PCB Connectors

Nov. 15, 2002
Most printed-board connectors are either a one-piece receptacle or a two-piece plug and jack.

Most printed-board connectors are either a one-piece receptacle or a two-piece plug and jack. With receptacle (edge) connectors, the printed-circuit board foils extend to an edge of the board and are mounted into the connector receptacle. With two-piece connectors, the connector jack is soldered or otherwise fastened to the circuit-board edge and mates with a receptacle. The most common style of PCB plug-and-jack connector is the Eurocard or DIN format.

Both edge and DIN connectors can accommodate single or double-sided-board foil connections. Contact current ratings range from 3 to 7.5 A for standard edge connectors and about 1 A for the smaller Eurocard contacts. Connectors can also be polarized through insertion of metal or plastic keys which fit into notches cut into the circuit board.

Circuit-board connectors have a variety of contact styles, which determine the number of insertions and withdrawals. Some high-priced contact systems provide as many as 100,000 contact matings without failure. Connectors attain these characteristics using highly compliant contacts, and are generally two-piece types.

When fewer than 100 contact matings are required over the connector life, one-piece receptacle connectors are generally used. These connectors contain fewer parts and materials than two-piece connectors, hence they generally cost less. Two-piece connectors are used to provide reliable connections in the presence of shock and vibration; simple one-piece connectors are often used in general-purpose applications.

Backplanes: Designers may choose either of two main types of buses, backplane or stacking. Standard motherboard backplane buses use one-piece edge connectors. To handle more complex circuits, designers usually prefer two-piece connectors.

Both card-edge and two-piece connectors are predominantly used with backplanes. Mating force limits the practical size of non-ZIF devices. In edge connectors, insertion force must range from 8 to 16 oz per contact to reliably connect thin boards while avoiding permanent damage to thick boards. This high force also limits edge connectors to 140 contacts or less. Most such connectors are designed for boards 0.062 in. thick and are not practical in bulkier multilayer PCBs.

Two-piece connectors overcome numerous drawbacks of edge connectors. Most versions use an 0.025-in. square pin and mating receptacle arrangement. Low tolerances minimize mating forces. Consequently, the connectors can contain up to 700 contacts. The use of pins and receptacles also allows the connectors to contain up to four rows of contacts.

Plug-and-socket connectors may cost more than other types but also require less board preparation. For example, two-piece units need no gold-plated PC pads, as do edge connectors. They also can do without the chamfered lead-ins that help guide boards into card-edge connectors.

Plugs and sockets make contact on two, or even four, sides of each male pin. The multiple contact makes the devices relatively insensitive to vibration. Edge connectors, on the other hand, rely on spring contacts that can eventually lose strength with repeated use.

Backplane connectors usually attach to a motherboard either by soldering or by using compliant pins. These pins provide a reliable gastight connection to plated through holes on the motherboard in lieu of soldering.

To handle a large number of signals, designers often employ a split backplane. This configuration divides bus connections between two motherboards, one on either side of the daughterboards. Using two motherboards shortens the path of connections to daughterboards and often improves performance. One drawback, however, is that replacing a daughterboard requires complete disassembly of the two motherboards.

It may be advantageous to employ side-entry ZIF connectors in frequently disassembled split-backplane systems. Contacts on these connectors are cammed open so daughterboards can slide into the connector. Thus, in split backplanes, ZIF devices serve as both electrical connectors and as card guides. This may eliminate other card-cage hardware and somewhat offset the higher cost of ZIF components.

Although ZIF connectors have zero-insertion force, some force is required to cam the contacts open. This force is proportional to the number of contact positions and limits the practical number of contacts. Nevertheless, these connectors are frequently used on boards containing 300 contacts or more.

The length of signal paths on standard backplanes may limit system speed and performance. Unequal delay times are particularly troublesome, resulting in distorted signals, and are difficult to prevent.

One kind of connection that alleviates such design problems is called a stacking bus. It uses connector tails extending through one board and plugging into a connector on an adjacent board.

The key feature of these connectors is that the connector can be located anywhere on the board, not just at the edges. Careful placement of connectors can eliminate the need for support hardware. Stacking connectors also free board edges for other uses and may shorten signal paths.

Boards in a stacking bus can have various sizes, shapes, and thicknesses. Additional boards can plug directly on the end of a stack.

The principal disadvantage of stacking connectors is that boards inside the stack are difficult to remove because their extraction requires some disassembly. However, a special two-piece connector called a stacking ZIF device can minimize some of these difficulties. Many stacking connectors also require boards with plated through-holes that increase the cost of some systems.

Surface-mount connectors: About a dozen manufacturers now supply surface-mount connectors. Most designs are horizontal or vertical headers and receptacles. Board-to-board and wire-to-board types, including D-subminiature and card-edge connectors are common. Many suppliers also have surface-mount sockets for ICs. Specialty connectors such as fuse block or screw terminated are generally not available because the size of these connectors defeats the benefits of surface mounts.

A through-mount connector joint can distribute forces over an area approximately five times larger than surface-mounted connectors (SMCs). To compensate for the smaller area, some manufacturers hold the SMC to the board using mechanical or chemical-bonding devices such as stakes, screws, or glue. But other types of connectors rely solely on solder joints to withstand outside forces. Each method has special advantages.

Even when solder is the only attachment used, the resulting joints have considerable strength. Strength depends on lead and solder type, the height of the lead above the board, and the quality of the joint. Individual joints have tensile strengths over nine pounds.

Depending on how the SMC is placed on the board, nonbonded connectors can often be mounted at faster rates then bonded types, and are often smaller and less expensive. And because of their size, they are self-aligning during reflow.

SMC leads are typically either L or J type. The L type is sometimes referred to as gull wing. It offers slightly more tensile strength than the J type, but also consumes more board area. Some designs have modified J leads, called J wings, which increase lead surface area.

L and J-type leads have similar tensile strengths. The tensile strength of both of these leads is a function of height above the board. Strength decreases as height increases. When selecting a lead type, factors other than board area and strength are important. Since the quality of a joint may influence both mechanical and electrical characteristics, ease of inspection is also a consideration. Gull-wing joints are easier to visually inspect than J leads.

Besides insertion and withdrawal forces, SMC joints experience forces exerted by thermal expansion and contraction between the connector and substrate. If the thermal coefficients of expansion (TCE) of these two materials is not similar, the induced stress may cause either electrical or mechanical failure. The effects of thermal shock can be considerable if the TCEs are not matched.

Housings are an important part of any connector design. Besides insulating individual leads, housings provide a reaction plate for preloaded leads and a means for pick-and-place machines to grasp the connector. Some housings also contain alignment pins which aid in accurate placement.

Housings are usually made of glass and mineral-filled thermoplastics. Polyethylene terephthalate (PET) polyester is often used as the thermoplastic because of its chemical and heat resistance and relatively low cost. When a connector must survive extended high temperatures, polyphenylene sulfide (PPS) may be used. PPS resists all solvents below 400°F but is affected by some amines and halogens. It is also more brittle than PET.

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