The need for more complex and compact electronic devices has increased the need for PCB miniaturization. PCB manufacturers need to design smaller PCBs that can fit smaller devices. Therefore, this great demand drives the need for the production of microvias. Microvias play a crucial role in the performance of high density interconnects HDI PCB.
In this article, we will be discussing the role of microvias in PCB design. Also, we will shed more light on microvias types.
What is a Micro vias in PCB?
A micro via is an interconnection between layers in HDI substrates and PCBs to enable the high input/output density of advanced package. Micro vias are different from standard vias because they are drilled with lasers. Therefore, they are much smaller than the regular vias. Also, a micro via features a small diameter and takes less space on the printed circuit board. Therefore, it gives more space for routing.
Also, this via has a low parasitic capacitance which is critical for high speed PCBs. However, the production process of this via is more complex and expensive than the regular vias. Microvias have a diameter that is less than 150 microns. Also, these vias can reduce the feasibility of a manufacturing defect. This is because via manufacturers use lasers to drill them. Therefore, this reduces the chances of leaving any residue after the process.
Micro vias allow the manufacturing of denser PCBS due to their ability to connect one internal layer to another. Most high density interconnect boards use microvias. Also, microvias help to connect one layer of a PCB to its adjacent layer. The IPC defined a micro via as any drilled hole in a PCB with 1:1 aspect ratio. According to the IPC, microvias are determined based on if their size was smaller than 0.006 inches or equal to.
With the advent of hand-held electronic devices like smartphones, microvias have evolved from single-level to stacked. Also, the microvia features a maximum ratio of 1:1 between the diameter of the hole vias and depth. In addition, the total depth should not be more than 0.25mm. Buried vias
Types of Microvias
Microvias are available in different types. PCB designers can drill any of these microvias on a printed circuit boards. Also, each microvia type features the same aspect ratios. However, the difference between these microvias is their placement.
Buried microvias
These microvias span between two internal layers and don’t get to any of the PCB surface. The best aspect ratio of buried microvais stays low. Also, this ratio spans a single layer to enable ease of fabrication and ensure reliability. PCB designers fill these microvias with copper by using a plating process with a combination of epoxy and copper resin or with pure copper. Also, this helps to ensure a good connection across the microvia’s head. Buried vias are crucial in the performance of a board.
Blind microvias
These microvias are primarily drilled to enable connection from the PCB surface to another inner layer. Therefore, these holes don’t completely go through the circuit like a through-hole. Also, blind microvias begin in the surface layer and remove 1 layer below the PCB surface. However, if the aspect ratio remains low, they could terminate two layers below the surface layer. These microvias could be unfilled or filled.
Staggered microvias
Staggered microvias are several buried microvia connecting layers. Also, PCB designers place these vias offset from one another. Staggered vias are crucial for multilayer boards.
Stacked microvias
Stacked microvias comprises several buried or blind microvia stacked on top of each other. This enables connection between the inner layers. Also, stacked microvias are a blind microvia stacked on buried microvias. PCB designers use conductive paste to fill the internal buried microvias in the stack. Also, they plate these microvias to ensure strong contact. Stacked vias help to ensure signal integrity in circuit boards.
The Importance of Microvia Aspect Ratio in PCB Design
The aspect ratio defines the ratio between the hole depth and hole diameter. For example, the aspect ratio of a standard PCB at 0.062 inches inch with 0.020 inch through-hole would be 3 to 1. Therefore, this ratio serves as a guide to ensure the PCB fabricator works with the best capabilities when drilling holes.
For standard vias, aspect ratios shouldn’t be more than 10:1. This would enable a 0.062 inch PCB to have a 0.15mm hole drilled in to. However, the ratio of a microvia is much different from a standard via. This is because of the size and depth of a microvia. It can be difficult to plate smaller holes. Also, trying to plate a small hole passing through 10 layers PCB can cause problems for a PCB manufacturer.
Plating can be much easier if this hole goes through two of those layers. The IPC once defined a microvia according to its size which was smaller than or equal to 0.006 inch. As this size became popular, the IPC changed this definition to prevent updating its specifications as technology evolved. The IPC has now defined a microvia as a hole with a 1:1 aspect ratio and a hole depth of 0.010 inches or below.
Are Microvias Filled or Unfilled ?
Microvias can be unfilled or filled with copper. Some PCB designers use copper to fill the via hole. This depends on the type of microvias. It is important to fill buried microvias with copper, particularly if there is any stacking. If there are any voids left over in the inner area of the via plating, there will be high stress degree along the wall of the via. Therefore, this could result in premature fracture during operation or reflow.
Also, blind vias can be unfilled. This was common in the early days of microvia implementation and fabrication. Also, these microvias used in-pad would require a standard process for filling. The PCB manufacturer will make use of pure copper or a combination of epoxy and copper resin for plating.
The plating processes that don’t integrate some additive in the filler material lead to void formation inside the body of the plating volume. Also, conformal coating can cause uneven copper deposition along the via body. This can result in voiding.
Advantages and Capabilities of Microvia PCB
Saves more space
The diameter of a microvia is smaller than that of plated-through hole and other via types. Therefore, a microvia helps to save more space in a printed circuit board. Also, space is a crucial factor during PCB design. Microvias interconnect several layers with shorter spaces. Therefore, microvias are space-savers for PCB designers.
Lowers the risks of manufacturing defects
Microvia reliability helps to reduce risks of manufacturing defects. Most times, PCB designers use lasers to create microvias in a circuit board. So far, this is the best technique as regards to safety and reliability. Also, the use of microvias prevents any issues during PCB fabrication. Bigger vias can result in issues during fabrication process.
Impacts signal integrity
As regards high density interconnect (HDI), microvias enhance signal integrity in these circuits. Also, large vias are more likely to create much radiation for high-speed circuits. This is because they function as antennas that have high radiation.
Shorter trace lengths
Microvias have shorter trace lengths compared to the trace length of other standard vias. This is a result of the sizes of the microvias and shorter diameters. Also, the shorter size of a micro via improves FR capability and EMS features.
Great reliability
Considering microvia reliability, microvia is much more innovative and efficient than through-hole. Irrespective of the circuit board’s size, microvias are much more reliable than other vias. Microvias are a reliable option for the maintenance of thermo-mechanical reliability of a circuit board. Also, microvia reliability affects the performance and functions of a PCB.
Promotion of future-oriented technology
This another great benefit offered by microvias. Microvias take less space in a PCB. Therefore, they allow the manufacturing of smaller and compact PCBs. Also, compact PCBs are now used in today’s modern devices.
Factors that Affect Reliability of Buried/ Blind Microvias and Stacked Microvias
There have been some arguments about the reliability of blind and buried vias and stacked microvias. There are several factors that determine the exact time to failure. These factors include:
- Copper plating thickness
- Aspect ratio of every microvia in the stack
- The size of any voids that create in the filling material while plating
- Number of microvias in a stack
Some certain criteria that can judge reliability still remain unclear. However, there are a few point people agree on:
- Voiding seems to be a result of failure. However, this depends on the void shape and volume fraction. It is impossible to state that voids always increase the rate of failure for all void sizes.
- Copper wrap plating on the top don’t seem to have a relationship with failure. Instead, the plating thickness at the butt joint indicates failure.
- Small aspect ratios can be more reliable. Also, smaller aspect ratios could survive accelerated life testing.
- As a result of inconsistencies in reliability of staggered vs. stacked microvias, there shouldn’t be more than two microvias in a vertical stack.
Conclusion
Microvias has gained popularity in the PCB industry over the years. It is a great alternative to through-hole vias. Due to the small size of these vias, it allows the production of more complex devices. HDI PCBs require the use microvias for advance functionality. Microvia boards are widely used in the electronics industry. Buried vias and stacked vias are ideal for HDI boards.
