Microvia Reliability Design Guide
A microvia is a small hole, usually below 0.15 mm in diameter, that connects two adjacent layers in a high density board. It is drilled with a laser rather than with a mechanical bit, and it is the feature that allows a fine pitch device to be routed at all. Microvia reliability is therefore a central question for any product that uses a high density interconnect, and the failure modes are different from those of a conventional through hole.
What a Microvia Is
A microvia connects two adjacent copper layers and has a diameter that is typically between 0.05 and 0.15 mm. It is formed by a laser that ablates the dielectric down to the target pad.
The depth of the hole is only one dielectric layer, so the aspect ratio is low, which is what makes the small diameter possible. A depth of 60 microns with a diameter of 100 microns gives an aspect ratio well under one.
The via is filled and plated over, and the next layer is built on top of it. The stack is therefore built up in sequence, with a lamination and a drilling step for each layer pair.
The construction and the ways of stacking the vias are described in the blind and buried via guide and in the HDI method description.
Laser Drilling
The laser removes the resin and the glass by vaporising them, and the process stops at the copper pad below because copper reflects the beam. That self limiting behaviour is what makes the process practical.
The pad below has to be large enough to catch the beam and to survive the small amount of overdrilling. A pad that is too small gives a hole that misses the target or a wall that is thin on one side.
The dielectric has to be formulated for the laser, since a glass fabric that is too thick or too coarse does not ablate cleanly. HDI materials use a finer glass or a resin coated copper foil.
The hole wall is left with a residue that has to be removed before plating, and the desmear step is part of the reliability of the via. A residue that is left behind becomes a void at the interface.
Plating and the Fill
The barrel is plated with copper, and the plating has to cover the wall evenly including the bottom corner. A thin deposit at the corner is where the thermal cycle crack begins.
A filled via is plated and then filled with a conductive or a non conductive paste so that the next layer can be laminated over it without a dimple. An unfilled via leaves a depression that traps resin and weakens the layer above.
The fill has to be cured and planarised, and the surface has to be flat enough for the next lamination. A bump or a dimple propagates upward through the stack and appears as a defect several layers later.
The process is described in the via filling material, and the quality of the fill is one of the strongest influences on the reliability of the finished board.
Aspect Ratio Limits
The aspect ratio of a microvia is the depth divided by the diameter, and the plating process sets the limit. A low ratio gives an even deposit, while a high one gives a thin wall at the bottom.
The limit for a laser drilled via is usually around one, and a design that exceeds it forces the shop to use a longer plating time and a chemistry that throws better into a small hole.
The dielectric thickness therefore sets the minimum diameter, and a designer who wants a smaller via has to use a thinner dielectric. The two numbers are chosen together rather than separately.
The same reasoning applies to the through holes that connect the whole stack, and the aspect ratio guide gives the limits that a shop can hold in production.
Failure Modes
The first failure is a crack at the corner between the barrel and the target pad, caused by the expansion of the dielectric in the thickness direction during a thermal cycle.
The second is a separation between the fill and the barrel, which appears as a void that grows with each cycle. The two are related, since both start at a weak point in the plating or at the interface.
The third is a crack between the stacked vias, where a via is placed directly on top of another without an intervening copper layer. The stack concentrates the stress at the junction.
The failures are invisible from the surface, so they are found by a resistance measurement on a daisy chain coupon rather than by inspection. A coupon with a long chain of vias is the standard test structure.
Design Measures
Keep the aspect ratio low, which means using a thin dielectric for a small via and accepting a higher layer count to achieve the routing.
Stagger the vias where the space allows, so that a via lands on a copper layer rather than directly on another via. A staggered stack spreads the stress and is more reliable than a stacked one.
Increase the target pad size, because a larger pad gives a better corner geometry and a stronger junction. The pad is often enlarged beyond the minimum that the drill requires.
Keep the via away from the edge of the board and from a high strain area, since the mechanical stress from handling and from the panel break adds to the thermal stress.
Practical Rules
Choose the via diameter and the dielectric thickness together, and check the aspect ratio against the capability of the shop before the stackup is released.
Prefer a staggered stack to a stacked one where the routing allows it, and enlarge the target pad as far as the space permits.
Test with a daisy chain coupon that has the same via geometry as the product, and record the cycle count with the design.
Keep every special requirement in the fabrication notes, since a microvia stack is a process commitment rather than a drawing detail.
Process Control and Verification
On a design of this kind, thermal cycling is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.
Process Control and Verification
On a design of this kind, thermal cycling is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.
FAQ
What diameter is a microvia? Typically between 0.05 and 0.15 mm, and the diameter is limited by the thickness of the dielectric that it passes through.
Why do microvias fail in thermal cycling? Because the dielectric expands in the thickness direction and the barrel at the corner concentrates the strain. The crack starts at the corner and grows across the wall.
Is a stacked via less reliable than a staggered one? It is, because the junction between two vias concentrates the stress. Staggering the stack gives a copper layer between the vias and spreads the load.
