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Grasping semi-additive fabrication

Posted: 22 Jun 2015 ?? ?Print Version ?Bookmark and Share

Keywords:IC? lithography? PCB? fabrication? impedance control?

From a system perspective, continued advances in circuit integration now depend less on improvements in semiconductor IC lithography than on better processes for fabricating printed circuit boards.

The smallest tolerance for trace width that can be reliably achieved in subtractive PCB fabricationthe process used by virtually all board manufacturersis within 0.5 mil of design value. A 0.5-mil variation is insignificant when traces or spaces are 3 mils or wider and signal edge rates are relatively slow, but for thinner traces would have a significant effect on impedance control. The practical limit has been reached for resolving fine lines for high-speed circuits beyond very short distances with the subtractive process.

The issue is etch chemistry. Basically, PCB fabrication starts with a cured laminate material with copper on one or both sides. That clad laminate is called a core and is produced by the laminate suppliers in a wide variety of substrate materials and thicknesses, which have a broad range of tailored dielectric and mechanical properties.

Figure 1: Chemical etching, no matter how tightly controlled, removes more material at the top of PCB trace walls, beneath the resist, than at the base.

The copper cladding comes in a wide range of thicknesses (weights) and several surface finishes. PCB manufacturers likewise create laminates by bonding copper foils to uncured substrate materials, which are known as prepregs, in a press. Traces and other features are formed by coating the copper with a photoactive etch resist, selectively exposing it to light where the copper should be retained, then etching away (subtracting) in an acid bath the unexposed resist and copper underneath. The goal is traces that have a rectangular cross-section but the bath not only removes copper in the vertical direction but also dissolves some horizontally from the trace walls.

A tightly controlled subtractive process results in traces that in cross-section are approximately a trapezoid with sides at a 25-45 degree included angle from the base and the base is at design width. However, if not closely regulated, bath chemistries can over-etch traces such that their top is much narrower than their base. The ratio between the height of the trace after etch (t) and how far the trace has been eaten away at the top of a wall (x) is the etch factor (F). The greater the etch factor, the closer a trace cross-section approaches rectangular. Bath chemistry can also vary from optimum such that not enough copper is removed, resulting in traces whose bases are wider than designed.

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