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The overall
connector design can be considered a package, with variations within each design
that determine its ultimate configuration and attributes. To this end, the following is an elaboration
on the individual parts that are selectively chosen to make up the various
connector packages (designs) discussed under Technologies.
Materials
Materials for
the connector contact, insert, and shell, are all chosen to accommodate very
specific design and cost requirements for the intended use application.
Contacts
Contacts are
the heart of the connector design and the material choices are:
a. Beryllium copper – The best electrical
conductor of any spring alloy of comparable hardness. It is stronger and more resistant to fatigue
than any other copper alloy. It has excellent corrosion resistance (comparable
to pure copper) and a wide operating temperature range (sub zero to
+149°C).
b. Phosphor Bronze – Alloy grades A and C are
widely used for its good corrosion resistance and fair electrical
conductivity. It is a good
general-purpose material for those applications below +107°C. Alloy C is preferred for its moderately high
mechanical properties. Alloy A is chosen
for its lower cost when technically acceptable.
c. Spring Brass – Popular for
low-cost applications where high temperature or high mechanical stresses are not
a consideration. It is often used in
combination with Beryllium Copper contacts to enhance the spring tension contact
when mated.
d. Low Leaded brass – Often the preferred rod
stock for male pins chosen for ease in its machining. It has good electrical properties and is
resistance to corrosion and stress cracking.
It is often used in combination with Beryllium Copper contacts to enhance
the spring tension contact when mated.
Inserts
Insert material is chosen for the lowest cost per unit
volume of the connector, and the shortest molding cycle, while still meeting all
of the electrical, mechanical, thermal and chemical parametric requirements of
the intended use application. An
important consideration often overlooked is the chemical compatibility of the
insert, with the cleaning agents used in the subsequent manufacturing soldering
processes. MIL-M-14 is a valuable
reference for severe environments and can be useful in less demanding
applications. Table 1
lists some of more the popular insert materials and their recommended
temperature ranges. The use of insert material beyond published ratings by the
manufacturer is not recommended. For
those special applications that may tolerate over-heating of materials for
shorten operational life, the manufacturer should be consulted as an additional
insight may be offered.
Shells and
Associated Hardware
Shells and
associated hardware for mounting, indexing, and securing are manufactured from a
limited number of materials. Shells and hoods are usually sheet or die-cast
Aluminum, but sometimes cold-roll or stainless steel is used, which are often
the preferred materials for locking devices, cable clamps, guide pins and jack
screws. Brass is also used for guide
pins and miscellaneous assembly screws.
Plating and
Coatings
Plating and
coatings are selected for both durability and corrosion resistance, and
sometimes, appearance. Connector shells
are often anodized but chromate, tin, cadmium plating, and paint are also
used. For appearance, a clear chromate
over cadmium plating provides a bright, silver like finish. Contact plating is necessary to ensure
continued acceptable conductivity as the connector ages and is repeatedly mated
and disengaged. Both wear and corrosion are factors affecting the useful
connector life. The plating choices are
dependent on the connector design and its intended use. A common plating methology uses hard gold on
the sockets and soft
Table 1. Common Insert Materials
|
Material Types (Common
Names)
|
Temperature Range*
(C)
|
|
Diallyl
phthalate
|
-50 to
+200
|
|
Ceramic
|
-55 to
+590
|
|
Epoxy resin
|
-40 to
+157
|
|
Ethylene
Propylene
|
-40
to
+105
|
|
Kel-F (CTFE)
|
-55 to
+150
|
|
Fluorosilicone
|
-55 to
+200
|
|
Melamine
|
-55 to
+130
|
|
Mica-filled
Bakelite
|
-55 to
+150
|
|
Neoprene
|
-55 to
+120
|
|
Nylon
|
-55 to
+120
|
|
Polyamide
|
-40 to
+100
|
|
Polyethylene
|
-55 to
+80
|
|
Polyimide
|
-55 to
+250
|
|
Polystyrene
|
-55 to
+85
|
|
Polysulfone
|
-55 to
+150
|
|
Rexolite
|
-55 to
+85
|
|
Silicone rubber
|
-55 to
+200
|
|
Teflon (FEP)
|
-55 to
+200
|
|
Teflon (TFE)
|
-55 to
+250
|
|
Vitreous glass
|
-55 to
+250
|
|
Polychloroprene
|
-55 to
+120
|
*NOTE: Values are based on best available data from a variety of
sources. Manufacturers should be consulted for specific design applications.
Also, see listing in the Connector Design and materials section for supplier
names and related trade
names.
gold on the pins, which provides a burnishing action as the
interfaced contacts move. The under
plating or flash is also important, as thin or porous gold over nickel or silver
is a problem, with long time storage of contacts in high sulfur atmospheres,
and/or for connectors exposed to high ambient operating temperatures (e.g.
automotive under-hood). Nickel has an
advantage over silver, as it does not oxidize as readily as the silver sulfides
when exposed. Nickel oxide and silver
sulfide is always a conductivity concern, and silver sulfide is a problem in RF
applications as the sulfides that tend to form on the skin of the plating induce
significant RF impedance into the circuit, especially at microwave frequencies.
Contact/Insert
Assemblies
Contact/insert assemblies with captive inserts and
solder cups are still used, however, crimp-type insertable/removable contacts
are now preferred in many applications to eliminate solder problems and to
facilitate repair or changes in contact arrangement. These designs are available
with variations in insertion/withdrawal techniques and contact entry
configurations. Circuit board edge connectors and some blade-type contacts have
a single tine that snaps against a shoulder in the molding and is easily
inserted by hand and removed with a simple tool. The retention systems for pin
and socket contacts and entry designs are described below.
Retention
Systems
Two popular retention systems
are described in paragraph a. and b.
Other retention systems are described in c and d.
a. A collar with spring tines is incorporated
into the contact and when inserted, snaps against a shoulder in the insert.
Insertion is accomplished by hand or with a tool, and withdrawal is done with a
tool.
b. The contact is grooved and inserted into a
resilient insert material, which fills the groove, and retains the contact.
Simple insertion and removal tools are required.
c. REMIä has a unique design, consisting
of a metal retention sleeve, molded into the insert that accepts either male or
female contacts, which snap in place with finger pressure and are removed with a
tool. This feature places
insertion/withdrawal stresses on the two metal pieces rather than the insert
material.
d. The Little Caesarä retention system incorporates a
resilient wafer with a cone configuration that expands and then snaps back
against the rear contact shoulder to hold the contacts in place. Insertion and
withdrawal are from the rear using a simple tool.
Entry Configurations
Entry
configurations are either closed or open; each provide spring pressure ensuring
continuity, by keeping the pin and socket in intimate contact. Open entry designs are more susceptible to
damage of the female contact when poor alignment occurs or by insertion of a
test probe. Closed entry designs reduce
this possibility of damage, as they incorporate a solid ring that guides the pin
into the socket. Spring pressure is
accomplished either by incorporating a spring-loaded feature in the socket that
presses against the inserted pin, or by a pin that has a slightly larger
diameter than the socket, but is of a compressible configuration.
Crimp
Connections
Crimp connections are a cost effective and reliable method for
attaching the electrical connector contacts to applicable electrical wiring.
Since there is no heating at the juncture, thermal damage to the insulation or
small wire/contacts is eliminated. The proper tool selection and set-up for a
given style crimp of the correct size for a given wire size is imperative. When
appropriate tooling is used, operator error and variations in crimp integrity is
also virtually eliminated. The wrong tool, tool set-up, or size can result in
over or under crimping, that will damage the wire or leave it loose. Both are unreliable connections that may
result in premature failure or a latent defect. An effect check on crimp
integrity can be achieved by adjusting the crimp depth, to allow the wire to
withstand a minimum of 75% of the wire’s tensile strength (see Table 2).
Table 2. Tensile
Strengths of Crimped Wire
|
Wire
size
|
Tensile strength, Lbs. (After
crimping)
|
|
26
|
7
|
|
24
|
10
|
|
22
|
15
|
|
20
|
19
|
|
18
|
38
|
|
16
|
50
|
|
14
|
70
|
|
12
|
110
|
|
10
|
150
|