Microvia Reliability in HDI Boards
What a Microvia Is
A microvia is a small laser drilled via, usually with a diameter below about a tenth of a millimetre, that connects an outer layer to the layer immediately beneath it. It is the element that makes a high density interconnect possible, because it can be placed inside a pad and it occupies far less space than a through hole. Its small size also means that its geometry, its capture pad and the interface with the target are all scaled down, and the reliability behaviour differs from a mechanically drilled via in ways that the design has to account for.
The Failure Mechanism
The characteristic failure is a separation at the interface between the via and the target pad, which appears after thermal cycling. The laminate expands far more than the copper when the assembly is heated, and the via, which is short and confined, is stretched. The strain concentrates where the via meets the pad, and a crack initiates there and propagates around the interface until the connection opens. The failure is often not visible on the surface and may not be detected electrically until it is nearly complete, which is why the qualification uses continuous monitoring during cycling.
The Design Factors
Several design choices control the strain. A larger capture pad on the target layer spreads the load and reduces the stress concentration; a shallower via has a shorter column and less strain; and a via that tapers, as a laser drilled via naturally does, behaves differently from a straight one. The arrangement matters as well: a stacked microvia, where one via sits directly on the one beneath, concentrates the strain at a single point, while a staggered arrangement spreads it. Where the routing allows, staggering the vias and avoiding a stack improves the reliability substantially, and the design rule should say so.
The Process Factors
The process determines whether the design’s intent is realised. The laser’s energy and its pulse shape determine the via’s profile and the amount of debris; the desmear and the preparation determine the interface’s quality; and the plating’s thickness and ductility determine how much strain the copper can absorb. The target pad’s surface condition matters because the interface is where the failure occurs, and a pad that has been oxidised or contaminated produces a weak bond. The via’s bottom should land on a pad with an adequate capture and should not penetrate it, which is a process control that the design’s rules assume.
Qualification and Test
The qualification is by thermal cycling with continuous electrical monitoring, using a coupon that contains the smallest via and the most demanding arrangement the product uses, including a stacked via if one is present. The coupon should be built with the production process, and the cycles to failure should be recorded rather than only the pass or fail result. A comparison between a stacked and a staggered coupon shows the design’s effect, and a comparison between two processes shows the process’s effect. The result is a number of cycles to failure, and the design’s acceptance should be expressed against the product’s requirement rather than against a generic standard.
Repair and Detection
A failed microvia cannot be repaired, and it is difficult to detect before it fails. An intermittent connection may pass a continuity test, and the crack may only open when the board is warm. Where a product has a field failure that appears after a period of use and the failure is intermittent, a microvia is a candidate, and the analysis requires a section at the suspect location. The detection is therefore retrospective, which is the reason the qualification is done at the design stage with the coupon rather than being left to the field.
Practical Rules
The rules that serve most designs are the following. Use the largest capture pad the routing allows. Keep the via as shallow as possible and avoid a via that passes through more than one dielectric layer. Prefer a staggered arrangement to a stacked one. Specify the plating’s thickness and its ductility. Qualify with thermal cycling and continuous monitoring on a coupon that represents the worst case. And keep the design’s vias within the fabricator’s demonstrated capability, since a via that is at the limit of the process will show a reliability behaviour that the design’s margin cannot cover.
The Interaction with Via in Pad and Via Fill
A microvia is often used inside a pad so that a component can be placed over it, and the fill and the cap then become part of the reliability question. A filled via with a copper cap presents a flat surface and a continuous copper path, which is the most robust arrangement but also the most process intensive. An unfilled via under a component traps flux and can outgas during reflow, which produces voiding in the joint above it. The design should therefore state whether the via is filled and plated, filled and capped, or tented, and the choice should follow from what has to be placed over it rather than from the fabricator’s default.
FAQ
Where does a microvia fail? Usually at the interface with the target pad, from the strain of thermal expansion during cycling.
Why is a stacked microvia less reliable? The strain concentrates at a single interface rather than being distributed between two.
What design change helps most? A larger capture pad and a staggered instead of stacked arrangement, where the routing allows.
How is reliability qualified? By thermal cycling with continuous monitoring on a coupon that contains the worst case via.
Can a failed microvia be detected early? Rarely, since the connection may measure continuous until the crack is nearly complete.
Conclusion
A microvia’s reliability is set by its capture, its depth, its arrangement and its plating, so design within the process and qualify with a coupon. Test the worst case. HDI design belongs to PCB design and layout, the laser and plating processes are described under PCB manufacturing, and the qualification is part of quality management. HDI boards are first built during prototype PCB assembly in 2026.
