Microvia: Design Rules and Process Limits

Microvias are the feature that makes high density interconnect possible, and they are also the feature with the least tolerance for process variation. Understanding how they are made explains why the design rules around them are stricter than for a conventional through hole.

How a Microvia Is Formed

A microvia connects two adjacent layers and is formed either by laser ablation of the dielectric or by photoimaging, with laser ablation being the common method. The hole is small, typically under a tenth of a millimetre, and it is tapered rather than cylindrical because the laser energy is highest at the surface.

The taper is useful, because it gives the plating a surface that is easier to cover. A cylindrical hole of the same diameter would be far harder to plate reliably. Our HDI via process notes describe the manufacturing sequence.

Plating Coverage in a Small Hole

The plating in a microvia has to cover the bottom of the hole completely, because that is where the connection to the target pad is made. An incomplete coverage produces an open that may pass a continuity test and fail under thermal cycling.

The difficulty is that the chemistry has to reach a very small volume, and the aspect ratio of the hole is measured against its depth rather than against the board thickness. A microvia with a depth greater than its diameter is significantly harder to plate. Our plating thickness notes describe the control.

Stacked and Staggered Vias

Where a connection has to pass through more than one layer, the microvias can be stacked directly on top of each other or staggered so that each one lands on a pad rather than on the via below. Stacked vias give a smaller footprint but concentrate the stress in one column.

The stress concentration is the reason stacked microvias have a lower thermal cycling life. Where the product sees significant temperature excursions, staggering the vias spreads the stress and improves the reliability at the cost of some routing area.

Dielectric Materials and Filled Vias

A microvia is usually filled with a conductive or non-conductive paste and then plated over, so that the next layer has a flat surface to build on. The fill is what makes stacking possible, because a hollow via would leave a depression that the next layer cannot bridge.

The fill material has its own thermal expansion, and a mismatch with the surrounding plating is a source of failure. The choice belongs to the shop, but the design should not assume that any fill will behave identically.

Laser Parameters and Target Pads

The laser removes the dielectric and stops at the copper beneath it, which requires the target pad to be large enough and thick enough to act as a stop. A pad that is too thin can be penetrated, and a pad that is too small leaves the laser with nothing to land on.

The registration between the via and the pad is therefore part of the design rule. A microvia that lands off the pad will remove the dielectric beside the pad and produce an unreliable connection, even when the hole itself is perfect.

Design Rules for Microvias

The rules cover the via diameter, the pad diameter, the capture pad on the target layer, the spacing between adjacent microvias and the distance from a microvia to the board edge or to another feature. Each of them is tighter than the equivalent rule for a through hole.

The capture pad deserves particular attention, because it is what the laser stops on and what the plating bonds to. A pad that is only just larger than the via gives no registration margin, and the yield falls quickly.

Reliability Under Thermal Cycling

The weakest point of a microvia is the interface between the plated copper and the target pad, because the two are joined over a small area and experience the full strain of the thermal excursion. Failures appear as a crack that propagates around the base of the via.

The reliability is improved by a larger capture pad, a thinner dielectric, a staggered rather than a stacked arrangement and a fill material whose expansion is closer to that of copper. Each of those is a design decision.

Inspection and Verification

A microvia cannot be inspected optically, and its small size makes X-ray interpretation difficult. The usual verification is a cross-section from a coupon that carries the same via geometry, examined for plating coverage and for fill quality.

The coupon should be built on the same panel as the product, so that the section describes the product rather than a separate build. Our test coupon notes describe the patterns that are used.

Process Control and Verification

On a design of this kind, microvia is the item that decides how the rest of the board is arranged. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.

The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.

Process Control and Verification

On a design of this kind, microvia is the item that decides how the rest of the board is arranged. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.

The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.

Process Control and Verification

On a design of this kind, microvia is the item that decides how the rest of the board is arranged. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.

FAQ

How small can a microvia be? The practical limit is set by the laser spot and by the plating, and it is generally larger than the theoretical minimum. A via that is smaller than the process can plate reliably will produce a yield problem rather than a density gain.

How many times can a microvia be stacked? More than the reliability of the product usually allows. The count should be chosen from the thermal cycling requirement rather than from the available routing area, and the analysis should treat a stack as a single column.

What does gopcb confirm for an HDI stack? We confirm the via diameter and the capture pad against our process, the number of lamination cycles the stack requires, the fill material and the coupon geometry. Where a stack would exceed our reliable limit we propose an alternative arrangement before the panel is released.