October 1969 Electronics World
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
from
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
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Printed circuit boards
(PCBs), especially high density and many layered, were still a relatively new technology
in 1969 when this "Printed-Circuit Kits for Short Runs" article appeared in
Electronics World magazine. Point-to-point wiring was still a large part of most products in the commercial,
military, industrial, and aerospace worlds. Producing artwork and contracting for
a lot of PCBs was rather expensive, especially in small quantities. Many people,
including me, made quite a few single- and two-sided boards using photoresist pens
and self-adhesive tape and pads, then bathing them in an etchant solution of ferric
chloride. Through holes were made on a drill press and then traces and pads were
tinned with flux and solder laid down on all exposed copper. Pseudo plated through
holes were made with short pieces of bare wire running through the top-to-bottom
holes and clinched down to the pads. Solder mask was not needed for small quantity
runs, especially at a time when the smallest distance between component leads (i.e.,
pitch) was 1/10 inch for DIPs.
Here is a rundown on some of the PC kits that are readily available
for use by small laboratories or in the workshop.
The author has been associated with various
satellite programs utilizing the Agena vehicle since joining Lockheed Missiles and
Space Company in 1963. Areas of responsibility have included guidance and control,
communications and command, and electrical power systems. He was previously employed
by Montronics Inc., now a subsidiary of John Fluke Mfg. Co., while attending Montana
State University (BSEE). He has written for various electronics magazines.
By Donald L. Steinbach / Senior Research Engineer Lockheed Missiles & Space
Co.
Professional-quality etched-circuit boards may be produced in the small laboratory
or in the workshop. The process is neither difficult nor particularly hazardous,
and special equipment is not required. The purchase of one or more of the several
inexpensive printed-circuit kits that are available affords an excellent means of
becoming acquainted with the various materials and techniques. The kits contain
all of the constituents (including instructions) necessary to produce a limited
quantity of quality boards. The kit materials may be replenished, and the per-board
cost will be reduced significantly if larger quantities of the individual items
are purchased.
Selecting & Using a Kit
Selecting the most appropriate kit for an intended application requires some
initial familiarity with the commonly used materials and processes. Board materials,
resists, and etchants may then be chosen to best suit the technical requirements
or interests of the user.
Copper-clad PC board material is readily available in either NEMA grade XXXP
phenolic paper or grade G-10 general-purpose epoxy-glass laminates, 1/16" thick,
covered uniformly on one or both sides with copper foil, 1 or 2 ounces per square
foot of board material, and with or without a photosensitive resist coating. Grade
XXXP is less expensive than G-10, is easier to machine, and is entirely adequate
for most applications. Grade G-10 should be used, however, for high-frequency r.f.
applications. The foil thickness required is a function of available conductor width
and required current-carrying capacity. A 0.100" wide conductor of 1-ounce (0.00135")
foil will safely carry 3 amperes, and the same width of 2-ounce foil (0.00270" thick)
will carry 5 amperes. Both figures are for a 20°C maximum temperature rise.
The resist is applied mechanically to an unsensitized board, or created photographically
on a sensitized board. Mechanical resists are available in tape (stick-on), dry
transfer (rub-on), or liquid form; the most suitable mechanical resists have an
asphalt or vinyl base. The more expensive photographic process requires the preparation
of a mechanical or photographic negative of the PC board layout. It is ideal when
several identical boards are required, when the artwork is to be prepared on a larger
format than that of the finished board, or when the ultimate in finished appearance
is required. Both the dry-transfer resists and the photo-resists have a limited
storage life (approximately one year).
Typical example of printed-circuit kit showing all the components.
The kit shown, the Techniques No. 5003, at around $10, has ample supplies for several
projects.
Ferric chloride or ammonium persulfate are the two most commonly used etchants.
A typical ferric-chloride etchant is 34% (by weight) FeCl3 in water,
although the concentration may range from approximately 28% to 42%. The mixed solution
will contain a small amount of hydrochloric acid; proprietary etchants may be modified
to contain up to 5% HCl. Ferric-chloride etchant has an indefinite shelf life, may
be reused, and has a useful capacity of around 12 to 15 ounces of copper per gallon
of solution. The etching time decreases with increasing temperature; the solution
may be used from room temperature up to about 160°F. Ferric chloride attacks
solder, steel, cast iron, and many other metals.
Typical ammonium-persulfate etchants contain about two pounds of ammonium persulfate
per gallon of water. A mercuric-chloride (1 ml of 27 g/l solution per gallon) or
sulphuric acid (60 ml of 96% H2SO4 per gallon) catalyst is
usually added. The mercuric chloride is poisonous, but the etchant should not be
stored in a tightly stoppered bottle because of gas release. The ammonium-persulfate
etchant has a useful capacity of about 7 ounces of copper per gallon of etchant
and should be used warm (115°F typical).
When the concentration of copper reaches 5 ounces per gallon, the etch temperature
should be kept above 130°F. It is unstable above 150°F and should be used
within a few days after mixing. Spontaneous decomposition may occur if it comes
in contact with ferric chloride. Its chief advantage lies in its compatibility with
most of the plated-metal resists.
All traces of the etchant must be removed from the finished board. Ferric chloride,
especially, has a tendency to contaminate the laminate substrate with insoluble
forms of ferric oxide. The etchant can normally be removed from the board by thorough
rinsing and scrubbing immediately after etching. Tape resists are simply peeled
off; liquid resists, dry-transfer resists, and photoresists can be removed by scrubbing
the board with an abrasive household cleanser or washing with a suitable solvent.
Typical Available Kits
Printed-circuit kits are available from several sources and range in price from
$3.49 to $42.95. Seven kits, representing five major vendors and priced under $10.00,
were examined and tried by the author. They represent a comprehensive selection
of board materials, foil configurations, resists and etchants. In addition to the
seven PC board kits, two special kits designed for the production of photographic
artwork and contact negatives are included in the brief summary that follows.
Editor's Note: There are many other PC kits available for use for short runs
or in the workshop. However, we are including the following as representative examples
of those kits that are most readily available from the larger electronics parts
dealers.
Amidon Associates (12033 Otsego Street, North Hollywood, Calif. 91607).
E-Z Etch Printed-Circuit Kit No. 246-D ($5.95) contains dry-transfer resist.
48 square inches (2 pieces) of 1/16" epoxy fiber glass board clad on one side with
2-ounce copper foil, and dry ammonium-persulfate etchant crystals.
A single-board kit is available at a corresponding reduction in price. The small
pads and narrow conductor strips included in this kit will be of particular interest
to IC users.
The dry-transfer resists are applied easily and rapidly. The
ones shown being used here are from Amidon Association.
The complete line of Amidon products may be obtained from Amidon, or from World
Radio Laboratories. Many of the local Lafayette Radio Electronics stores stock the
printed-circuit kits.
Injectorall Electronics Corp. (4 North Rd., Great Neck, N. Y. 11024).
Printed-Circuit Kit No. 500 ($5.95) consists of 2 3" x 4 1/2" 1/16" XXXP phenolic
boards clad on one side, a resist-ink pen, resist-ink solvent, a 6-oz bottle of
ferric-chloride etchant, a 1/16" drill bit, and a 5" x 7" x 2" plastic case in which
the boards are etched and the kit is packaged. Kit is available from Lafayette Radio
Electronics and other parts dealers. Kepro Circuit Systems, Inc. (3630 Scarlet Oak
Blvd., St. Louis, Missouri 63122).
Standard Kit No. S-101A ($3.95) contains both liquid and tape resists, 54 square
inches (3 pieces) of plain and perforated 1/16" XXXP phenolic board clad on one
side with 1-ounce copper foil. and liquid ferric-chloride etchant.
Professional Kit No. P-101A ($5.85) utilizes the photographic process. It contains
45 square inches (3 pieces) of 1/16" XXXP phenolic board clad on one side with 1-ounce
presensitized copper foil mechanical negative material, photoresist developer, and
liquid ferric-chloride etchant.
Both kits are available with G-10 glass epoxy boards instead of phenolic boards
at a corresponding increase in price.
Photo Layout Kit No. PL-2A ($3.95) is designed for making photographic artwork
layouts for contact or photographically reduced negatives. It contains mechanical
negative material, clear acetate film, and pressure-sensitive tapes, pads, and letters.
Photo-Reversing Kit No. FK-701 ($7.20) is a unique kit for producing high-contrast
line negatives from inked or taped artwork. The artwork is contact-exposed on the
film provided (480 square inches total) and developed with the developer supplied.
Kepro products may be obtained from Allied Electronics, Newark Electronics Corporation,
and Burstein-Applebee.
Techniques, Inc. (235 Jackson Street, Englewood, New Jersey 07631).
Experimenters Printed Circuit Kit No. 5002 ($4.75) contains tape resist, 72 square
inches (6 pieces) of 1/16" XXXP phenolic board clad on one and both sides with 1-ounce
copper foil, and the liquid ferric-chloride etchant.
Technicians Printed Circuit Kit No. 5008 ($9.75) is an enlarged version of the
5002 kit, containing both tape and liquid resists, 128 square inches (10 pieces)
of board material, and proportionately larger amounts of the other supplies.
Techniques products may be obtained from Lafayette Radio Electronics. Do not
order small quantities of material directly from Techniques in Englewood.
Vector Electronic Company, Inc. (12460 Gladstone Ave., Sylmar, Calif. 91342).
Etched Circuit Kit No. 27XA ($6.50) contains dry-transfer resist, approximately
80 square inches (4 pieces) of plain and prepunched XXXP phenolic board clad on
one side with 2-ounce copper foil, and dry ammonium-persulfate etchant crystals.
Vector products may be obtained from Allied Electronics, Newark Electronics Corporation,
or Lafayette Radio Electronics.
The prospective PC board user would do well to obtain catalogues from the listed
vendors and examine the other products they have to offer.
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