Microvia Reliability And Failure Modes

A microvia is a small hole, usually no more than a hundred and fifty micrometres across, that connects two adjacent layers in a high density board. It is formed by a laser rather than by a drill, and its geometry is measured in tenths of a millimetre, which makes it the most delicate interconnect structure in common use. Its failure modes are correspondingly subtle, and they appear after thermal cycling rather than at the moment of manufacture.

This article explains how microvias are made, which failure modes are seen in service, what drives them, and which design and process rules improve their reliability.

How Microvias Are Made

The dielectric is opened by a laser, either through a copper foil that acts as a conformal mask or directly onto the target pad, and the resulting hole has a conical profile with a wider entrance than base. The hole is then desmeared to remove the resin residue left by the laser, and copper is plated into it, or the hole is filled with a conductive paste and capped with plated copper. The target pad, the capture pad below the hole, is what the plated barrel lands on, and the quality of the interface between the barrel and that pad is what decides the reliability of the structure.

The process window is narrow. Laser energy that is too low leaves resin at the bottom of the hole, which blocks the connection and produces an open circuit that may pass a continuity test but fail later. Energy that is too high damages the target pad and reduces the copper thickness under the via, which weakens the very interface the structure depends on. The subsequent desmear chemistry also has to match the resin system, because a treatment that works on one laminate leaves residue on another.

Failure Modes Seen In Service

The most common failure is a crack in the plated copper at the bottom corner of the barrel, where the barrel meets the target pad. That corner is a stress concentration, and it is also the location where the plating is thinnest, so a thermal excursion that expands the dielectric in the z axis pulls the barrel away from the pad and starts a crack. The crack then grows with each cycle until the resistance of the link rises and eventually opens.

Other modes include separation between the target pad and the underlying layer, also known as pad lifting or land pull out, and interfacial separation between the plated copper and the pad when the desmear was incomplete or the pad surface was contaminated. Voids inside the plated barrel concentrate the current and reduce the copper cross section at the same time, and a stacked structure in which one microvia sits directly on top of another places the weakest points of each layer in a single vertical column, which is the least forgiving arrangement available.

What Drives The Failures

The driving force is the difference in thermal expansion between the dielectric and the copper. Laser drillable materials are often filled with silica to reduce expansion, but the resin still expands several times faster than copper along the z axis, and at the peak of a lead free reflow profile the board sees more than two hundred and forty degrees. A microvia that survives assembly has already experienced one severe excursion, and each subsequent thermal cycle adds damage until the crack reaches the critical size.

The geometry also matters. A higher aspect ratio, meaning a deeper barrel for the same diameter, produces a more constrained structure in which the barrel is less able to flex, and a thick plated barrel resists the stress better than a thin one. The plating distribution along the barrel is not uniform, and the bottom corner receives less copper than the top, which is why the failure concentrates there. Moisture absorbed by the dielectric adds an internal pressure at temperature, and a filled and capped via behaves differently from a plated barrel because the fill supports the structure from the inside.

Qualification And Testing

Reliability is demonstrated on coupons rather than on the product. The interconnect stress test passes a current through a daisy chain of vias and measures the resistance while cycling the coupon between two temperatures, which turns a slow crack into a measurable resistance rise. Thermal cycling between minus fifty five and plus one hundred and twenty five degrees, and a highly accelerated thermal shock test, are the other common sequences, and they are applied to coupons that represent the real stack with the same via geometry and the same materials.

The result is judged by the change in resistance, and the coupons are then microsectioned to confirm where the failure occurred. A crack at the bottom corner, a separation at the pad interface, and a plating void each point to a different corrective action, so the section is as important as the electrical result. The coupon design should include the smallest via, the highest aspect ratio, and the stacked structures that the product uses, because those are the cases that will fail first.

Design Rules That Improve Reliability

The first rule is to keep the aspect ratio low, and a ratio of about one to one between the depth and the diameter is a practical working limit even though the standard permits more. The second is to avoid stacking microvias directly on top of one another where the design allows, and to stagger them instead, so that the stress is distributed through the layers rather than concentrated in one column. Where stacking is unavoidable, the lower via should be filled and capped so that the next via lands on a solid copper surface rather than on an open barrel.

The capture pad geometry is the third rule. The target pad should be large enough that the laser has a margin against misregistration, typically about a tenth of a millimetre larger than the via diameter on each side, because a via that is drilled at the edge of its pad produces a weak and unreliable connection. A microvia placed directly in a ball pad has to be filled, plated over, and planarised, since an open via in the pad traps flux and solder during assembly and can produce a void or an outgassing blowhole. The structures themselves are described under blind and buried via selection, the filling process under via filling in HDI, and the in pad case under via in pad or plated through.

Process Controls

On the fabrication side the laser parameters are the first control, with the pulse energy and the beam profile set to open the dielectric without penetrating the target pad, and the hole inspected by cross section on a sample basis rather than by visual inspection alone. The desmear chemistry is the second, because the residue that remains after an incomplete treatment is invisible until the joint fails. The plating bath is the third, with the throwing power and the additive balance set so that the copper at the bottom corner reaches the specified thickness.

Verification is a combination of cross section, which shows the barrel profile and the copper thickness at the critical corner, and electrical test, which confirms continuity. Where the product is intended for a demanding environment, the coupon results and the section photographs are the evidence that the process is capable, and they should be repeated whenever the laminate, the laser parameters, or the plating chemistry changes.

FAQ

What aspect ratio is safe for a microvia? Around one to one between depth and diameter is a practical working limit. Higher ratios are made in production, but they reduce the ability of the barrel to accommodate z axis expansion and they shift the failure earlier.

Why is the bottom corner the usual failure site? Because the plating is thinnest there, the corner concentrates stress, and the interface with the target pad is where the z axis expansion is transferred from the dielectric into the copper.

Is a filled microvia more reliable than a plated barrel? In a stacked structure it is, because the fill supports the barrel and provides a solid base for the next via. In a single layer structure the difference is smaller, and the choice is driven by the surface finish and planarity requirement.