Generally, nonrotating wire contacts on screw-terminated connectors handle 12 through 30 awg solid or stranded wire. Similarly, the connectors handle a variety of current capacities, making them useful in power as well as high-speed signal-handling applications.
Screw-terminated connectors include one-piece units and two-part plug-in assemblies. One-piece connectors contain a barrier strip and accept wires that need not be soldered or attached to intermediate connectors such as crimp-on lugs.
Dual-contact fingers on some one-piece connectors provide redundant contact for reliability. Other units employ a cantilevered design that allows wires to be inserted and removed frequently without scoring the bond pads.
One-piece screw connectors also offer screw contact orientations at 90 or 45° from the board surface or parallel. The feature facilitates wiring in restricted locations.
Two-part connectors come in the form of male and female halves. The male half is soldered to the circuit board and female part serves as a socket. Wires are terminated into the female. This modular configuration permits several wires to be connected or removed with one operation.
Two-part connectors are particularly convenient in assemblies needing frequent removal for field servicing. The devices minimize the chance of errors in the field because they keep wires in the correct sequence when disconnected. Typical two-piece connectors terminate between two and 20 wires.
A variety of two-part connectors is available. One type has plug-in mating parallel to the PC-board surface with integral lock-in tabs that resist shock and vibration. Another design incorporates locking tabs and stacks the two connector halves vertically. Both designs conserve board space and permit increased circuit density. As in single-piece devices, connectors offer a choice of horizontal or vertical headers and 90 or 45° screw orientation.
Special connectors are available for installations with height limitations. These use low-profile female halves. Even lower profiles are available through use of flush-mount designs where the entire terminal strip rests directly on the circuit board. This configuration significantly reduces the stress on solder joints encountered when tightening screw terminals on ordinary connectors.
Standard miniature connectors are normally available in either 5 or 10-mm centerline versions to meet specific creepage and clearance requirements. The option permits a design density of up to five circuits per inch or, where board space is severely limited, up to 15 terminations/in.2 Headers may also be ordered separately in the exact length needed.
A number of optional accessories are common in screw connectors. For example, latching pins can be specified to lock connectors onto the board in high-vibration conditions. Polarizing pins may be used to prevent connector cross wiring.
The specification of screw-terminal connectors involves a variety of factors that may not be obvious at first glance. For one thing, the distance between the last connector pin and the end of the plastic housing should be less than the spacing between pins. This allows two connectors to be butted together end to end without wasting board space.
Use of wire protectors is another important feature. High-quality connectors clamp wires under such protectors rather than clamping wire directly under the screw. Wire protectors basically allow the connectors to hold fine-stranded wire directly. However, some low-cost connectors contain no wire protectors and allow the screw to come directly down on the wire.
So-called bias-design wire protectors can be specified to hold wire snugly in the box contact before fastening. The wire protector acts as a spring biased downward so that it holds a wire in the connector without tightening the screw. Connectors can also be obtained with "dead-front" construction. Here, the screw is recessed in the plastic to reduce the possibility of accidental shorts to adjacent parts or personnel.
Connector housings are available in 6/6 nylon or similar materials that resist damage from solvents. The property is particularly important in connectors that must undergo flow soldering. ABS/PVC blends or other less-expensive connector materials are prone to attack from solvents used to remove excess solder flux.
Contact styles in female two-piece connectors vary widely among brands. Connectors can be found with as few as two and as many as seven points of contact between male and female parts. Contacts are essentially formed from cylinders cut lengthwise into individual spring fingers. The number and shape of the fingers are proportional to the insertion force required to connect or disconnect the connector. The number of contact fingers also determines the electrical integrity of the joint. Because contact area is critical to connector efficiency, contact style is an important issue.
Mechanical wire termination is the most widely used method for attaching wires to connectors. This includes clamping, riveting, cold welding, and ultrasonic bonding. However, the two most common methods are solderless wrapping and crimping.
Solderless wrapping produces a reliable gastight connection that has a large contact area with low contact resistance. The wire is wrapped around a long terminal post with a square or rectangular cross section. These posts are available on many connectors and are long enough to accept at least three wire-wrapped connections, which can be rewrapped if necessary. Stranded wire cannot be used.
Solderless-wrap connections can be made by hand-operated, semiautomatic, or automated machinery. In operation, a wire with a stripped end is inserted into the tool wrapping bit, which is then positioned over the terminal to be wrapped. The wire is wrapped around the terminal in about one-tenth of a second.
Crimping requires minimal operator skill and produces consistent and reliable terminations at high production rates. Stranded and insulated wire are crimped with ease; even coaxial cable and flexible flat cable pose no serious problem. However, tooling is critical and the right combination of wire, terminal (or contact), and crimping tool is necessary for ensuring reliable joining. The cost of specialized tooling is offset by the convenience of crimped contacts in production and field repair. One tool or machine crimps a range of contact sizes by using interchangeable dies.
Insulation displacement connectors (IDCs) typically terminate ribbon cables, and are available with terminals matching the number of conductors in all popular cable widths. Cables are trimmed square on one end (but not stripped) and inserted into an IDC connector. All connections are then made simultaneously by squeezing the conductors into connector terminals using manual or automatic equipment.
Designs for IDCs vary widely, but all connections are based on forcing each conductor between the prongs of a split terminal. The prongs cut through the insulation and squeeze the bared conductor. The result is a high-pressure, gastight, solderless connection.
Several IDC types are available. Solder-transition connectors, for example, typically have two rows of pins on 0.1-in. spacing for direct soldering to PCBs. These IDCs generally can be soldered into PCBs either before or after a cable is terminated in the connector.
DIP plug connectors for ribbon cables generally contain from 14 to 40 gold-plated pins on 0.1 X 0.3-in. centers for inserting into standard DIP sockets. Rugged versions are available for applications involving excessive insertions and removals.
Card-edge connectors for ribbon cables join flat cables to double-sided PCBs having termination pads on 0.1-in. centers. Connectors are also available for use with European standard DIN headers. Other connectors join 0.05-in. flat cable with 9, 15, 25, and 37-pin D-type connectors with 0.545-in. spacing.
The impedance of most connectors matches that of flat cable within a few ohms. Flat cables terminated in IDCs generally are more reliable than cables terminated by other means because connections are less subject to operator skill and wiring errors are unlikely. Flat cables, moreover, typically distinguish one edge from the other with different colors, and each fifth conductor is usually color coded to ease wire identification and troubleshooting.
Because of industry's increased concern with flame resistance, most IDC manufacturers now use plastic compounds in the connector housing that have UL 94 V-0 rating. The V-0 rating is given to material that will not support self-combustion. The ratings are assigned by testing material to UL Standard 94, a flammability test procedure.
Most IDC housings are made from a 15 to 30% glass-filled polyester or nylon. These materials provide the strength and rigidity needed to hold contacts firm and to maintain dimensional tolerances. The choice of one material over the other is dependent upon the application in which it will be used. Polyester is used because of its resistance to a wide variety of industrial solvents and chemicals. It also is more stable under high humidity conditions, both mechanically and electrically. Nylon is used primarily in applications where toughness and flexibility are required, in addition to withstanding higher operating temperatures.