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.
Here is another in a collection
of articles on printed circuit board technology that appeared in the October 1969 edition
of Electronics World. Amphenol has been making and driving the evolution of electrical cable, sockets, and
connectors since 1932. I have seen their advertisements in many of my vintage electronics
magazines, including this one that appeared in these Radio News issues from
and October 1945. A lot of
the innovation was driven by military and aerospace quality, reliability, pin density, and
weight requirements, and as is always the case, a lot of the benefit redounded to the
commercial realm as well. Many of the connector types appearing in this article were
still being used during the years I worked in the defense electronics business. During my
time as a technician at Westinghouse Electric and Simmonds Precision Instruments, I
soldered thousands of wires to those things.
Connectors for PC Boards
Mr. Hecker is responsible for the marketing of all standard Amphenol Industrial
Div. PC connectors as well as development of new products. He has had twenty years of marketing
experience combined with a close association with engineering and production. Previously associated
with Cessna Aircraft and E.C.I., he joined Amphenol in 1961.
Mr. Novak is responsible for marketing special PC connectors that are developed
for specific application programs. With 14 years experience in electronics, he has worked
directly with production, industrial, and design engineering departments. He was formerly
associated with Hotpoint, joined Amphenol in 1960.
By Philip C. Hecker and Charles T. Novak / Product Managers
Standard and Special PC Connectors, Amphenol Industrial Div., The Bunker-Ramo Corp.
The great variety of connectors makes selection difficult, but it does away with the need
for expensive, custom designs. Here is guidance in making the proper connector selection.
The constant demand for printed-circuit board connectors to meet specific circuit requirements
has resulted in a most awesome collection of configurations, sizes, contact designs, and spacings.
Although the variety can make connector selection difficult, it does allow use of existing
or "standard" connectors in a large number of applications that might otherwise call for custom
designs requiring expensive tooling.
Evaluation of several circuit parameters helps to reduce the number of connectors that
can be used in a given application. Each connector can offer the right qualities for a wide
range of uses. To realize maximum benefit from a given PC connector, however, it must be employed
within its best range.
One of the most important factors to consider - and one that is often overlooked-is the
probable number of insertion and withdrawal cycles.
Group of typical Amphenol printed-circuit board connectors.
Types of Contacts
Contact design and plating determine the number of cycles that a connector can undergo.
For applications with 500 cycles or less, several contact designs are used. One type resembles
a tuning fork in which the extra-long spring base of the contact and the contact material
resilience provide a wide flexing range with elastic reserve. A base plating of copper or
nickel is followed by gold plating. These contacts have been tested for 500 cycles of insertion
and withdrawal plus a 48-hour salt-spray exposure with only negligible change in contact resistance.
Tuning-fork-type contacts can effectively accommodate printed-circuit boards from 0.054-
to 0.71-in thickness. Micro-finished contact surfaces afford excellent insertion characteristics
and controlled lateral float of the contacts permits accommodation of warped boards with positive
continuity of all circuits.
For applications requiring more than 500 insertions and withdrawals, bellows-type contacts
are recommended. These contacts, formed from phosphor bronze or beryllium copper strip, have
a convex contour that concentrates bearing pressure on the mating surface, displaces semiconductive
film that may exist, and reduces voltage losses to a minimum. Plating is usually gold over
copper or gold over nickel. The bellows action of the contacts absorbs the actual deflection
force so that mating surface can withstand in excess of 1000 insertions and withdrawals.
Many bellows-type contacts have bifurcated mating surfaces which provide two points of
electrical contact per circuit even on irregular surfaces. The bifurcated design also helps
to keep the contact on the board during any vibration that might occur.
Also, for applications requiring more than 500 insertion and withdrawal cycles, a recently
developed connector with precious metal contacts (usually a gold/silver alloy), is applied
to a spring member at the point of contact. Thickness and shape of the tip can be varied to
suit the application. The entire contact 'itself, except for the tip, can be solder- or tin-plated,
eliminating costly over-all gold plate. These contacts have been tested to more than 1000
insertions with no increase in resistivity. The gold is literally where the action is, at
the point of contact.
Although for most applications the printed-circuit board is inserted directly into the
connector receptacle, a plug connector or male contact pins can be fastened, most often by
dip soldering, to the board. This is done when the cost of a board with components is particularly
expensive or perhaps non-repairable, as in the case of welded or encapsulated components.
This board with its large number of PC connectors has been designed for
Navy's multiple-warhead Poseiden.
Specifiers of PC connectors should have a general knowledge of materials available and
how they affect not only connector performance but also connector cost.
A number of different dielectric materials are used.
Phenolic, the most common and least expensive, is intended for applications requiring good
electrical qualities with mechanical qualities better than the acceptable minimum. It will
withstand a maximum operating temperature of 300° F. See Table 1.
Mica-filled phenolic, a low-loss and low-water-absorbing material, is used for applications
requiring superior dielectric properties. It has a thousand times the insulation resistance
of general-purpose phenolic, but costs only 50 percent more.
Cellulose-filled melamine has excellent electrical and mechanical properties and should
be considered when resistance to surface creepage is needed. Costs are approximately the same
as mica-filled phenolic. Ceramic steatite withstands high temperatures - 1400° F vs 325°
F for melamine - is non-hygroscopic and has a low loss factor, but it costs about three times
as much as phenolic.
Mineral-filled diallyl phthalate is used where dimensional stability of the connector and
close tolerances are important. Insulation resistance is excellent - 4 x 107 megohms
- and it withstands extremes of humidity and temperature, but it is almost four times more
costly than phenolic. Glass-filled diallyl phthalate has generally the same electric properties
as its mineral-filled counterpart, but has higher flexural strength. For this reason, it is
specified by the military for rugged environments. The cost is approximately six times that
The thermoplastic resins, which include polysulfone, nylon, polycarbonate, and ABS, are
often used where resiliency is required. Additionally, glass reinforcement can be applied
for increased strength and heat resistance. Thermoplastic resins exhibit considerably more
flexural strength than the thermosets. From a procurement standpoint, reduced costs can often
be realized through shorter cycling times that are inherent in the thermoplastic molding process.
Contact Materials & Plating
Table 1. Characteristics of popular dielectrics that are used for printed-circuit
Three basic contact materials are used in PC connectors: brass, phosphor bronze, and beryllium
copper. The right choice for a given connector application is a compromise between electrical
and mechanical properties. Brass, although it has the best conductivity and is relatively
low in cost, will not stand up under excessive insertion and withdrawal cycles. Spring qualities
of brass, good at the outset, become less effective as the material age-hardens or crystallizes
Phosphor bronze, twice the cost of brass, has good spring qualities over connector life,
but conductivity is relatively low.
Heat-treated beryllium copper contacts of the proper design will withstand more insertion
and withdrawal cycles than most other non-ferrous spring materials. Conductivity is almost
as good as brass but contact cost is about five times that of brass due to both material and
In addition to these three basic materials, a number of other alloys can be used to fit
the specific requirements of an application.
So many variables, not only the different types of plating materials and combinations,
but also plating thickness and contact shape, make meaningful cost comparisons more involved.
Gold, probably the most common plating used, is applied in many thicknesses: commercial
applications require 20 to 30 millionths of an inch; military specifications call for 50 millionths;
and some special applications call for 100 millionths or more. Gold over copper or nickel
is generally used in dry-circuit applications requiring low contact resistance and good corrosion
resistance. Silver over copper or nickel is used in higher power applications, but corrosion
resistance is not as high as gold. Naturally, heavier plating and the number of plating operations
increase connector cost.
Contact Spacing vs Cost
The two main material costs in a PC connector are, of course, contacts and dielectric.
Increasing the contact density in a given size and dielectric block results in some saving
in dielectric cost but it is relatively minor. The limiting factors in increasing density
are dimensional stability of the dielectric material, contact true position requirements,
unusual connector shapes, and termination methods used.
Cost of a given size connector will rise as contact density increases, but on a per-contact
basis cost actually decreases. For example, assume a certain size connector dielectric block
costs 25 cents. This block with 15 contacts at 5 cents apiece would cost $1.00; the same block
with 30 contacts would cost $1.75. Cost of the connector on a pre-contact basis with 15 contacts
is about 6.7 cents, with 30 contacts it is about 5.8 cents. (The saving in dielectric cost
in the 30-contact connector is small enough to be considered negligible.)
Crosstalk in a PC connector is usually insignificant.
Tests have shown that connectors, because of their fixed contact spacing, present fewer
crosstalk problems than wiring and cabling.
Popular "card-guide" printed-circuit board connector.
Collection of AMP connectors, showing variety available.
A military specification covering many printed-circuit connectors is MIL-C-21097B which
outlines electrical and environmental tests as well as connector dimensions to insure interchangeability.
For example, PC connectors meeting the spec must withstand thermal shock, vibration, and 50
G's physical shock tests in accordance with MIL-STD-202. An insulation resistance of 5000
megohms at a high-potential test (no breakdown at sea level with 1800 V a. c. and at 70,000-ft
altitude with 450 V a. c. applied) are also specified.
Although the electrical and environmental requirements of MIL-C-21097B necessarily limit
the contact and dielectric materials that can be used, certain materials are specified by
type. For example, dielectrics are specified under MIL-M-14F and MIL-M-19833: plating requirements
Contact resistance specified is for a voltage drop of 30 millivolts at a current of 5 amperes.
What is most important, however, is that contact resistance, whatever the particular value,
remain reasonably constant over the specific connector's rated number of insertion and withdrawal
Some PC connectors are available for commercial applications that meet the performance
and environmental requirements of military specifications although they do not necessarily
have the exact dielectric or plating thickness specified. These connectors, since they are
not restricted to the spec, can provide greater flexibility for design.
Choosing the Right Terminating Method
Military specifications cover standard solder and crimp pin terminations. Commercial PC
connectors can also employ taper-pin type (similar to standard crimp type except that a separate
pin is required), percussive welded terminations, and mechanical terminations. The terminating
method chosen does not greatly affect the initial cost of the connector, however termination
cost variations can be significant.
Connectors with solder terminations are designed for direct wiring or for installation
on printed-circuit boards, panels, or chassis for dip-solder applications. Taper-pin terminations
provide flexibility of assembly procedure and easy modification and repair. Crimp terminations,
available with the flexibility of removable contacts, are very economical in applications
where programming changes for PC connectors are anticipated. In general, mechanical termination
methods - such as solderless wrap and automatic crimp - offer the user significant assembly
time savings where large quantities of connectors are involved. For short-run PC board production
or applications with a very small number of connectors, soldered terminations are usually
the most economical.
Individual insert blocks with from one to well over 500 contacts are available with contact
centers ranging from 0.025 to 0.250 in. PC connectors are usually referred to as miniature,
subminiature, microminiature, and ultraminiature but at present there are few standards that
cover these types.
In addition to the standard lines of PC connectors on the market, a number of special types
have been developed that are available as stock items. For example, card-guide units provide
alignment in blind-spot installations and prevent PC board "rocking." Connectors with coaxial
contacts, with alternating contact tails that provide more space for terminating, and double-sided
connectors are only a few of the special-purpose units available to the designer.
This one-piece, economy-type 40-terminal, card-edge connector is employed
in a new automatic writing machine.
Two outstanding examples of special-purpose connectors are direct-entry and indirect-entry
plate assemblies. Both types utilize an aluminum plate punched to accept thermoplastic modules
containing one or more contacts. Direct-entry plate assemblies utilize three- and four-position
modules in addition to two-position "L"-shaped card guides; enabling the connector manufacturer
to use standard components to produce special configurations. The plate assembly makes framing
unnecessary, providing both rigidity and true positioning of contact tails for automatic termination.
Indirect-entry plate assemblies are made up of individual single-contact insulators pressed
into the plates, creating complete versatility in terms of wrap-tail number, limited only
by a designer's imagination. This provides maximum use of contact density, reduction in size
of the end product, and economical contact termination. Several manufacturers have been soldering
ground and voltage planes on the contact tails and wrapping the balance of the tails. Provisions
for grounding contacts are made in both versions.
Other special-purpose connectors include PC headers, crimp headers, and male wrap-tail
headers allowing for use of any of a wide range of economical termination equipment currently
available. Selection of such a special also precludes additional expense of a mold charge.
Additionally, these techniques permit repair or replacement of the male and female solderless
wrap contact subassembly by simple removal and insertion of an individual contact sub-assembly
with hand tools.
If a designer keeps the connector in mind at the time he first begins laying out the printed-circuit
board, he will most likely to be able to adapt his PC board leads to conform to the contact
spacing and over-all width of a standard PC connector. If the connector is not considered
until the board is completely designed, there may be no choice but to specify a special connector
that not only will require longer delivery time but also involve expensive tooling costs.
Designers should provide the PC connector manufacturer with as much information as possible
about the unit required: not only the obvious factors of size, number of contacts, current/voltage
capacity, but also environmental aspects of the application, contact finishes, dielectric
materials, and terminating method preferred.
In addition, knowledge of the mounting requirements into the specific end product may enable
the connector manufacturer to provide services the user may not have been aware of. Several
connector manufacturers are tooled to provide plate assemblies with connectors mounted and
to completely wrap, crimp, or solder-terminate the assembly.
Posted September 25, 2017