Stacked Microvias: Structure, Process and Reliability
When a board runs out of room, the vias are usually the first thing to blame. A through-hole occupies every layer it passes through, so a dense design ends up paying for routing area it cannot use. Stacked microvias solve that by making the layer transition as small as physically possible, and then placing several of them directly on top of one another.
What a Microvia Is
A microvia is a small hole, typically under 150 micrometres in diameter, formed by laser drilling rather than by mechanical drilling. Unlike a through-hole, it penetrates only the dielectric between two adjacent layers, which is why it can be so small: the aspect ratio is short, and the hole does not have to be drilled through the full thickness of a panel.
The short barrel has an electrical benefit as well as a spatial one. A via contributes inductance and capacitance, and the unused portion of a through-hole barrel acts as a stub that resonates at some frequency. Removing the stub removes that resonance, which is why microvias improve signal integrity at high frequency.
Stacked Versus Staggered
The simplest high-density structure is the staggered microvia, where a connection from layer one to layer two, then from layer two to layer three, is made with holes that are offset from one another. Each via lands on a separate capture pad, so the structure is easier to plate and easier to inspect.
A stacked structure places the microvias directly on top of each other, sharing a single capture pad at each interface. That removes the offset pads, which frees routing area and shortens the connection, but it also concentrates the mechanical stress of every layer transition at one vertical column. The trade between density and reliability is the central design question.
Why Stacking Is Used
The first reason is space. Removing the intermediate capture pads frees significant area on the inner layers, which often allows the same circuit in fewer layers or a smaller outline. On a dense design that saving can pay for the increased processing many times over.
The second reason is electrical. Stacking shortens the total path and eliminates the pad-to-pad transitions that add impedance discontinuities, which matters most in RF and high-speed digital designs where a via is often the weakest point in the channel.
The Fabrication Sequence
Production is inherently sequential. The core layers are imaged, etched and laminated; microvias are then laser drilled through the outer dielectric to the layer beneath, desmeared and plated; and the next layers are laminated on top before the process repeats. Each cycle adds drilling, plating and inspection steps.
Alignment between cycles is the critical control. Every subsequent laser drill must land on the capture pad created in the previous cycle, and the tolerance stack accumulates with each lamination. The capture pad cannot simply be made small, because it must accommodate the accumulated registration error as well as the hole itself.
Copper Filling and Capping
A stacked via structure must be filled before the next layer is laminated on top of it, or the void left inside the barrel will cause delamination or a blowout during the press cycle. Filling is done with conductive copper or with a non-conductive paste followed by plating over the top.
Filled and capped vias also support via-in-pad designs, where the via sits directly under a component pad. That practice removes the short trace that would otherwise connect the pad to the via, which improves the electrical performance but requires the fill to be flat and the cap to be plated to a uniform thickness.
Design Rules
The microvia aspect ratio, meaning the drilled depth divided by the diameter, should stay around 0.75 to 1.0 for reliable plating in production. That constrains the dielectric thickness between the layers being connected, which in turn constrains the achievable trace widths for a given impedance on those layers.
The capture pad diameter needs to cover the hole plus the drill registration tolerance plus the pad-to-layer registration tolerance, and it should be agreed with the fabricator rather than assumed. Stacked structures also need a solid landing area on the layer below, since a via landing on a trace rather than a pad creates both a stress concentration and a reliability risk.
Reliability and Test
The failure mode to design against is barrel cracking under thermal cycling. Because the copper in the via and the dielectric around it expand differently, they are governed by different thermal expansion behaviour, and a stacked column concentrates that mismatch at one point rather than distributing it across several staggered transitions.
Verification uses thermal cycling with electrical continuity monitoring, and interconnect stress testing, which applies repeated thermal excursions and measures resistance change through the via chain. Cross sections from a test coupon show plating thickness and fill quality directly, and they remain the most convincing evidence that the process is under control.
Cost and Where to Stop
Every stacked microvia adds drilling, plating, filling and inspection to the process, and each lamination cycle multiplies the risk. The structure also usually forces sequential lamination, which raises the cost of the whole board rather than only the vias.
The practical guideline is to stack only where the density or the electrical performance genuinely requires it, and to use staggered microvias elsewhere. Consolidating connections onto fewer stacked columns and keeping the deepest stacks in the least mechanically stressed regions of the board reduces both cost and reliability risk at the same time.
Where Stacked Microvias Sit in a Stackup
They are almost always confined to the outer layers, connecting the surface to the first or second inner layer. Deeper connections are normally made with plated through-holes or with buried vias, because pushing a stacked structure deep into the board multiplies the lamination cycles without a proportionate gain in density.
A common arrangement is a ten-layer board with stacked microvias on layers one to three and on layers nine to ten, a buried via core in the middle, and plated through-holes only where a connection genuinely has to cross the entire stack. The inner layer routing is then planned around the blind and buried via structure rather than adapted to it afterwards.
That ordering matters because the via structure determines where inner layer routing is blocked, and a layout that ignores it discovers the blockage only when the routing is nearly complete and the stackup can no longer be changed cheaply.
It is also worth agreeing the structure with the fabricator before the layout is released. The pad diameters, the dielectric thicknesses and the number of sequential cycles all depend on that fabricator process capability, and a structure that suits one line may sit outside the window of another.
FAQ
When should a design use stacked rather than staggered microvias? When the inner layer routing area is the limiting factor or when the electrical path length matters. Otherwise staggered vias cost less and are more tolerant of thermal stress.
Do stacked microvias need to be filled? Yes. Any void left inside a via that will be laminated over becomes a delamination risk, and filling is also what makes via-in-pad construction possible.
How many layers can be stacked reliably? Two or three stacked microvias per column is common in production. Deeper stacks are possible but concentrate stress further, so they should be justified by a specific requirement and verified by test coupon.
