October 1969 Electronics World
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
This is the last of a series of articles
on printed circuit boards (PCBs) that appeared in the October 1969 issue of
reporting on the latest and greatest advances in printed circuit board technology. Author
Gaetano Viglione, of Sanders Associates (bought by Lockheed Martin in the 1980s and now owned
by BEA Systems), reported on the state of the art of flexible printed circuit wiring.
Sanders did a lot of aerospace and military electronics systems and was a leader in the
field. In those days, the larger electronics manufacturers had their own in-house PCB
design and fabrication capability.
Flexible Printed Wiring
By Gaetano T. Viglione / Manager, Product R & D
Flexiprint Division, Sanders Associates, Inc.
The author is a graduate of Columbia University (Chemical Engineering).
He holds several patents in metal processing and chemical engineering. He has been a consultant
to industry and federal government in process engineering problems.
This wiring can be preformed, folded and rolled, twisted and turned to fit any conceivable
irregular configuration. It is particularly useful in very dense electronic packages.
Flexible printed circuitry consists of flat etched copper conductors bonded between layers
of pliable insulation. While this sounds simple enough, flexible printed circuitry has had
placed upon it many demands, some of which are due to the numerous design features which appear
to be attainable. The use of flexible circuitry as an interconnection medium has begun to
inspire the design engineer because of its many advantages.
For example, the significant difference of less weight per unit of area coupled with the
reduction of volume over conventional cabling techniques, results in reductions in mass of
2:1 and, in some cases, 8:1. The feature of flexibility allows circuits to be preformed, folded
and rolled, or twisted and turned to fit any conceivable irregular configuration in the more
dense electronic packages, especially those typical of aerospace applications. Further, flexible
printed circuitry is readily shielded and inherently more reliable due to its design and the
adaptation and use of materials having exceptional physical properties.
Table 1 - Characteristics of the most commonly used insulation material
for flexible printed wiring.
The real dollar savings come at the time of assembly of the electronic package because
of a reduction in man-hours required for assembly. Another benefit is the reduced possibility
of error when connecting by flexible printed circuitry as compared to the greater possibility
of error when using point-to-point wiring.
Listing of Advantages
The benefits resulting from the use of flexible printed wiring and cabling include the
1. Each circuit is a finished unit, ready for component assembly.
2. Handling of individual wires is eliminated because there is no need to measure, cut,
strip, tin, route, solder, and lace.
3. Circuits are custom-designed for each job; therefore, wiring errors are eliminated.
4. Each circuit of a particular design is mechanically and electrically identical and completely
5. Solder pads are in one place, rendering them ideal for automatic processing.
6. Wiring requirements no longer limit package geometry. Circuits can be run flat, bent
around sharp corners, folded, and twisted.
7. Conductor breakage is nil. High-reliability hinge, spring return, and extensible interconnections
can be readily designed.
8. Flexible printed circuitry can be bonded to rigid circuit boards to create a complete,
one-piece interconnection assembly, eliminating unnecessary solder joints.
9. Single and multilayer circuits are closely spaced and held to close tolerances; therefore,
high wiring and internal package densities are possible.
10. Flexible printed circuitry has a high volumetric efficiency. Close-tolerance conductor
location is possible because each circuit is a precision etched unit.
11. Material normally needed in the form of a relief loop for the bend radius using standard
wire cable can be eliminated, resulting in shorter wiring runs.
12. Thin, flat, two-dimensional geometry permits cable routing through narrow slots and
along smooth surfaces, eliminating the excessive bulk of round wire.
13. Depending on the specific application, flexible printed circuitry can save approximately
75% of volume and weight over conventional round-wire cable.
Fig. 1 - Conductor widths and spacings for various thicknesses.
14. Foreign material, such as moisture, flux, and gases which could "wick" inside the insulation
of wire, cannot degrade flexible printed-circuit performance because all conductors are completely
15. Tension loads are carried by the entire cable, not by individual wires; therefore,
each circuit part is a solid mechanical structure.
16. High reliability in demanding environments is inherent because the entire circuit flexes
as a unit under stress of vibration and shock.
17. Distributed capacitance and cross coupling do not vary from unit to unit of a single
design, resulting in constant electrical characteristics.
18. Circuits are easier to solder and inspect than a tangle of conventional wires; therefore,
quality-control operations are more accurate.
Design Criteria & Costs
When considering the use of flexible circuitry for interconnecting either printed-circuit
boards or black boxes, certain basic design criteria should be remembered. General guidelines
to keep the number of layers in flexible circuitry to a minimum are: set-up pin address to
reduce or eliminate crossovers; use the freedom of pin address as a method of reducing layers;
consider the use of narrower conductors and spacing; and use fold-outs to increase density
at the terminal area.
There are many factors that affect the cost of a flexible circuit. Major cost advantages
result if standard flexible-circuit materials and sizes are used and if non-critical tolerances
are loose enough to allow economical, automated production. The following items will also
serve to keep costs to a minimum:
1. Specify 0.0027" thick (2-ounce) copper conductors if possible. These are the most economical
because raw materials are purchased, handled, and stocked in large quantities. Other sizes,
such as 0.00135" (1 ounce) and 0.0040" (3 ounces) are available should the need arise.
2. Specify enlarged punched-out areas in the covercoat at solder-pad areas rather than
tight-fitting, pad-sized individually punched areas. This eases registration in manufacturing
and thereby reduces cost.
3. Try to keep punched-out bare copper areas on the same side of the circuit. If it is
necessary to present bare copper on the reverse side of a circuit, try to eliminate the extra
cost involved in punching the base insulation by folding the circuit.
4. Keep large punched-out areas in simple shapes to lower the cost of intricate dies and/or
to avoid excessive hand-cutting and punching.
5. Design terminal pads somewhat oversized to allow for slight drift.
6. Specify insulation as shown in Table 1. These are usually stocked in quantity.
7. It is recommended that conductor widths be specified larger than the minimum shown in
8. It is recommended that conductor spacing be specified larger than the minimum shown
in Fig. 1.
9 . Hold over-all flexible-circuit length and width to a minimum.
10. Use the fewest number of layers possible.
This complex flexible printed circuit, which is shown here along with an
18-in rule for size comparison, demonstrates how a complicated interconnection problem has
A multilayer flexible circuit with layer-to-layer interconnections along
with connectors attached is illustrated.
Another important design consideration is the selection of the termination method. Recently,
connectors for flat cable that accommodate circuits of 50-mil centers have become available.
The type and size of terminating pins and connectors and the method of attaching the circuits
to them is important in keeping down the size and weight and permitting greater accessibility.
The method of terminating using a pad with a hole or a "lap" soldering technique utilizing
pins from a connector are two of the most commonly used fabricating techniques.
For reasons of greater economy and increased reliability, a new generation of terminations
welded to flexible circuitry has evolved. Materials such as tinned copper, nickel, and gold-plated
Kovar are being satisfactorily welded to flexible circuits and potted as a substitute for
specifically designed connectors. All of these terminal lead materials are easily solderable
and, in some cases. they are also welded to weldable printed-circuit boards. The advantages
here are: lower attrition due to the fact that leads do not become unsoldered upon secondary
soldering; faster processing, reducing the possibility of process effects such as delamination
due to less time at high temperatures as when a solder joint is made.
The materials used in the fabrication of flexible printed circuitry depend upon the application
and environment in which the equipment will be expected to operate. Normally, Kapton-F film,
to which copper has been laminated, is used because of its desirable high-temperature properties.
(Kapton-F is the trademark of the duPont Company for its plastic film consisting of a layer
of Teflon FEP resin bonded to one or both sides of a polyimide film.)
Covercoat materials can be varied depending on the humidity characteristics and high-temperature
properties expected in the system operation. To some extent, the choice of materials is dictated
by the amount of flexing anticipated prior to and during assembly of the units. Generally.
the polyimide materials are most popular since the physical influences, mentioned above, are
most easily overcome. However, copper, due to its inherent nature when subjected to excessive
flexing and vibration, can eventually result in fatigue and electrical failure unless proper
support of the connector areas and the crucial bend radii are carefully observed and practiced.
Testing Flexible Printed Circuitry
While much information is available on the applications aspects of flexible circuits, very
little data has been presented about the environmental characteristics of the finished product.
Sanders, as well as other manufacturers of flexible printed wiring, has set up test programs
to run environmental checks on its products to make sure they meet customer specifications.
The following government and industry standards have been used for such tests: MIL-T-5422E.
MIL-STD-354, MIL-STD-202, MIL-P-55110A, MIL-STD-810, MIL-E-5272, MIL-P-13949, IPC-CF-150 copper
foil specification, ASTM-D-635, and AST-1-D-150.
The parameters evaluated include: operating temperature; moisture absorption; flexing and
tensile strength; resistance to chemicals, abrasion, and fungus; aging and weathering effects;
distributed capacitance and require shielding; flammability; dielectric constant and strength;
conductor and insulation resistance; and current-carrying capability.
The results of the test data will be supplied to customer - and potential customers by
the manufacturers of flexible printed wiring. Such test data, while considered to be evidence
of the present and potential quality of flexible circuitry, is really only a start. Much testing
of new material - and advanced processes is continuing, insuring that advanced technology
is made available to the user of flexible printed circuitry as new materials are developed
and placed on the market.
Posted November 7, 2017