Types of Microvias: A Guide to Microvias for Technical Professionals
What are Microvias?
As modern technology continues to advance, PCBs (printed circuit boards) are becoming smaller and increasing in density, allowing engineers and designers to use smaller traces and fit even more components onto a single compact circuit board.
Microvias are the perfect solution for getting traces into the inner layers of a multi-layer PCB, allowing higher interconnect densities and larger layer counts.
A microvia, as implied by its name, is a smaller version of your typical mechanically drilled via with a slightly different structure. They commonly have a tapered shape that slopes inward as it transitions layers within a PCB and usually terminates at the pad of the next layer.
Regarding their size, A microvia is any via that has a diameter of less than approximately 150 µm or 6 mils. With new advances in microvia laser drilling technology, some modern microvias can be as small as 15 µm – an incredible feat of engineering. Each microvia will have a low aspect ratio; Anything with an aspect ratio greater than 1:1 may not be officially considered a microvia.
Filled or Unfilled Microvias
Each microvia can either be unfilled or filled with copper, depending on where it is located within the HDI PCB.
When a microvia is buried and connects internal layers, it is essential that the hole be filled with copper or other conductive resins, especially if stacking is used instead of staggering. Any voids that remain in the interior of a plated via will experience high concentrations of stress along the via wall, increasing the probability of premature fracturing.
Types of Microvias
A blind microvia is one that starts at the surface layer of the PCB and terminates one layer below the surface.
Blind microvias could potentially span two layers if the aspect ratio is low enough, but stacked microvias are the ideal solution for multi-layer functionality.
Can either be unfilled or filled: Blind microvias can be left unfilled unless they are being used in-pad.
Buried microvias have a structure very similar to blind vias but do not reach the circuit board surface.
A buried microvia spans two internal layers of a PCB and is most reliable with low aspect ratios and single-layer spans.
This is a filled via: To ensure the connection across the head of a buried microvia is strong, these vias are filled with copper using either an epoxy copper resin or pure elemental copper.
When microvias need to span every layer of a multi-layer, high-density PCB, a designer would quickly realize that using large numbers of blind or buried microvias isn’t ideal for resilience and longevity. The solution is staggered microvias:
Staggered microvias are used as one of the standard ways HDI PCBs span multiple layers.
On every successive layer, the microvia in the current layer is offset from the microvia in the previous layer, creating a staggered formation.
Just as with staggered microvias, stacked microvias are ideal for HDI PCBs and multi-layer designs.
A stacked microvia will contain stacks of buried vias and a blind via on the very top.
This is a filled via: To ensure that the connections between buried vias are reliable, a microvia stack must be plated over and filled with conductive paste. This is done sequentially as each successive layer is plated and deposited.
Microvia Fabrication Processes
Due to the incredibly small size of microvias, precision manufacturing methods such as mechanical drilling or laser drilling are used. The ideal method is determined by the size of the microvia as well as desired precision. With laser-drilled microvias, for instance, there is a lower chance of potential defects compared to mechanically drilled vias.
Once a microvia is drilled and cleaned, the hole can be plated using electrolytic deposition, a sputtering process, or electroless copper plating processes. This plating process ensures that there are no voids, bumps, or other structural defects in the via that can cause high-stress concentrations.
Microvia Drilling Services from Micron Laser
Micron Laser is one of the largest laser service providers in North America and is well known for its PCB laser services and laser part manufacturing expertise.
If you’re looking for premier microvia drilling services, PCB services, or a reliable manufacturer that can perform precision laser drilling and cutting, contact Micron Laser today.