Blind via and Buried via Production: Process Limits

A blind via reaches only some of the layers, and a buried via connects inner layers without reaching either surface. Both exist to free routing space, and both change the way a board is built: the layers are laminated in stages, the holes are formed between the stages, and every additional cycle brings its own registration and plating constraints.

Why Blind and Buried Vias Are Used

A through hole consumes connection space on every layer it passes, which is wasteful when the connection is only needed between two of them. On a dense board with fine pitch devices, that lost area is often the difference between fitting the routing and not fitting it.

Removing the unused portion of the barrel also improves electrical performance, since a long stub reflects energy at high frequency. The construction is described in general terms in the rules for via and stack selection, and it is the standard approach for high density interconnect boards.

Laser Drilling

Small holes are formed with a laser rather than a drill bit. The beam vaporises or melts the dielectric, so the process depends on the material absorbing the energy rather than reflecting it, and it produces a hole with a defined depth controlled by the energy and the number of pulses.

Because a laser cannot cut through copper easily, the copper above the intended hole is opened first by etching a clearance window of the finished hole diameter. The laser then removes the dielectric beneath it and stops on the copper of the target layer, which acts as the bottom of the hole.

Materials That Suit Laser Drilling

Resin coated copper is commonly used for the outer build-up layers because it contains no woven glass, so it absorbs the beam evenly and does not reflect it. The resin thickness and the copper foil thickness are selected from the range the material is made in.

Resin coated copper is available in higher and lower glass transition grades, and its dielectric constant is generally lower than that of ordinary FR-4, which matters when the layer carries controlled impedance traces. The values should be taken from the specific grade rather than assumed, since the HDI stackup calculation depends on them.

Registration and Tooling

The laser has to find each hole position accurately, so the panel carries registration marks placed on an inner layer rather than on the surface being drilled. Those marks are added during the inner layer imaging and must be identified in the fabrication data.

The artwork for the blind via sites is produced by direct imaging, which also determines the panel size used at that stage. Because the clearance window and the drill position share the same reference, any discrepancy between them shows up as an offset hole or an incomplete connection.

The Build Sequence

The layers below the build-up are processed first using the normal multilayer flow, since they are complete before the outer layers are added. The cores are laminated, the outer edges are routed, and the panel then enters the HDI sequence.

From that point the order is fixed: registration holes are drilled, dry film is applied, the copper above each blind via is opened by etching, the laser drills the holes, through holes are drilled, and the panel enters plating and the remaining steps in the usual way.

Mechanical Drilling for Larger Vias

Where the hole diameter is around 0.2 mm or larger, mechanical drilling can be used instead of a laser. The tool is more economical and more widely available, but the minimum size is limited by the drill bit, and the depth control is less precise than with a laser.

Blind holes produced mechanically still need to stop on the correct layer without damaging it, which requires careful control of the drilling depth and the press thickness. The process is chosen for larger features where the tolerance is achievable.

Plating Restrictions

Blind via plating interacts with through hole plating because the outer surface is common to both. In the normal flow a panel receives one panel plating step and one pattern plating step, and the requirement that the through holes be properly plated constrains what can be done to the outer surface.

Depending on the line width and the board thickness, the outer surface may need to be protected by a film during the blind via plating so that it does not receive additional copper. That choice affects the process, not the design, but the design has to allow for it: narrow outer traces and thick boards both narrow the options.

Aspect Ratio and Filling

The aspect ratio of a blind via is the dielectric and copper depth divided by the hole diameter, and it governs how easily the hole can be plated and filled. A low ratio allows solid filling without voids, while a high ratio narrows the process window significantly.

Filling matters because an unfilled via under a pad traps chemistry and produces an uneven surface. The via filling process is therefore specified with the stackup, and the pad above a filled via must be large enough to cover the filled area.

Standards and Acceptance

Boards of this type are covered by the dedicated HDI standard, which sets the minimum copper thickness on a laser drilled wall and defines when a teardrop is required. Where the pad is only marginally larger than the hole, the teardrop is what preserves the connection if the via is slightly off position.

Acceptance also covers the tolerance between the hole and its target pad, and the amount of barrel that must be present. These are verified by cross section on coupons, because the surface appearance of a filled and capped via says nothing about what happened beneath it.

Design Rules That Keep It Manufacturable

Keep the blind via diameter within the capability of the laser being used, avoid stacking more microvias than the process can fill reliably, and provide enough pad around each one to survive registration error. Pad size, not hole size, is usually the limiting dimension.

The panel itself also needs space. A narrow board leaves no room for the handling and tooling required at several stages, so the outline should provide adequate margin on each side rather than being trimmed to the absolute minimum.

Choosing Through, Blind or Buried

The simplest option that connects the layers should normally win. A through hole is plated in the same step as every other hole, needs no additional lamination and receives no special inspection, so it stays the default wherever the space it consumes is not critical.

Blind vias become worthwhile when a connection must reach an outer layer without crossing the whole board, and buried vias when the connection is entirely internal. Each choice adds process steps, so the stackup should be justified layer by layer rather than adopted as a whole.

Cost and Lead Time

Every build-up cycle adds a lamination, a drilling operation and a plating step, and each of those adds cost and time. The material also costs more than a standard laminate, and the inspection requirement is higher because the critical features are hidden.

Lead time is affected by sequencing rather than by any single step. The layers must be processed in order, so the schedule is longer than the sum of the individual operations, and a change to the stackup after the first lamination cannot be accommodated without restarting.

FAQ

When should a blind via replace a through hole? When routing density demands the space or when the stub of a through hole would degrade the signal. If neither applies, the through hole is cheaper and simpler.

Why is resin coated copper used for laser drilling? Because it has no glass weave to scatter or reflect the beam, so the hole forms cleanly at the intended position and depth.

How many build-up cycles are practical? Each cycle adds lamination, registration and plating steps, so the count is limited by the material’s thermal history and by the accumulated tolerance rather than by the design alone.