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 another of a series of
articles on printed circuit boards (PCBs) that appeared in the October 1969 issue of Electronics World,
reporting on the latest and greatest advances in printed circuit board technology. Already
in production were rigid multi-layer laminates, flexible plastic laminates, and
special-purpose laminates for hazardous duty applications. Author Norman Skow does not
mention how many layers were routinely accomplished at the time. Plated-through holes were
a relatively recent thing for high volume manufacturing. Of course population of PCB components was still a completely manual procedure since pick-and-place machines were still
a couple decades away.
I remember sitting in classes in the mid 1980s at Westinghouse Electric in Baltimore
where engineers described their research with machine vision and robotics for inserting
both leaded and surface mount components on PC boards for manufacturing. Nowadays you can
build systems with greater capacity using
LEGO MindStorms and
Arduino Makeblock kits.
The author holds B.S., M.S., and Ch. E. degrees in Chemical Engineering
from the State University of Iowa and a Ph.D. in Chemistry from Cornell University. He was
a Fellow at the University of Iowa and at one time taught chemistry at Cornell. His entire
career has been focused on plastics - fifteen years as a research engineer for Bakelite Corp.
and, since 1945, as Synthane Director of Research until his appointment as a vice-president
By Norman A. Skow / Vice-Pres. & Technical Director
Research & Development, Synthane-Taylor Corporation
The laminate, the base material on which PC board components are mounted, must have the
proper electrical and mechanical characteristics. Here is what is available, along with guidance
in making the proper selection.
For the processor seeking high production yields of assembled printed-circuit boards, for
the circuit designer requiring good electrical and mechanical qualities, and for the ultimate
user looking for high reliability and good performance, the quality of the metal-clad laminate
used for the printed-circuit board is an important item. To get high yield, it is important
to use copper-clad laminates that are free from pits and dents and which are readily sheared,
punched, or sawed. If the laminate is free from defects like these, it will help to eliminate
open circuits and high resistance points, two faults which decrease printed-circuit board
Production of high-quality circuits can be achieved by photoengraving, silkscreening,
or offset printing if the boards are flat and the copper clean. In addition, the board must
be adequately bonded not only between the layers of laminate but also between the laminate
and the metal.
This bonding is important if deterioration of the board or separation of the board from
its circuitry is to be avoided. The laminate must be able to offer sufficient resistance to
acid, alkali, and solvent attack during the fabrication and processing.
The need for sufficient bonding of metal to laminate board is even more critical where
fine-wire circuitry or wiring must be produced. In addition, the board must have heat resistance
to stand up to the temperature of the solder bath without deterioration. This must be true
for boards that have been stored for long periods of time, as well as for those which are
To meet these requirements, the correct resin binder must be used, it must properly impregnate
the right reinforcement material, and from that point on the laminate must be given "white-glove"
handling in atmosphere-controlled clean rooms. Final inspection and shipment must be under
equally controlled conditions.
Top and bottom of a printed circuit on a nickel-clad glass-epoxy laminate
shows fine-line circuitry that can be etched.
Metals Used for Cladding
Of the cladding metals available, the only ones used to any great extent are copper, aluminum,
and nickel, and of these copper accounts for more than 99% of boards manufactured. Aluminum
is readily available in film form and adheres well to laminates, but is handicapped by relatively
poor soldering efficiency.
Electrolytically deposited nickel is also available and has several advantages, such as
scratch resistance, tarnish resistance, and weldability. But higher costs of nickel over copper
and aluminum represent a major disadvantage.
Copper of printed-circuit quality is used in large quantities because of ready availability
and because the metal can be made to have uniform thickness and density, and is free from
pits, pinholes, and scratches.
Copper for printed-circuit boards can be manufactured with one surface that is readily
solderable and the other treated to produce consistently good bonds to the laminate.
Selecting the Proper Laminate
In order to select the proper laminate, the end user needs to know the properties of the
grades that are available. Of the 80 grades of laminates, less than a dozen are used in printed-circuit
Laminates are made in three different classifications: rigid board, ultra-thin panels for
multilayer board, and flexible films for flat cables and flexible printed circuitry. The materials
that are commonly used to manufacture these types are shown in Table 1.
Table 1 - Materials commonly used to manufacture PC laminates. Various
resins (shown horizontal) are used to impregnate a number of different reinforcement materials
(shown at the left of table).
Table 2 - NEMA and military grade designations of laminates, along with
some of their typical applications.
Printed-circuit rigid boards of paper-phenolic construction have various trade names and
are designated by the National Electrical Manufacturers Association (NEMA) and the American
Society of Testing Materials (ASTM) as grades XP, XXXP, XXXPC, and FR-2. These grades are
relatively inexpensive and can be used in such commercial applications as radio and television
receivers where a flat, strong board is needed. These grades are relatively warp-free and
have good impact strength, high adhesion, and excellent insulation resistance, in addition
to good dimensional stability. Glass mat polyester type board also finds limited use in commercial
Flexible printed-circuit film offers advantages of lower cost, less space, and smaller
volume. High flexibility of pliable film permits creasing without breaking circuit.
The glass-epoxy and glass-polytetrafluoroethylene laminates maintain excellent electronic
properties even under severe humidity environments. They are therefore designated for more
sophisticated applications in computers, military electronic devices, and communications equipment
or any electronic device requiring the highest reliability even under extreme temperatures
While rigid boards are made from these few materials, they are manufactured in various
grades. These grades are shown in Table 2, which indicates both NEMA and military grade designations.
The thickness of rigid board is from 1/32" to 1/8".
In order to increase the circuit concentration in a small space. designers have gone to
smaller and multilayer boards necessitating the manufacture of ultra-thin laminates. These
thin laminates may vary in thickness from 0.002" to 0.031" (1/32").
The two laminates generally used in making multilayer boards are NEMA grades G-10 and FR-4,
which are designated GE and GF by the military.
Multilayer board measuring 0.07-in thick consists of five layers of ultra-thin
glass-epoxy copper-clad laminate. Four of the layers have copper on both sides, the fifth
has copper on one side. The registration of each layer is held to within 0.003 in of the first
layer. Over 800 plated-through holes are used, ranging from 0.028 to 0.144 in in diameter.
Flexible printed circuitry offers advantages of lower cost, lighter weight,
and smaller space and volume without loss of reliability. One added advantage of flexible
printed wiring is the fact that its manufacture poses no particularly difficult production
Atmospherically controlled white-room conditions help keep PC laminates
free from defects. Here pre-pregnated materials and copper for cladding are stored and weighed
Flexible printed-circuit laminates are flat, pliable, copper-clad films manufactured from
high-temperature thermoset adhesive and a flexible plastic film substrate. Among the films
used in this type of laminate are polyamide, fluorinated ethylene propylene (FEP), polyvinylidene
fluoride (PVF), polyethylene terephthalene, polyimide, polyvinyl chloride, flexible epoxy
glass, and tetrafluoroethylene.
The basic difference between rigid and flexible wiring is in the base material to which
the copper foil conductors are bonded. In rigid board it is stiff laminated plastic, in flexible
wiring it is a pliable plastic film.
The films possess excellent dimensional stability and resistance to soldering temperatures.
The basic properties of a flexible laminate film coupled with the thermoset adhesive permit
circuit fabrication with extreme ease due to the high chemical and heat resistivity of the
copper-clad film. It is also possible to use high pressures in multilayer circuit production
without circuit rupture. The extreme flexibility of this laminate, available with copper cladding
on one or both sides, permits 180° creases in any direction without cracking, rupturing,
or separating the foil from the dielectric film.
(Editor's Note: For further information on this type of material, refer to the article
"Flexible Printed Wiring" in this Special Section.)
Sizes and Dimensions Available
Printed-circuit laminates are available in standard sizes and thicknesses. Rigid circuit
board comes in 36" by 36", 36" by 72". and 24" by 96" sizes. Some grades are available in
36" by 42" and 36" by 48" sizes. Some grades may be supplied in 24" widths and in lengths
longer than 96". Standard rigid board thickness ranges from 1/32" to 1/8". Copper foil varies
in thickness from 0.0014" to 0.007" and is bonded to one or both sides of the board.
Ultra-thin laminates for use in multilayer boards come in thicknesses ranging from 0.002"
to 0.031". The same copper thicknesses are used here as on rigid board. and the copper can
be bonded to either one or two sides. Ultra-thin laminates are produced in standard sheet
sizes of 36" by 36" and 36" by 48". Some PC-laminate manufacturers will cut panels to size.
Flexible copper-clad film laminates are manufactured in maximum roll widths of 59" and
maximum roll diameters of 14" (2000 feet) . Flexible film measures from 1.5 to 9.9 mils
thick with copper on one side, and from 2.5 through 14.8 mils with copper on two sides. In
addition to copper, other metals can be used in metal-clad laminate construction, among
these are steel, Nichrome, Kovar, and nickel.
PC Laminate Costs
The cost of printed-circuit laminates depends on the type laminate selected. Further variation
in costs results from the different materials from which the laminate is manufactured. In
general. the more sophisticated and critical the application, the higher the cost of the laminate.
Printed-circuit laminates were developed and used successfully for the first time about
15 years ago. The first efforts were to develop the proper mechanical and electrical properties
and adequate peel strength of the copper during soldering.
These problems were solved in the early years. But more recently attention was directed
to improving copper-clad material in the face of today's trend toward miniaturized electronic
Meeting these more exacting requirements has created a demand for laminates with greater
dimensional stability and stronger resistance to processing techniques. Great strides have
been made in these directions so far and more will be made in the future.
* He is a member of Sigma Xi and active in the ACS, SPE, A.I.Ch.E, ASTM, NEMA, and IPC.
He is active in the American delegation of ISO (International Standards Organization), and
has made many trips abroad in the interests of this group. He holds several patents. Articles
he has authored have appeared in twenty-six publications.
Posted September 14, 2017