Microvia Reliability: Stacked, Staggered and Filled

A microvia is a small hole with a thin wall, and its reliability depends on the shape of the wall rather than only on its thickness. The copper that plates a blind hole tapers, and the taper is where the cracks begin.

Why Microvias Are Different

The depth is short, so the plating reaches the bottom easily and the aspect ratio is favourable. The difficulty is the geometry: a laser drilled hole has a tapered wall and a curved bottom, and the copper is thinnest where the wall meets the target pad.

That corner is the stress concentration. A microvia that passes a thermal cycle test has usually been tested with a favourable geometry rather than by luck. Our plating notes describe how the thickness is verified.

Stacked and Staggered

A staggered arrangement places each microvia over the pad of the previous layer, so the load passes through solid laminate between them. It is the more robust arrangement and it costs routing area.

A stacked arrangement places each microvia directly on the one below, so the load passes through a column of copper that ends in the corner of the lowest via. Stacked vias are used where the routing density demands them, and they require a fill and a planarisation between layers. Our HDI notes describe the process sequence.

Filling and Plating

A stacked via must be filled before the next layer is built, or the plating of the next via sits on a void. The fill must be cured and planarised, and the copper that covers it must be continuous.

Where the fill is incomplete, the subsequent plating can crack at the interface. The defect is invisible from the surface and it appears as an open after thermal cycling.

Design Rules

The microvia diameter, the capture pad and the target pad are set by the laser and the registration capability. The landing pad must be large enough that the via lands on it with the registration tolerance of the process.

The number of layers that may be stacked is a process limit rather than a design preference, and it should come from the fabricator for the specific build. Our layer assignment notes describe how the stackup is arranged.

Thermal Cycling Behaviour

The failure mode is a crack around the base of the via, in the copper that connects the via to the target pad. The crack grows with cycles and it starts at the outside of the corner.

The behaviour depends on the board thickness below the via and on how much the laminate moves in the direction of the via axis. A thick board with a thick dielectric below the microvia loads it more. Our reliability notes describe how the load is assessed.

Verification

The verification is a cross section of the via on a coupon from the same panel, showing the wall thickness at the base, and a thermal cycling test on a daisy chain that includes the via structure.

The daisy chain should include the stacked or staggered arrangement used in the design rather than a simplified pattern, because the arrangement is what decides the result. Our test coupon notes describe how the coupon is arranged.

When to Avoid a Microvia

Where the routing allows a through via or a staggered arrangement, the microvia adds process steps and a reliability question for a density benefit that may not be needed.

The decision should be made against the routing requirement rather than against the layer count alone. Our layer count notes describe how the two are compared.

Process Control and Verification

The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.

Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. 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.

Checks Before Release

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.

Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.

Verification and Records

The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting. Where an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record.

Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected.

FAQ

Is a stacked microvia always worse than a staggered one? It is more demanding rather than always worse. With a complete fill and a sound base, a stacked via performs; without them it fails.

How is the base thickness verified? By cross section on a coupon. The surface inspection cannot see the corner where the crack starts.

What does gopcb provide for microvia reliability? We provide microvia and pad rules against the laser and registration capability, stacked and staggered arrangement definition with the fill and planarisation sequence, cross section verification of the base wall, daisy chain thermal cycling of the actual arrangement, and a documented decision where a through or staggered via can replace it.