OL1.OCT–Oct–SC
Flexible-circuit process forms integral terminations
Nickel barrier enables process to control copper plating thickness and
achieve fine-line traces
Flexible-circuit makers are engaged in an ongoing quest to build thinner
circuit traces for today's denser, higher-speed circuits. This process is
the first to be able to image terminations along with fine-line traces.
Traditional flexible printed circuits have incorporated rigid
terminations, which add weight and cost to the circuit. Enter the
Tri-Metal process from Advanced Circuit Technology. This process combines
the ability to image thin traces with the ability to image thick conductor
areas to provide terminations, eliminating the need to add space-consuming
rigid interconnections as with other flexible-circuit processes. The
process is an enhancement of the company's Sculptured flexible-circuit
process, introduced in the late 1970s. Like the original process,
Tri-Metal uses a three- dimensional etching technique to build
terminations and raised areas into the flexible circuit (see diagram).
However, the original Sculptured process could only image fairly coarse
traces. By contrast, the Tri-Metal process can build thinner flexible
circuits–typically with conductors 0.7 mil thick and conductor/terminal
areas 3 mils thick. Any commercial, industrial, or military application
requiring thin, flexible circuit traces could benefit from the Tri-Metal
process, according to Chris Lampron, applications specialist for Advanced
Circuit Technology. The one drawback of the process is the added cost of
being able to image the termination areas as part of the flexible circuit.
Typical cost per circuit will be about $0.75 to $1 per in.
least 50% higher than for other flexible-circuit processes. Lampron points
out, however, that the space and weight saved by integrating the
terminations into the fine-line traces could outweigh the added cost in
some applications. The process can achieve the fine-line traces because
a nickel barrier layer is added to allow precise control of copper-plating
thickness. The nickel layer, 50 μin.thick, is sandwiched between two
copper layers whose thicknesses can vary. Plated copper thickness can be
as little as 0.5 mils with a +/-1-mil tolerance. After artwork
registration holes are drilled into the copper, the circuit pattern is
imaged onto the thinner copper side of the "sandwich." The sheet is then
etched and laminated with an insulation layer. A second imaging step
forms the circuit termination features on the thicker side of the copper.
During the subsequent etching step, the presence of the nickel barrier
ensures that only the thick copper layer is etched. The exposed nickel
barrier is then removed with a stripping solution that does not affect
copper. After cleaning, an insulation layer is laminated to the etched
side of the Tri-Metal sheet. The exposed terminations are then plated with
solder or gold. For more information on the Tri-Metal process, contact
Chris Lampron at Advanced Circuit Technology (Nashua, NH) at 603-880-6000,
or . –Spencer Chin
CAPTION:
The Tri-Metal process from Advanced Circuit Technology forms thin,
flexible circuit traces with integral termination areas, eliminating the
need to add separate terminations later.
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