycles.

Voiding

Although voiding is related to failure, the shape and volume of the void, together with the aspect ratio of the microvia, have a greater contribution. Therefore, there cannot be a general statement that voids always increase the rate of failure.

Stacked vs. Staggered

There are inconsistencies in defining the reliability of stacked vs. staggered microvias. As a thumb rule, designers tend to stack no more than two microvias vertically, staggering the other microvias in the vertical interconnect.

Copper Wrap Plating

Although the copper wrap plating does not contribute to failure, the plating thickness at the butt joint does.

Advantages of Microvias Space

Although presenting some less-advanced manufacturers with fabrication difficulties, microvias are indispensable in HDI PCB technology, as they are primarily responsible for making space available for routing in these boards. In dense boards using fine-pitch components, microvias ensure that designers can pack components into small spaces. As circuit board space is valuable, microvias help lower the costs. Along with smaller traces, microvias allow denser connections between layers.

Additionally, more space is made available by using in-pad microvias. They help to save space by making connections from inside the pad of an SMT. The small form factor of microvias makes them especially suited for this type of interconnection. Where regular vias are too big to fit within the tiny pads of fine-pitch BGAs, microvias, being tiny, can easily fit inside the pad without causing fabrication issues.

Apart from being useful for BGA breakout, in-pad microvias can fit within the pads of even the finest pitch components, making them useful for all types of breakout channels. Although it is possible to generate surface space using in-pad microvias, designers still need several layers to route the traces to different parts of the circuit board. Using microvias increases the number of channels for routing, resulting in fewer overall layers.

EMI

Microvias contribute significantly to signal integrity in HDI circuits. Because of their smaller size, microvias carry smaller parasitic inductance and capacitance, allowing them to be packed close together, without causing an increase in noise coupling and crosstalk. Even when microvias and HDI traces are close together, their smaller parasitic features compensate for any strong electromagnetic fields.

Vias are a major concern in high-speed circuits, as they radiate signals and cause reflections. Often a large via stub can resonate, coupling with high field strength into neighboring vias. Typically, large vias can be equated to good high-frequency radiating antennas, specifically with the stub resonating. The best way to reduce the power of an antenna is to reduce its size.

As microvias are smaller, they behave like smaller antennas, with smaller emissions and absorption cross-sections. Moreover, as they are fabricated layer by layer, there are no stubs to resonate. They also have greater shielding, as their thinner dielectrics allow them to be closer to ground and power planes.

The above are the primary reasons for HDI boards with microvias design tending to lower problems with radiation.

Read More: Understanding PCB Vias: Types, Construction, and Applications

Conclusion

In the complexities of modern HDI PCB design, microvias are powerful tools. Rush PCB Inc. recommends using a modern PCB CAD software tool to provide a comprehensive solution to all microvias design aspects.