HDI PCB: Blind and Buried Vias, and When They Are Worth It
An HDI PCB is usually requested for a reason that has nothing to do with fashion: a package is too dense, a product is too thin, or a bus is too fast for the routing area available. The process is the answer to a space problem, and the decision should be made by measuring that problem rather than by defaulting to a more advanced stack-up.
The distinction that matters is simple. A conventional board connects every layer with a plated through hole that passes through the entire stack. An HDI board replaces some of those holes with structures that reach only as far as they need to, which frees the layers they no longer obstruct.
What Blind and Buried Vias Change
A blind via starts at an outer layer and terminates at an inner one, which is what the term describes and what the fabrication capability has to be able to produce. A connection from the first layer to the third consumes no space on the second, so that layer keeps its routing area. A buried via exists only between inner layers and is invisible from the surface, leaving the outer layers completely free.
Both structures also permit smaller holes. A microvia is typically formed by laser and can be well under 0.15 mm in diameter, which in turn allows narrower traces and tighter spacing around the same escape pattern. The combined effect is a board with fewer through holes, more usable routing area per layer, and often fewer layers than the same design would need on a conventional stack.
The Main Process Variants
The simplest variant is often called one step HDI. Blind vias connect the outer layers to the adjacent inner layer only, and the remaining inner layers are still joined by through holes. Lamination is straightforward, the yield is close to a conventional board, and the cost increase is modest. It is the natural choice when a design needs more density than a through hole stack can provide but does not need a fundamental change in architecture.
At the other end is any-layer HDI, where every connection uses a blind structure and no through hole remains. Every layer can reach every other layer, so routing is limited only by geometry rather than by where a hole can be drilled. The price is sequential lamination: each additional build-up step adds processes, and the yield falls accordingly. The cost is typically a multiple of the conventional equivalent, and the lead time is measured in weeks.
Between them sits the substrate-like board, a stack whose line width and spacing approach those of an IC substrate. It is used where the mechanical envelope is extremely tight, such as a camera module or a memory socket, and it inherits both the fine geometry and the strict process control of that class of product.
When the Density Justifies the Cost
Thin products are the first case. A handheld device has a thickness budget, and reducing the layer count while increasing the density per layer is often the only way to stay inside it. High pin count packages are the second: below roughly 0.4 mm pitch, escaping a BGA on a through hole stack becomes progressively awkward, while a microvia can be placed where the routing needs it.
High speed content is the third reason. Routing a dense bus on inner layers against solid reference planes is easier when vias do not consume the space around each pin, and fewer transitions means fewer impedance discontinuities in the path. The fourth reason is architectural: where some inner layers carry only power and ground and never need to reach the surface, buried structures remove surface vias that would otherwise fragment the outer routing area.
Lead Time and the Fabrication Conversation
HDI fabrication is not simply a conventional process with smaller numbers. Each build-up step adds lamination and plating, and the thin dielectric layers demand tighter control of registration and of copper thickness. Lead time for a one step board is measured in a few extra working days; an any-layer stack can take several weeks, and the schedule has to accommodate that rather than discovering it.
The capability of the fabricator is the deciding constraint, and it belongs in the conversation during board manufacturing planning, and it varies more between suppliers on HDI than on conventional boards. The minimum line width, the smallest hole that can be reliably filled, the achievable aspect ratio for a blind structure and the number of sequential laminations the process can support are all specific to the plant. Confirm those numbers against the actual design before the stack-up is frozen, and ask for an impedance coupon and a cross section on the production panel: a microvia that is not fully filled, or a dielectric layer that is thinner than specified, will not appear in a visual inspection.
The layer assignment itself belongs with the rest of the layout review, and it is also worth checking whether the design needs HDI at all. Re-planning the fanout, moving a group of signals to a different layer, or accepting one more layer on a conventional stack occasionally solves the same problem for less money. HDI is the correct answer when the mechanical envelope, the package pitch or the bus density genuinely requires it, and the wrong answer when it is used to avoid a routing exercise.
What HDI Changes for Assembly and Inspection
The benefits do not stop at the layout stage, and neither do the requirements. A finer feature set means the paste deposit and the placement accuracy have to hold a tighter tolerance, since a small pad leaves less room for the deposit to spread. The stencil aperture and the pad geometry become a matched pair rather than two drawings produced by different departments, and the assembly partner needs the HDI stack-up information as part of the documentation, because the thermal mass of a thin build-up board differs from that of a conventional multilayer panel.
Inspection is the other half. A buried via is by definition invisible, and a microvia sits under a surface pad, so the failure modes that matter cannot be seen with optical inspection alone. Cross sections from the production panel and the electrical continuity data on the coupon become the evidence that the layer-to-layer connections are sound, which makes those records part of the acceptance criteria of quality management rather than a laboratory exercise. Where the same supplier handles fabrication and assembly, the stack-up, the coupon data and the assembly process all sit in one place, and a change to any of them can be traced to the boards it affected.
Finally, expect the documentation to be read by more people. An HDI stack-up carries information that a conventional board does not: which layers are laser drilled, which are plated, and how many lamination cycles the board has already been through. Sharing that openly with the partner who assembles the product prevents a well-intentioned change, whether in handling or in reflow, from damaging a structure the design depends on.
FAQ
Is a blind via the same as a microvia? No. A microvia is defined by its size and by the laser process used to form it. A blind via is defined by the fact that it terminates at an inner layer.
How many layers does HDI require? There is no minimum, but HDI is generally applied to multilayer boards where the additional density is needed.
Can a conventional line build an HDI board? Only if it has the laser drilling, the lamination steps and the registration control the stack requires. Most HD boards come from plants that specialise in it.
Does HDI always cost more? Per square metre, yes. Per design, not always, because the same function can sometimes be achieved with fewer layers.
Summary
HDI PCB technology exists to solve density, thickness and package pitch, and it does so by replacing through holes with structures that reach only as far as required. Choose the simplest variant that solves the actual problem, and when the density genuinely demands it, match the stack-up to the fabricator capability rather than to a marketing description. Blind and buried vias are a tool with a price, and the diagnosis should come before the prescription.
