Stacked Microvias: Design Rules and Reliability

What a Microvia Is

A microvia is a very small plated hole, conventionally defined as no more than 0.15 millimetres in diameter, that connects two adjacent layers of a board. It is produced by laser drilling rather than by mechanical drilling, because a mechanical bit of that diameter cannot survive the process economically, and it is usually formed in the thin dielectric between two copper layers rather than through the full thickness of the board.

Because the hole is shallow, its aspect ratio, the ratio of depth to diameter, is low, which is what makes it possible to plate reliably. That is the whole point of the microvia: it gives the designer a connection between layers that costs very little board area and can be placed inside a component pad, while remaining a plated hole that the plating process can fill.

Stacked and Staggered

Where a connection has to pass through more than one dielectric layer, two arrangements are used.

Staggered. Each microvia lands on a copper pad that is offset from the one below, so the stack steps sideways. The pad between the vias acts as a landing pad and distributes the stress. Staggered microvias are the more forgiving construction and are the default where the routing allows.

Stacked. The microvias sit directly on top of one another, sharing a pad at each interface. This consumes far less area, which is why it is used in the dense escape regions under a fine pitch package, and it also shortens the vertical connection. The price is mechanical: the load path is continuous through the stack, so the stress from thermal expansion concentrates at the copper to dielectric interface rather than being spread sideways.

Stacked and filled, plated over. Where a via sits in a pad, the hole is filled with a conductive or non-conductive paste, planarised and plated over, so that the pad remains flat and the via does not draw solder away from the joint. This is the construction that allows the densest routing, and it adds an extra process sequence to the build.

Why Stacked Microvias Are Used

• Escaping a fine pitch package. Thousands of connections in a small area cannot be routed out on the surface, so the signals drop through stacked microvias to the layers below in the smallest possible footprint.
• Via in pad. Placing the via inside the component pad frees the routing channel around it. It requires the fill and planarisation step, and it removes a stub from the connection.
• Any layer interconnect. The most advanced construction, where a via can be placed between any two adjacent layers, which removes the constraint that a signal must return to the surface to change layer.
• Layer reduction. A stack built with stacked microvias can carry the same routing in fewer layers than one built with mechanical through holes, which is the trade that justifies the extra process steps.

Reliability

The reliability question is whether the copper in the microvia survives the thermal cycles of assembly and operation. The failure modes are a barrel crack, a separation at the interface between the via and the pad, and a delamination of the dielectric around the stack.

Four factors govern the outcome.

The dielectric. The resin between the layers expands more than the copper when it is heated, so it pulls on the via. A resin with a lower expansion and a higher glass transition temperature reduces the strain.

The via geometry. A lower aspect ratio, a larger landing pad and a smooth barrel all help. So does the offset of a staggered stack, which converts a tensile load into a shear load spread over two interfaces.

The plating. The copper in the barrel has to be uniform and ductile. A thin or brittle plating fails first, and the plating quality is checked on a microsection and in a thermal cycling test.

The assembly process. A board with stacked microvias is baked before reflow, because absorbed moisture turns to steam and delaminates the stack. The reflow profile is controlled, and the number of reflow cycles is limited.

The industry practice is to qualify the construction rather than the individual board: a test vehicle with the same stack and the same via arrangement is built and thermally cycled, and the number of cycles to failure is measured. That number, not the visual appearance of the board, is what the design is qualified against. Our notes on quality management describe the qualification records that should accompany the product, and our notes on PCBA testing cover the inspection that detects the defects.

Design Rules

Keep the microvia inside its pad. The landing pad has a minimum annular ring that the fabricator defines, and a via placed too close to the pad edge can break out during laser drilling.

Never place a microvia directly on a through hole. A mechanically drilled hole and a laser drilled microvia have different tolerances, and stacking one on the other creates a cumulative registration error that no plant can hold.

Limit the stack height. Most fabricators will not stack more than two or three microvias in a column without the reliability data to support it, and some will not stack at all. The limit is a process and reliability matter, not a layout preference.

Fill and plate over via in pad. A via inside a pad that is not filled will wick the solder paste during reflow and leave a void in the joint. If the via cannot be filled, move it out of the pad and accept the routing penalty.

Plan the stack with the fabricator. The number of lamination cycles, the dielectric thicknesses and the via arrangement are all fixed by what the plant can build and qualify. Our notes on PCB manufacturing describe the sequential lamination and laser drilling that produce the structure, and our notes on PCB design and layout cover the layout that goes with it.

Cost

A microvia adds process steps rather than material cost. Each lamination cycle, each laser drilling operation, each plating sequence and each inspection adds to the price, and the yield falls as the number of steps rises. A board with stacked microvias and via in pad filling typically costs several times an equivalent board built with mechanical vias, and the multiple grows with the layer count and the number of sequential lamination cycles.

The justification is area. Where a design cannot fit in the available space with mechanical vias, the microvia construction is the difference between a product that exists and one that does not, and the comparison is against the alternative of a larger board or a different package rather than against a cheaper board. Our notes on PCB capabilities describe the process window that applies.

Inspection

Microvias cannot be inspected optically from the surface, so the verification is done in three ways. The inner layers are inspected before they are buried, since a defect inside the stack cannot be repaired. A microsection is taken from the panel to measure the plating thickness, the barrel shape and the interface between the via and the pad. And X-ray inspection is used on the finished assembly where the via is under a component and affects the solder joint.

Where via in pad is used under a ball grid array, X-ray with automated void analysis is standard, because a void in the thermal pad raises the junction temperature of the device and the electrical test will not reveal it.

FAQ

What is the difference between a microvia and a normal via? Size and method. A microvia is at most 0.15 millimetres in diameter and is drilled with a laser, and it normally connects two adjacent layers rather than passing through the board.

Are stacked microvias less reliable than staggered ones? They concentrate the thermal stress into a single column, so they are less forgiving and are qualified with thermal cycling data rather than assumed to work.

Why fill a microvia? To keep the pad flat so that solder paste does not wick into the hole, which is essential where the via sits inside the pad of a fine pitch component.

How much more does the construction cost? Several times an equivalent board with mechanical vias, because of the extra lamination, laser drilling, plating and inspection steps and the lower yield.

Conclusion

Stacked microvias are what allow a dense package to be routed out of a small area, and they are a qualified construction rather than a layout trick. Keep the via inside its pad, avoid stacking on a mechanical hole, limit the column height to what the fabricator can support with data, fill and plate over any via that sits in a solderable pad, and manage moisture before reflow. The area saved is real, and so is the process discipline required to keep it reliable.