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Advances in technology have been clear to see within the component packaging industry, as the ball grid array (BGA) package sizes reduce from 1.0 mm pitch to 0.8 mm, 0.4 mm, and even beyond. However, while these improvements have occurred with component packages, it has become increasingly more difficult to break out and route the dense circuitry associated with these parts. Currently, the high-density interconnect (HDI) method typically used for the breakout of such parts has been to create the smallest possible subtractive-etched traces with microvias to allow for connections and escapes on the innerlayers of your PCB.
Now there are new fabrication processes that change how we can approach some of these layout difficulties. Additive and semi-additive construction now allows us to get down below 0.075 mm trace and gap sizes easily and reliably, however, utilizing it brings its own series of challenges to the table. Before designers and engineers can start to use this technology, we need to understand the difference between the standard subtractive fabrication method and these new additive fabrication methods, as well as their respective design requirements for trace impedance, and the signal integrity impact from routing traces closer together.
Subtractive vs. Additive
Let's start by taking a brief high-level view of the different fabrication processes. With subtractive fabrication, our PCBs start with a base layer of copper of some thickness already laminated to the substrate. Then copper is electrolessly plated onto the board’s outer layers, including inside the drill and via holes. A design image is then applied, an etch resist plated onto the exposed traces and holes, after which etching will occur. This is our subtractive step, where we remove the copper in areas where there was no image applied. This is also the limiting step in the subtractive fabrication process, because as we etch vertically down through the copper, the etching agents also remove copper in a horizontal direction, under the applied design image. The result of this process is a final copper trace cross-section with a trapezoidal shape. The critical concern here is that if the trace height is half as tall as its width, likely the etching process will remove the trace.
With additive fabrication, the process can be imagined as similar to 3D printing. The PCB starts with no copper on the laminate material and is instead “built” up on top of a thin seed layer of electroless copper, or on top of a thin laminated copper foil. This not only allows for trace and gap sizes down to 0.010 mm; it also creates a trace cross-section that has a rectangular shape.
To read this entire article, which appeared in the April 2022 issue of Design007 Magazine, click here.