Microvia Plating Reliability: Filling and Cracking
A microvia is a small hole, typically 100 micrometres or less in diameter, formed by laser and plated to connect adjacent layers. Its reliability depends on how completely it is filled and how well the plating adheres to the wall and to the target pad below. Both properties are difficult to judge from the surface, so microvia control is largely a matter of sectioning and of process discipline. A microvia that looks perfect from above can hide a thin wall or a void, and no optical method will reveal either one.
As build-up counts rise, the number of microvias per board rises with them, and each one is a potential open or a potential latent crack. The inspection strategy therefore combines a small destructive sample with process parameters and electrical continuity checks on the coupons that are built alongside the product.
How a Microvia Is Formed
The hole is produced by a laser, most often a ultraviolet source, that ablates the dielectric down to the copper target pad. The laser energy has to remove the dielectric without damaging the pad, which sets both the pulse parameters and the number of pulses used. Energy that is too high scores the pad and creates a weak interface, while energy that is too low leaves dielectric at the bottom of the hole and produces an open.
The remaining dielectric at the bottom must be removed completely, but the copper beneath it must not be pierced. That narrow window is why laser parameters are qualified per material and per thickness, and why any change of dielectric requires a fresh parameter set. A change of copper foil roughness or of pad plating is equally significant, because both alter the way the laser couples into the target.
Via Filling and Planarisation
After the laser step the via is plated, and the objective is a solid copper fill rather than a thin lining. A completely filled via supports the next layer without a dimple and avoids the entrapment of plating solution inside a partially closed hole.
Filling is achieved by a combination of chemistry and current profile. Additives that accelerate deposition in the small volume of the via, together with a stepped current waveform, produce bottom-up growth. A simple direct current usually gives a conformal lining with a seam in the middle. The additive system is specific to the via geometry, so a bath that fills a 75 micrometre via well may leave a seam in a 150 micrometre one.
Target Pad and Interface Quality
The interface between the plated via and the target pad is the weakest point in the structure. Any residue left by the laser step, any oxidation of the pad, and any inadequate pre-treatment will reduce adhesion exactly where the current has to pass.
The target pad should also be large enough to contain the via with a defined annular margin. A via landing at the edge of a pad creates a thin section that cracks under thermal cycling, and the defect is invisible until the section is prepared. Annular margin should be checked on the artwork as well as in section, because a design that places a via at the pad edge cannot be rescued by process control.
Plating Thickness and Distribution
Plating thickness on the via wall is normally specified between 12 and 25 micrometres, measured at the mid-wall rather than at the surface. The distribution across a panel is as important as the average, because a via in a low-current-density area can be well below the specification while the panel average looks acceptable.
Current density and bath agitation control that distribution. Thickness should be checked on coupons at several panel positions, and the results compared with the plating current density record for the same lot. Where the distribution is poor, agitation and shield design are examined before the total current is raised, because raising the current makes an uneven distribution worse rather than better.
Failure Modes and Their Signatures
The two dominant failure modes are the open via, where the plating does not connect to the target pad, and the cracked via, where the wall separates under thermal stress. Both begin at an interface rather than in the bulk of the copper. An open is usually a process escape and appears at electrical test, while a crack develops over time and appears in the field. The two have different owners, which is why the electrical limit and the section schedule have to be defined together.
A third mode is the partially filled via, which traps plating chemistry and later releases it during reflow or in a humid environment. It shows up as a void or a dark region in section rather than as a resistance change, which makes it a quality issue rather than an immediate electrical one. Entrapped chemistry can also cause a slow rise in leakage current, which is why a partially filled via is treated as a reliability risk rather than as a cosmetic defect.
Incoming Material and Coupon Strategy
The dielectric and the copper foil both influence the way a microvia forms, so a change of either should be treated as a process change. Foil roughness affects how much laser energy couples into the pad, and resin chemistry affects how cleanly the dielectric ablates.
A coupon built alongside the product is the cheapest way to monitor the process, provided it carries vias of the same diameter and at the same density as the densest area of the board. A coupon that is easier to process than the product will report a yield the product never reaches.
Microsection Method
Microvias are inspected by preparing a polished cross section through the centre of the via. The plane must pass through the axis, because a section taken slightly off centre makes a sound via look thin and a marginal via look open. Locating the centre accurately is the main skill in the method.
Etching after polishing reveals the grain structure and the interface with the target pad. Filling defects, seams and thin wall sections all become visible, and the results should be recorded with the same discipline as any other via process measurement.
Process Control and Monitoring
Routine control combines a periodic section, an electrical continuity coupon and the laser and plating parameters recorded for each lot. The section is the only method that shows fill quality, so it cannot be replaced by electrical testing alone. Continuity coupons confirm that the connection exists, but only a section shows how much copper is actually carrying the current.
Sampling should cover the panel corners as well as the centre, because laser energy and plating current density both vary across a panel. A monitoring plan that samples only the middle of a panel will miss the very defects that vary with position. Coupons built into the panel border are the cheapest way to cover those positions without giving up product area.
FAQ
Why must a microvia be filled rather than lined? A filled via supports the next build-up layer without a dimple and prevents plating chemistry from being trapped inside the hole.
How thick should microvia plating be? Between 12 and 25 micrometres at the mid-wall is a common specification, checked across several panel positions.
Can microvia cracks be found without sectioning? Not reliably. Resistance changes appear only after the crack has opened, so sectioning remains the primary method.
