Fine line flexible cable fabrication process

A method for fabricating supported fine line electrical conductors comprises the steps of forming a pattern (16) which defines a configuration (18) of the desired conductors on a surface (12), placing electrically conductive material on the surface in a configuration defined by the pattern to form electrical conductors (20), removing only the pattern from the surface and thus leaving the configured electrical conductors thereon, adhering a support (22) to the electrical conductors with an adherence which is greater than that existing between the conductors and the surface, and separating the assembled conductors and support from the surface. One of the surface and of the support and assembled conductors is flexible and the other is relatively rigid, so that the flexible one can be peeled away from the rigid other.

BACKGROUND OF THE INVENTION 
The present invention relates to a method for fabricating fine line 
electrical conductors. 
Fine line electrical conductors are fabricated both on flexible cables and 
rigid backings, and are widely used in and with a wide variety of 
electronic equipment for land, sea, air and space applications. 
Fabrication of such flexible and rigid devices is generally effected by 
preparing a fine line circuit on a substrate material, such as of Kapton, 
and then providing a laminate therefrom to seal the conductor lines. To 
provide electromagnetic protection, a shield layer is then deposited on 
the laminated structure. Both the layered constructions are also made for 
high density interconnections. Modern electro-optical devices demand high 
packaging density, which may be met by using multilayers and/or by 
reducing the line width of the conductors. A typical line width is 4 mil 
or more, with future needs being toward finer lines. 
The major problems in producing such fine line cables are associated with 
the preparation of the conductor lines. Conventionally, interconnecting 
circuit patterns are formed by a photlithographic process and by a 
chemical etching process, which involve many steps. Since the handling in 
any one particular step can create defects, therefore, as the steps are 
increased, the overall yield correspondingly decreases. 
Conductors and fine line applications must have fine geometry and minimum 
voltage drop, which requires a high aspect ratio, that is, a ratio of the 
conductor's thickness to its width. In chemical processes, which are 
subtractive processes in that material is removed from a conductive layer 
to produce the fine line conductors, undercutting becomes a common 
problem. As the aspect ratio descreases, undercutting becomes an even 
greater problem and eventually cannot be tolerated. As a result, 
undercutting is a prime reason for rejecting articles. 
Additional sources of rejected cables result from openings in or short 
circuiting between fine line conductors and poor adhesion of photoresist 
and metallization on the base substrate to which the fine line conductors 
are secured. Openings in the circuit increase exponentially with a 
decrease in line width. 
While both additive and subtractive processes require use of thin film 
stock, in the subtractive process special care must be used to prevent 
such defects as dents, scratches and creases which affect the production 
yield and product quality. 
Access holes or vias are provided in the proporting dielectric film for 
access to solder, wire bonding and multilayer interconnects. When these 
holes are prefabricated in the film, the lack of mechanical support for 
the metal foil in these areas provides vulnerability to mechanical damage 
and to chemical attack during fabrication processing. 
Such components as infrared detectors which are operated at cryogenic 
temperatures, require that one end of the interconnection cable for the 
detectors be at such a temperature, while the other end is at ambient 
temperature. This temperature differential or gradient creates a problem 
in heat loss by conduction. Thus, the thermal conductivity and electrical 
conductivity balance must be considered in the material selection. 
SUMMARY OF THE INVENTION 
The present invention overcomes and avoids the many problems associated 
with subtractive processes, by utilizing an additive process. Briefly, a 
pattern, which defines a configuration of the fine line conductors, is 
formed on a surface which, preferably, is polished stainless steel and 
which is used as a fabrication aid. Electrical conductive material is then 
placed on the surface in a configuration which is defined by the pattern 
configuration to form the electrical conductors. The pattern itself is 
removed from the surface, leaving the configured electrical conductors in 
place. A support dielectric material, such as a polyimide, preferably of 
Kapton (trademark of E. I. DuPont de Nemours & Company), is adhered to the 
thus formed electrical conductors, with an adherence which is greater than 
that existing between the conductors and the surface so that, in the next 
step, the assembled conductors and support may be easily separated from 
the surface. Thereafter, the conductors on the support may be further 
laminated into a finished product. 
Several advantages accrue from such an additive process. Finer conductor 
lines are produceable because there is no undercutting involved in the 
inventive process. Fewer handling steps are involved, which enable higher 
yield and higher quality cables to be obtained. The fewer handling steps 
also enable the cost of the products to be reduced. Wider choices of line 
height are also obtainable. 
Other aims and objects as well as a more complete understanding of the 
present invention will appear from the following explanation of exemplary 
embodiments and the accompanying drawings thereof.

DETAILED DESCRIPTION OF THE INVENTION 
Accordingly, in a first embodiment of the present invention, as illustrated 
in FIG. 1, a fabrication aid disposed as a base 10 is provided with a 
surface 12 to which anything deposited thereon will only weakly adhere. 
The preferred base is a plain polished stainless steel plate having a high 
chromium-nickel composition. Accordingly, a 300 series stainless steel 
plate is prefered. In addition, electrodeposited and electrolessly 
deposited metals, acting as different surfaces 12 on base 10, are also 
acceptable. Such metals include copper, nickel and silver. Because such 
electro or electroless deposition requires later removal in the process, 
the plain polished stainless steel plate is preferred to minimize the 
number of steps required. 
A resist film 14 is then placed on surface 12. Resist 14 comprises any 
suitable dry or liquid resist for photolithographic processing. The type 
of photoresist is selected according to the intended application. For 
etching, a 4-5 micron thick liquid resist is adequate to cover the rough 
surface of most substrates. However, for electroplating on a thin, 
liquid-resist pattern, resist thickness limits the ultimate conductor 
thickness. Conductor thickness of 7 microns is obtainable by using Shipley 
1375, which is capable of providing a height of 10 microns. However, 
because liquid resists do not provide as well a defined perpendicular 
resist wall as may be required, a dry resist film, such as Riston 
(trademark of E.I. DuPont de Nemours & Company) for electroplating. Such a 
dry film resist is capable of providing a thickness in the 25-50 micron 
range with a well defined, perpendicular resist edge profile which is 
excellent for plating. In addition, a dry film resist also requires less 
exposure energy than liquid resists. 
A circuit pattern of the desired fine line electrical conductor 
configuration is placed atop resist 14 and exposed to light energy, 
followed by removal of the resist, all utilizing conventional techniques, 
to provide a pattern 16 as illustrated in FIGS. 2a and 2b. Pattern 16 is 
provided with openings 18 extending to surface 12 which are configured 
according to the desired circuit pattern. 
The circuit pattern is electroplated through openings 18 as shown in FIG. 
3, leaving electroplated deposits of conductors 20 on surface 12. Resist 
pattern is then removed leaving plurality of conductors 20 weakly secured 
to surface 12 as shown in FIG. 4. 
A sheet of supporting dielectric material 22, such as of Kapton, with an 
adhesive layer thereon, such as of an acrylic polymer, e.g., Pyrolux 
(trademark of E. I. DuPont de Nemours & Company), which is a methyl 
methacrylate crosslinked with phenol, is laminated onto the electroplated 
pattern comprising conductors 20 as depicted in FIG. 5. As illustrated in 
FIG. 6, lamination occurs under heat and pressure, with arrows 24 denoting 
pressure at a preferred temperature of 250.degree. F. (121.degree. C.) at 
a pressure of 100-250 psi (70-176 kg./sq. cm.) for about 10 minutes. The 
dielectric substrate with conductors 20 secured thereto is then peeled 
from surface 12 on base 10 because the adherence between the conductors 
and their substrate 22 is much greater than the adherence between the 
conductors and surface 12, as shown in FIG. 7. 
If electroless coppper or nickel were deposited on base 10, it would then 
be removed by a quick etching process. In either case, the product would 
then have an appearance such as shown in FIG. 8. 
Further lamination of a protective dielectric cover 26 is applied as shown 
in FIG. 9 to protect the pattern comprising conductors 20 from the 
environment. 
For construction of rigidly backed circuit patterns, fabrication acid 
comprising the stainless steel plate depicted in FIGS. 1-7 was replaced by 
a dielectric film such as of Kapton, so that the base is flexible, which 
then will be peeled fro the nonflexible circuit pattern support. The 
Kapton base was first cleaned with a detergent and dried and placed in a 
plater in a deposition system, which was then evacuated. The film was 
plasma-cleaned in situ and ion plated with a thin adhesion layer, e.g., 
100-200 angstroms of titanium or nickel-chromium, and a conductor base of 
approximately 5000 angstroms of copper or nickel was placed on top of the 
thin adhesion layer. 
The metalized film was then taped on a rigid backed board, cleaned, and 
coated with a liquid or solid film photoresist. The photoresist was 
exposed, developed, and electroplated with a conductor, such as of nickel 
or copper, to a desired thickness, e.g., 8-20 microns. The photoresist 
were then stripped with a resist stripper. 
The unwanted conductor base was then removed by a short etchback to isolate 
the circuit pattern. The pattern was protected by laminating it with film, 
preferably of Kapton and Pyrolux. 
Although the invention has been described with reference to particular 
embodiments thereof, it should be made therein without departing from the 
spirit or scope of the invention.