Microvia PCB Technology: Structures, Laser Drilling and Cost
The Smallest Hole on the Board Does the Most Work
As products have shrunk and their silicon has become denser, the conventional through-hole has become a bottleneck. It occupies routing area on every layer it passes through, it adds stub inductance that degrades high speed signals, and on a fine pitch package there simply is not enough room to place one per net. The microvia is the answer that made high density interconnect practical, and it is now the defining technology of modern high end board design.
This guide covers what a microvia is by definition, the structural variants and what each is good for, the stackup and design rules, the manufacturing process, the advantages and the cost bands.
What Counts as a Microvia
IPC definitions are specific and useful. A microvia has a diameter of 150 microns or less, commonly quoted as 6 mil, an aspect ratio no greater than 1:1, and it is formed by laser drilling rather than mechanical drilling. Those three constraints together define a process class, not just a hole size. A 100 micron hole drilled mechanically in a thick panel is not a microvia in the sense that matters; the technology is defined by the combination of small diameter, shallow depth and laser formation.
Microvias are used in HDI designs, ultra-thin boards, high speed boards and high frequency boards. Wherever fine line routing, dense component fanout or controlled impedance in a small area is required, microvia technology is what makes the design feasible.
The Structural Variants
Blind microvia. Connects an outer layer to the adjacent inner layer. This is the standard single-stage HDI structure and the one most products use.
Buried microvia. Located entirely between inner layers, not exposed at either surface.
Stacked microvia. Multiple microvias stacked vertically on top of one another to achieve very high interconnect density. It is common in the most demanding consumer products, and it is the structure that carries the greatest reliability risk under thermal cycling because stress concentrates at the stacked interfaces.
Staggered microvia. Microvias offset between layers rather than stacked directly. This distributes stress and improves structural reliability at the cost of some routing density.
Single-stage versus multi-stage. Single-stage microvia construction is lower cost and more reliable. Multi-stage construction delivers higher density but requires more lamination cycles, tighter process control and correspondingly higher cost.
Stackup and Structure
Microvia boards are built with sequential lamination, in which the core is fabricated first and build-up layers are added one stage at a time. Common constructions include four layer HDI with blind microvias, six to ten layer multi-stage microvia boards, and hybrid structures combining microvias with buried vias. Base materials include high-Tg FR-4, low loss high speed laminates and ultra-thin dielectric layers of 100 microns or less.
Thin dielectric is what allows a microvia to meet the 1:1 aspect ratio rule, and it is also what makes the laser drilling and plating steps more delicate. Design review here is worth doing properly: the interaction between stackup, microvia type and routing density determines both yield and cost, and it is best settled before fabrication rather than after. Our overview of HDI PCB capability describes the construction families in more detail.
Design Rules
Five points carry the design risk. Aspect ratio must stay at or below 1:1, which constrains the dielectric thickness for a given via diameter. Via-in-pad structures need a fill and cap process, because an open microvia in a pad will wick solder away from the joint; the design rules for that are covered in our notes on SMT assembly for fine pitch packages. Impedance control must be maintained through the microvia transitions, which means the stackup has to be designed for the target impedance rather than adjusted afterwards. Stackup symmetry prevents warp during sequential lamination. And the construction should be checked against the relevant IPC standards for high density interconnect and rigid board performance, which establish the acceptance criteria the supplier will build to.
Manufacturing Process
Microvia fabrication is materially harder than conventional multilayer work. The sequence runs through laser drilling with ultraviolet or carbon dioxide sources depending on the material and via size, desmear to prepare the hole wall, chemical copper deposition to create a conductive seed layer, electroplating or via filling, sequential lamination, and then inspection with automated optical inspection, X-ray and reliability testing. A failure at any one of those steps produces voids, cracks or opens that may not be visible until the board is in service, which is why process qualification and inspection depth matter more here than at any other board class.
The Technical Advantages
Four benefits justify the cost. Routing density rises substantially, because a microvia consumes a fraction of the area of a through-hole and blocks none of the layers it does not connect. Signal integrity improves, since shorter interconnect paths reduce reflection, crosstalk and loss, and removing the through-hole stub removes the resonance that limits high speed channels. Reliability and thermal behaviour can improve, because a well designed microvia structure conducts heat between layers and can be arranged to distribute mechanical stress. And design flexibility increases, enabling via-in-pad, fine pitch ball grid array fanout and complex multilayer routing in a compact outline.
Where Microvia Boards Are Used
Smartphones and consumer electronics. Wearables and IoT devices. 5G communications equipment and AI hardware. Automotive electronics and ADAS systems. Medical imaging and portable medical devices. All of these combine small size, high density and demanding signal performance, which is precisely the combination that microvia construction serves.
Cost Structure
Cost is driven by layer count and lamination cycle count, the microvia type, laser drilling density, material selection and order quantity. Reference bands in US dollars illustrate the shape of the market. A four layer HDI microvia board typically costs 25 to 60 per board at prototype quantity and 5 to 12 per board in production. A six layer microvia board runs 45 to 120 at prototype and 8 to 20 in production. An eight to ten layer stacked microvia board runs 90 to 220 at prototype and 15 to 40 in production. High end HDI with filled vias and fine pitch features runs 150 to 350 at prototype and 25 to 60 in production.
Two rules follow from that. The first is that cost scales with the number of lamination cycles far more than with layer count alone, so stackup optimisation is the highest leverage cost decision available. The second is that microvia cost falls steeply with volume, so a design that is expensive at prototype may be entirely reasonable in production. Our notes on custom PCB pricing explain how these fabrication factors translate into a quotation.
Selecting a Manufacturer
Five capabilities define a credible supplier for microvia work. Mature HDI and laser drilling experience, rather than occasional projects. In-house inspection including X-ray and cross-section analysis, because microvia quality cannot be judged from the surface. IPC certification and demonstrated yield control. Engineering support with meaningful DFM feedback, which is where most avoidable cost is removed. And stable volume production, since a design proven at prototype still has to yield in production. The wider process context is described under PCB manufacturing, and boards that will be populated benefit from pairing this with assembly testing capability.
Questions Engineers Ask
What is the minimum microvia diameter? Typically 75 to 150 microns, depending on the laser system and material.
Are microvias more reliable than through vias? With sound design and process control they offer better electrical and mechanical performance, but stacked microvias need explicit reliability validation.
How many stages should a design use? Single or double stage constructions are more reliable and cheaper. Multi-stage builds are justified only when routing density cannot be achieved otherwise, and they should be verified by thermal cycling test.
When is a microvia mandatory? When the product requires miniaturisation, high speed signal performance or routing density beyond what conventional through-hole construction can provide.
Outlook
Microvia technology continues to shrink. Smaller diameters below 75 microns, higher layer counts with ultra-thin dielectrics, and deeper integration with IC substrates and advanced packaging are the visible directions. As AI hardware, automotive electronics and high speed communications expand, microvia capability will remain one of the defining competencies of a high end PCB manufacturer rather than a niche specialty.
Summary
Microvias are what allow a modern dense, fast product to fit into the space available. The technology is defined by small diameter, shallow aspect ratio and laser formation, and it appears in blind, buried, stacked and staggered forms with quite different reliability and cost characteristics. Success requires stackup design that respects the 1:1 aspect ratio rule, via-in-pad fill and cap where pads must remain solderable, impedance controlled transitions, and a manufacturer with genuine HDI experience and the inspection equipment to prove the quality of what cannot be seen from outside.
