HDI Board vs Blind and Buried Via Boards

Two boards can both be described as high density and still be built by completely different processes. One may need a single laser drilling step on each side of a conventional stack; the other may need two lamination cycles and four laser steps before a single through hole is drilled. The words used to describe them overlap, and the difference in cost and lead time does not.

This article explains what an HDI board is, what blind and buried vias are, and how the order of an HDI construction is counted, so that a specification can say precisely which one is wanted. The distinction matters at the quotation stage, because the two boards are not interchangeable and neither price applies to the other.

What HDI Actually Means

High density interconnect is not a single construction but a family of them, united by the same idea: the connections between layers are made by vias that are much smaller than a conventional through hole, and those vias are formed after the layers are bonded rather than before. That lets the pad and via geometry shrink with the component pitch.

The driving force is the package. A fine-pitch ball grid array has no room between its balls for a through hole and no room under the package for the fan-out that a through hole would need, so the first layer of routing has to escape through small vias that stop at the layer below. Everything else about the construction follows from that requirement.

Blind Vias and Buried Vias Defined

A blind via connects an outer layer to an inner layer and does not pass through the whole board, so it is visible from one side and hidden from the other. A buried via connects two inner layers and is invisible from both outer surfaces, because the layers above and below it were bonded over it after the hole was made and plated.

The practical difference between them is the number of processes each one demands. A blind via requires the drilling to be done at a stage when one side of the target layer is still exposed; a buried via requires the sub-assembly to be drilled, plated and laminated before the outer layers exist. Choosing between them is the subject of the stack selection rules for blind and buried vias.

Why Portable Products Push Toward HDI

Portable products have a fixed small volume and a growing list of functions, so the board area per function keeps falling. Packages shrink to match, and once the pitch falls below roughly a quarter of a millimetre the escape routing no longer fits on the layer the package sits on.

HDI answers that by allowing vias to be placed inside the package footprint and by removing the through-hole clearance that ordinary fan-out would require on every layer. The same construction also shortens the return path, because a via that stops at the layer below creates a much smaller stub than one that passes through the whole board.

First-Order HDI: A Six-Layer Example

The order of an HDI construction counts how many times the stack is drilled with a laser and how many times it is laminated. In a six-layer first-order board the blind vias connect layer one to layer two and layer five to layer six, with the inner layers joined by conventional buried or through holes.

The process is short. The inner core is imaged, the buried vias are drilled and the core is laminated with the outer foils, and then the laser drills the two outer connections before the outer layers are plated and imaged. One laser step per side and one lamination pass are enough, which is why first-order HDI carries the smallest cost premium.

Second-Order HDI: Two Lamination Cycles

A second-order board adds a further set of laser-drilled connections, and the sequence grows accordingly. The buried via is drilled and the inner pair is laminated first, then the first set of laser vias is drilled and the next layers are laminated over them, then the second set of laser vias is drilled, and only at the end are the through holes drilled.

Two lamination cycles and two laser steps per side mean two chances for the registration to drift and two opportunities for the resin to flow where it should not. That is where the cost of the higher order comes from, not from the drilling itself: the extra process steps have to be aligned with each other, and the alignment is measured after the fact. The filling and plating of the vias has to be complete before each lamination, and the way the CAM data is prepared for HDI work reflects the extra stages.

Staggered and Stacked Vias

Within a given order there are still two ways to arrange the laser vias. Staggered construction offsets the via in each successive layer so that no two vias in the stack touch; stacked construction places them directly on top of each other, so the via from layer one to layer two lands on the pad of the via from layer two to layer three.

Stacked vias save routing area, which is why they are common in the densest products, but they are harder to fill and plate without voids, and the stacked column has to be planarised between cycles. Staggered vias are more forgiving and cost less, at the price of occupying two positions instead of one. A design that does not need the space should take the cheaper option. The decision is usually taken once per stack-up rather than per net, which is another reason to settle it before the fan-out is drawn.

What to Specify

A fabrication drawing for an HDI board has to state the order, the via types and their layers, the finished via diameter and the fill requirement. Saying only that the board is HDI leaves the fabricator to choose the construction, and the choice will be the one that meets the drawing rather than the one the design actually needs. The via diameter and the fill requirement are read together, because an unfilled via in a stacked construction traps air that expands in the next press.

The layer stack should be described in the same document, because the order of the construction and the arrangement of the layers in the stack-up are decided together. A stack that puts the fine-pitch devices on the layers with the smallest via span will be cheaper to build than one that forces the laser vias to reach further.

FAQ

Does a board with buried vias have to be HDI? No. Buried vias appear in ordinary multilayer boards where the inner connections are made before lamination, without any laser-drilled blind via. HDI implies the laser-drilled blind via that reaches an outer layer.

Is a higher order always better? No. Each additional order buys routing density at a cost in yield, lead time and price. Most designs are well served by the lowest order that allows the fine-pitch escapes to be completed.

Can an existing through-hole design be converted to HDI? Only by re-routing the fan-out, because the whole point of the construction is that the vias sit where the through holes used to be. It is a new layout, not a stack-up change.