Micro-Vias

A micro-via is a laser-drilled blind via with a finished hole diameter of 0.15 mm or less (per IPC-2226). Unlike conventional through-hole or blind vias which are mechanically drilled, micro-vias are formed by a UV or CO₂ laser that ablates the dielectric and stops on the target copper layer. This allows them to span only a single dielectric layer, making them far smaller than anything achievable with a drill bit.

The ability to use micro-vias is the defining characteristic of a High Density Interconnect (HDI) board. Because they consume so little board area they free up routing channels that allow higher component densities — particularly important when escaping fine-pitch BGAs or placing components on both sides of a dense board.

HDI construction is categorised by IPC-2226 using a shorthand notation. The most accessible HDI structure is 1+n+1, meaning one laser-drilled build-up layer on each side of a conventional n-layer core. For example, a 1+4+1 board has a four-layer mechanically drilled core with one additional laser-drilled copper layer laminated to each surface, giving six copper layers in total. More advanced constructions such as 2+n+2 add further build-up layers but involve additional lamination cycles and significantly higher fabrication cost.

Proteus 9.2 adds a dedicated 1+n+1 board style to the Layer Stack Wizard. Selecting this option alongside the desired core layer count configures the layer stack with the correct sequence of core, pre-preg and build-up layers automatically. The wizard creates both the standard mechanical drill spans for the core and the laser-drilled micro-via spans for the build-up layers in a single operation, ensuring that the resulting construction is manufacturable before any routing begins.

As with all Proteus stackup configurations, the completed HDI layer stack can be saved in a PCB template so that every new HDI project starts with the correct construction already defined.

For designs that require custom HDI constructions beyond 1+n+1 the layer stack grid remains fully editable. Cores can be changed to pre-preg and vice versa, and individual drill spans can be added or removed manually. Proteus performs sanity checks when the dialogue is closed and will warn about configurations that conflict or that could render the board un-manufacturable. We always recommend discussing any custom HDI stackup with your fabricator before committing to it.

The key distinction introduced in Proteus 9.2 is the formal separation of laser-drilled micro-via spans from mechanically drilled spans within the drill span list. Each micro-via span is associated with the specific build-up dielectric layer it traverses, and Proteus applies appropriate manufacturing constraints automatically:

Aspect Ratio — The ratio of drill depth to finished hole diameter. For laser-drilled micro-vias the IPC-2226 guideline is a maximum aspect ratio of 1:1 (depth should not exceed diameter). Proteus flags vias that violate the aspect ratio for the declared dielectric thickness.

Minimum Hole Size — Laser processes have a practical lower bound on the finished hole. The minimum is applied per span based on the declared build-up thickness, warning the designer if a via style is too small to be reliably formed.

These constraints are enforced passively during design: when a micro-via style is placed or when the via style is changed via the Edit Via dialogue, Proteus will check the style against the constraints for the relevant laser span. Violations are also reported by the DRC system so that they appear alongside all other rule violations in the error list and can be navigated to and corrected in the normal way.

Micro-via spans are fully integrated into the Proteus routing environment. During manual routing, double-clicking to drop a via will select the shortest available drill span between the source and destination layers — if a laser-drilled span covers the required layer transition it will be preferred over a longer through-hole span, exactly as with conventional blind or buried vias. The smart via behaviour in the autorouter follows the same logic on a per-net-class basis.

The floating via method (SPACE bar during routing) allows the designer to cycle through destination layers interactively. When micro-via spans are defined in the stackup the build-up layers appear as valid destinations and the correct laser span is selected automatically when the via is dropped. This is particularly useful when escaping fine-pitch BGAs where the build-up layer provides the first escape route before transitioning to the core layers.

The Via Inspector lists micro-vias alongside through-hole and conventional blind and buried vias, with the span type clearly identified. Via styles can be filtered, selected and bulk-edited from the inspector, making it straightforward to review all micro-vias on the board, check their aspect ratios and apply changes such as non-functional pad removal across a group in one operation.

Length Matching uses the declared dielectric thickness to calculate the physical length of a micro-via and add it to the total route length for the net. This ensures that length-matched net groups remain accurate even on HDI boards where the shorter micro-via depth is significant relative to the overall route length.

Fabricators of HDI boards need a precise and complete description of the intended construction. Proteus provides this automatically in its CADCAM output when micro-via spans are defined in the stackup.

The readme.txt file included in all Gerber X2 output sets describes the full HDI construction: the core stackup with mechanical drill passes, each laser-drilled build-up layer with its associated micro-via span, the declared dielectric thicknesses and the aspect ratio and minimum hole size constraints that apply. This gives the fabricator everything needed to quote and produce the board correctly without requiring any supplementary notes from the designer.

Micro-via drill data is exported in a separate drill file from the mechanical drill passes. In the drill plot (Excellon output) each laser span appears with its own tool code and is clearly annotated with the span type, source and destination layer and finished hole size. The distinction between laser and mechanical drill passes is preserved throughout so that the NC drill department can set up the correct machine for each pass without ambiguity.

For designs combining micro-vias with resin-filled through-hole vias, both via types export in their respective dedicated drill files alongside the standard plated through-hole file, keeping each fabrication step cleanly separated.