Circuit board technology has gone through significant changes in the past two decades. Currently, printed circuit boards hold superiority over their traditional counterparts due to increased functional performance and efficient design capabilities that they allow in electronic devices.
However, it is still amazing to see how designers are currently packing so much functionality into increasingly compact devices.
This has all been made possible by HDI PCBs, which are just one of the many different types of printed circuit boards.
HDI PCBs incorporate microvias technology in their design to increase the electrical density of the printed circuit boards, enabling multiple functions on a single board!
Here’s everything you need to know about microvias PCBs!
What Is Microvias PCB?
A microvias is a miniaturized version of your standard via technology, albeit with a different structure. They have a conical frustum shape, which slopes inward as the microvias make a layer transition.
It is recommended that the microvias only span one layer for maximum functional efficiency and reliability.
However, engineers frequently stack microvias to enable multiple-layer connectivity. Staggered microvias are preferred over the blind and buried variants to ensure reliability.
Characteristics
Microvias typically have a diameter of less than 150 µm. In addition to that, all types of microvias have at least two common characteristics, which are:
- Low Aspect Ratio: microvias PCB has an aspect ratio of 0.75:1. While a microvias PCB with an aspect ratio of 1:1 or 2:1 can be manufactured, it’s preferred not to over reliability concerns. Additionally, a microvias PCB with an aspect ratio greater than 1:1 also does not meet the criteria set by the IPC.
- Susceptibility to Neck Fractures: microvias PCBs are susceptible to neck fractures due to the plating method. As the copper turns inwards to the conical barrel region, it can induce fractures at the neck due to strong vibrations, thermal cycling, or even mechanical shock.
The Fabrication Process
Laser drilling is the preferred fabrication method for drilling holes in a microvias PCB. In fact, laser technology has enabled microvias size to be pushed down to tiny numbers.
After the holes have been drilled, the printed circuit boards require plating, either done by electrolytic deposition, sputtering, or other copper plating methods.
Additionally, engineers practice caution to avoid generating any dimples, voids, or bumps during the plating process. These can affect the structural integrity of the microvias PCB, causing reliability concerns that reflect poorly on the product design.
Laser-drilled microvias exhibit more minor manufacturing defects than mechanical drilling methods, which can have flaws due to vibrational and shock damage from the drilling. You can learn more about the microvias PCB fabrication guidelines here.
Types of Microvias
There are many different types of microvias depending on differences in diameter, location on the printed circuit boards, and their configuration.
On-surface microvias are typically utilized via-in-pad technology, filled with conductive epoxy, and then plated over using copper. Via-in-pad technology allows for the microvias PCB to be soldered.
Consequently, there are four different types of microvias as follows:
Blind Microvias
Blind microvias span a single layer. They start at the surface layer and terminate as they reach a layer below it. You can even fabricate them using more than one layer if the aspect ratio is kept within the defined range.
However, it’s best to use staggered or stacked microvias for spanning more than two layers. These can either be filled or unfilled.
Buried Microvias
Buried microvias follow a similar structural pattern as blind microvias. However, they are sandwiched between two interior layers beneath the surface – hence the name.
Again, the aspect ratio is kept low to prevent reliability concerns and facilitate the fabrication process.
Buried microvias are filled using a conductive copper-epoxy resin or pure copper; this ensures a solid conductive connection across the microvias head.
Stacked and Staggered Microvias
Stacked and staggered microvias are your average blind or buried microvias employing different conformations.
As microvias have diameters spanning in microns – really tiny sizes – they’re excellent to utilize in stacked applications, where microvias need to be traversed between multiple layers, such as in the case of HDI PCBs.
Stacked microvias comprise stacks of multiple buried vias or a blind via stacked upon buried ones. Stacked microvias need to be filled with conductive material, copper, or conductive resin mixture to ensure firm contact with the next via in the subsequent layer.
Staggered microvias are an alternative to the stacked conformation, and they involve buried and blind microvias that are offset from one another in successive layers. The aspect ratio of the microvias PCB defines which conformation will be used.
What Are Filled and Unfilled Microvias?
Microvias can either be filled or unfilled, depending on the type used. Buried microvias must be filled with copper, especially in the stacked conformation, as unfilled holes will leave voids in the design, which can subject stress across the via walls, leading to neck fractures and appliance failure.
Blind microvias can be left unfilled, although in-pad blind microvias should be filled using a standard filling process. The plating process utilizes additives as a pure copper filling does not efficiently concentrate along the via walls and might lead to void formation. Therefore, a mixture of epoxy resin and copper is typically used in the filling process.
Uses of Microvias PCB
Microvias PCB technology is typically used in high-density interconnect designs to serve various functions and allow interconnectivity. However, there are other applications as well, such as:
- Signal routing
- Stitching vias that offer multiple connections on a plane and provide EMI protection
- Thermal vias to conduct heat out from electrical mounts and components.
Microvias PCB technology allows engineers to save significant amounts of space, allowing components to be packed in dense circuitry spaces.
Reduction in space brings down the cost of manufacturing electrical appliances and allows the designer more creative freedom over the product design features!
Microvias PCB at Hemeixin Electronics
Electronics is one of the world’s leading manufacturers of printed circuit boards. It utilizes different technologies and methods to produce a variety of them, including HDI PCBs, and the engineers at the company are experts in their craft and dedicated to maintaining transparency.
As a result, you can view their detailed fabrication guidelines and product specs for different types of printed circuit boards. Visit their website, and get HDI PCB quote today!
