Additive PCB Process Routing Rules for Fine-Pitch BGA Fan-Out
Package pitches have fallen from 1.0 mm to 0.8 mm, then to 0.4 mm, and the escape routing problem has become the dominant constraint in the layout of large devices. At those pitches a conventional etch process cannot produce conductors fine enough to escape the ball field without consuming an impractical number of layers. The additive PCB process was developed for exactly this situation, and it changes the design rules rather than only the fabrication capability.
Understanding the difference matters because the electrical consequences are not identical. A trace formed by additive means has a different cross-section, which changes its impedance for the same nominal width, and the denser routing it enables changes the coupling environment around every signal.
What Changes Between Subtractive and Additive
The subtractive process begins with copper-clad laminate and removes the unwanted copper by etching. Because the etchant removes copper laterally as well as vertically, the finished conductor is trapezoidal and narrower than the artwork, and the process becomes impractical below roughly 75 microns.
An additive process builds the conductor instead of removing it. Starting from a laminate with no copper, or from a very thin seed layer, copper is deposited only where the conductor is required. The result is a rectangular cross-section rather than a trapezoid, and the achievable width falls to around 10 microns and below. The same geometry that limits the subtractive process simply does not exist in the additive flow.
The practical consequences of the rectangular profile are significant: a rectangular conductor of a given width carries more current than a trapezoidal one of the same nominal dimension, and its impedance is more predictable because the geometry is closer to the ideal used in the calculation.
The Impedance Problem With Very Fine Lines
Characteristic impedance is set by the conductor width and by the distance to the reference return path. A conventional subtractive process reaches 50 ohms comfortably with a 75 micron conductor over a 50 micron dielectric. When the conductor is reduced below 50 microns to increase density, the dielectric thickness must also be reduced to maintain the same impedance, because a narrower trace over the same dielectric height has higher impedance.
That requirement is where the material supply becomes the limiting factor. Ultra-thin dielectric materials exist, but their availability is limited and their cost is high, which makes the additive route attractive only when the density genuinely requires it and when the fabricator has a qualified material set.
Coplanar waveguide structures offer a partial mitigation, because the adjacent ground on the same layer contributes to the impedance and allows a slightly wider dielectric spacing. However, the dielectric thickness remains the dominant factor, so a coplanar arrangement improves the situation without removing the constraint.
For a design team the practical implication is that the fine-line decision cannot be made from the artwork alone. The stackup, the dielectric thickness, and the fabricator’s qualified material set all have to be confirmed before the trace geometry is frozen.
Effect on Signal Integrity
The purpose of fine-line routing is to fit more conductors into a smaller area. As trace width shrinks, the field around each conductor is not reduced, because the distance to the reference plane is unchanged. What does change is the spacing between the conductors: they are closer together inside the same field, so the coupling between them increases.
The result is predictable. Increasing density increases the risk of crosstalk and other signal integrity problems, and the routing rules have to compensate in three ways. Spread the conductors across a wider area where the design permits, keeping them as close as possible to their return path. Reduce the length over which conductors run parallel, which usually means changing the escape pattern so that fan-out turns away from a neighbor rather than sharing a long channel. Group the conductors that belong to the same interface, because signals that switch together and share a reference are less affected by coupling from outside the group than signals routed arbitrarily.
Where a group of fine-line conductors must run in parallel, the return path becomes even more important. A solid reference immediately below the group is what keeps the fields contained, and any interruption in that reference will convert the tight routing into a radiating structure.
Escape Routing for Fine-Pitch Packages
The decision to use an additive process usually starts with a large BGA fan-out. The design question is how many signal layers are required to escape the ball field, and how the escape is organized.
Via-in-pad escapes consume the least area, because the via is placed inside the pad and the conductor leaves the ball directly, but they require filled and capped vias to prevent solder wicking during assembly. Dog-bone escapes place a short trace from the pad to a nearby via, which is simpler to build but consumes more of the available channel. Where the pitch is very fine, the dog-bone approach runs out of room, and the design has to move to via-in-pad or to a finer process.
The escape pattern should also be planned by row. The outermost rows can frequently be escaped on the top layer, which frees inner layers for the rows further in, and the assignment of which rows go to which layer determines whether the layer count can be kept low. Rules for organizing that escape are described in escape routing and fan-out rules.
Working With the Fabricator
Additive processes differ between suppliers, and the capability varies enough that a design cannot be transferred between them without re-checking the geometry. Commercial semi-additive flows have different limits on conductor width, conductor height, and the aspect ratio of the plating, so the design rules have to come from the specific supplier that will build the board.
A useful working assumption is that 75 microns is the crossover point: above that width the conventional subtractive flow is adequate and cheaper, and below it the additive route deserves evaluation. The minimum achievable width is set by the supplier’s process rather than by the designer’s preference, and the design should be built around the fabricator’s certified capability rather than the best number that appears in a marketing document.
When the Additive Route Pays
The technology is best suited to designs with high component density or with packages whose pin count cannot be escaped otherwise. Its main benefit is reaching more device pins without adding board layers, which reduces cost and improves reliability compared with a stackup that would otherwise need additional lamination cycles.
The main caution is that the escape region itself becomes the critical area. If the routing inside the package area is uncontrolled, the density advantage is consumed by signal integrity problems that appear after fabrication. Where microvia structures are part of the escape, the filling and plating process has to be qualified, as described in blind and buried via stack selection. The routing practices that keep dense fine-line channels under control are covered in high-frequency trace and data bus routing.
FAQ
Does an additive process always produce better impedance control? It produces a more predictable conductor geometry, which makes the impedance calculation more accurate. The actual impedance still depends on the dielectric thickness and the distance to the reference plane, so a fine trace over an unchanged dielectric will have higher impedance than the wider trace it replaced.
How do I decide whether my design needs an additive process? Start from the ball pitch and the pin count. If a conventional escape would require additional signal layers, or if the required conductor width falls below roughly 75 microns, evaluate the additive route with the fabricator and compare it against the cost of the extra layers.
Is the additive process limited to rigid boards? No, but the material set and the process window differ for flexible and rigid-flex constructions, and the supplier qualification must cover the specific substrate. The design rules should be obtained for the material actually being used.
