April 2014 • The PCB Design Magazine 23
The semi-additive process is essentially
the opposite of the subtractive PCB fabrica-
tion process. It starts with a laminate that is
clad with much thinner copper than the thin-
nest conventional foils. For example, quarter-
ounce copper is approximately 8.5 µm thick,
but the ultra-thin foil is typically 2 µm or 3
µm thick. Such ultra-thin foils have a backing
that is removed after the foil is laminated to
the substrate. The holes are drilled and then a
bath of electroless copper coats the bores and
the foil surface. A "reverse" resist is applied,
developed, and selectively removed to expose
only the areas that will become the traces, vias,
and other conductive features. Those exposed
areas are plated up, the resist is stripped, and
the ultra-thin copper remaining in the well be
-
tween the conductors is etched away.
The plated-up traces and features are far
more accurately defined than those produced
by even the most-well-controlled subtractive
processes (Figures 2 and 3). Trace width and
spacing down to 1.25 mils can be maintained
over distance within extremely tight toler
-
ance. There is no penalty in manufacturing
time. The semi-additive process frees designers
who would otherwise be challenged to break-
out dense fine-pitch BGAs and also enables
very strict impedance control to ensure signal
integrity.
And finally, while I have the chance to dis-
pel myths harbored by some PCB designers,
let me clear up another misconception. If any
manufacturer tells you there's a setup charge
for board fabrication or assembly, tell them no
thanks, you'll shop elsewhere.
Remember, no special jigs or one-off fixtures
are necessary to build these boards. Tell them I
said so.
PCBDESIGN
References
1. Abdelghani Renbi, Arash Risseh, Rikard
Qvarnstrӧm, and Jerker Delsing, "Impact of
etch factor on characteristic impedance, cross-
talk and board density," IMAPS 2012.
Amit Bahl directs sales and
marketing at sierra Circuits, a
PCB manufacturer in sunnyvale,
California. he can be reached
by clicking here.
design for manufacturing
MyTHBuSTING: SEMI-ADDITIVE PROCESS MAkES SuB-3/3 A REALITy continues
Figure 3: This pair of traces designed to be 3 mils wide on 3 mil spacing was produced by the best
possible subtractive process.
