By Bruce Burroughs
Using these guidelines for choosing a PCB terminal block based on its metal parts assures a quality PCB-to-terminal connection.
 Figure 1. Fully extruded clamping bodies offer strength and corrosion resistance. |
Modular and pluggable printed circuit board (PCB) terminal blocks are crucial components to PCB production and applications. Typically constructed of two different components, the plastic insulating housing and the metal clamping and current-carrying bodies, the terminal blocks distribute power and signal to and from the board. Choosing a PCB terminal block that will ensure a reliable, effective, gastight connection is no small task.
Buyers and design engineers often hear about the importance of nonflammable construction of the insulating housings, but rarely are the metal materials or clamping designs sufficiently analyzed. To ensure a quality connection, design engineers must identify and select the appropriate PCB terminal block clamping body for the application, while not overlooking the importance of high-quality metal parts, such as screws and current-carrying parts, or the platings and base metals that give these components their durability and conductivity.
The first step in selecting a PCB terminal block is choosing a clamping body appropriate to the application. There are five different clamping body styles: 1) rising cage, 2) compression, 3) top-mount screw, 4) spring clamp and 5) insulation displacement contact (IDC). Each has proven to be effective in providing excellent metal-to-metal contact, but some of the technologies are more suited for one application over another.
Rising Cage Style
The rising cage clamping body features wire entry and clamping screws at a 90° orientation. The captive clamping screws rotate in their own housings to control the rising cage and to clamp and tighten the conductor against the current-carrying bar. There is no direct contact of screw to conductor and, therefore, no need for an accessory wire guard.
Two main methods are used for manufacturing the rising cage clamping body. In the first, the cage is manufactured of fully extruded nickel-plated copper alloy or zinc-plated steel, yielding a one-piece clamping body. The steel is used for strength and the zinc plating provides corrosion protection (see Figure 1).
The second design method creates the clamping body by stamping and folding the metal. A superior design incorporates the fold at the bottom of the rising cage, away from where the clamping screw engages the cage. This design prevents potential cross-threading of the screw, which could occur if the fold is at the top with the screw.
Regardless of extruded or folded design, it is crucial that enough metal is incorporated into the bottom of the rising cage to avoid a conductor being inserted underneath the clamping body while in the open position. Lower cost products often eliminate this design feature because of the added material costs, but with a corresponding decrease in safety. A conductor inserted underneath the clamping body will not be safely terminated, resulting in an open circuit or a dangerous short.
Compression Style
The compression clamping body is similar to the rising cage in that the clamping screw and wire entry relate at 90°. However, the clamping body is static in its housing, and the screw moves up or down to tighten or loosen the conductor. This clamping body is also manufactured from extruded nickel-plated copper alloy. The screw may be either nickel-plated copper alloy or zinc-plated steel.
A superior design incorporates a wire guard as a standard feature to prevent the screw from directly contacting the stranded conductor, eliminating potential damage to the conductor. The wire guard is manufactured of tin bronze to promote "memory" (i.e., the wire guard follows the screw back up, and does not remain in the closed position when the screw is backed out). The compression clamp may perform both mechanical and electrical functions when the solder tail is integrated into the clamping body.
Top-mount Screw Style
With the top-mount screw clamping body, the wire entry and clamping screw are in the same plane, facilitating wiring. The conductor is clamped against the current-carrying bar via a cam lever/screw mechanism. This orientation facilitates wiring in space-constricted applications. The clamping body is constructed of stamped and folded zinc-plated steel.
 Figure 2. Built-in test points ensure an accurate reading regardless of screw metal. |
The clamping screw is either zinc-plated chromated steel or nickel-plated copper alloy (brass). The PCB application, such as corrosive environments, the drive method (straight blade vs. Phillips head or posidrive) or the manufacturing method, such as forging or milling, may determine optimal screw material selection. Some manufacturers offer both types — steel for strength, and brass to avoid dissimilar metals (see Figure 2). The clamping screw must, in every instance, be captive to avoid loss of the component (i.e., if the screw falls out, the clamping body and/or the wire guard often follows, requiring the terminal block to be thrown away).
The captive clamping screw also prevents inadvertent loosening caused by vibration. A superior PCB terminal block incorporates a braking mechanism at the fully open position to facilitate use of automatic screwdrivers. A screw-activated clamping body offers the ability to maintain a precise torque and retention of the conductor over the life of the product, regardless of cold flow or temperature cycling effects on the conductor and clamping body.
Spring Clamp Style
Spring clamp PCB wire entry and tool entry are in the same plane as a top-mount version for easy wiring access or at 90° or other angled versions based on the application. Spring clamp technology is screwless, with constant tension of a spring forcing the conductor against the current-carrying bar. Proponents value the antivibration retention and the decrease in wiring time. The spring is typically made of corrosion-resistant spring steel, with memory and strength to prevent loss of retention over time or because of temperature cycling.
IDC Style
The IDC PCB terminal block eliminates the need for cutting and stripping wire conductors. A connection is facilitated by inserting an unprepared (unstripped) conductor into the clamping body manufactured of copper alloy. Superior design allows the clamping body to move to the wire while the conductor is stationary, facilitating a connection even when wire length is maxed out. Optimally, the clamping body includes special plating for strength and corrosion resistance. This clamping body performs both electrical and mechanical functions.
Solder Tails and Platings
Separate from the clamping bodies, but handling most of the electrical function of a terminal block, solder tails and current-carrying bars must be examined in the quest for quality metal parts. These two components are often integrated into one piece, and transmit the power or signal from the conductors to the PCB. Solder tails and current-carrying bars are base material copper for conductivity, with various prime and underplatings, depending on the application.
Normally, PCB terminal blocks have an underplating in conjunction with the prime plating to provide a diffusion barrier to improve durability and to prevent corrosion and solderability problems. The most common material for underplating is nickel because of its superior ability to act as a barrier and improve durability. Some manufacturers used to include 2 to 5 percent lead (Pb) underplates to prevent "tin whisker" growth. This has been eliminated by many because of environmental concerns over lead.
 Figure 3. A secure pluggable design provides a gastight connection for superior metal-to-metal contact. |
Often, underplatings are eliminated to reduce costs. However, risks are involved with this cost-saving measure. Copper migration, corrosion, contamination and dewetting all decrease solderability and conductivity. Specialty surface platings such as gold and silver are also employed for specific applications. Gold is generally used where superior plating integrity and low, stable resistance are required, and in applications where arcing may occur, such as hot-plug applications. Silver is used in ultra-high-frequency and power applications because of its conductivity.
The plating process itself must ensure a smooth, clean, impervious surface to prevent problems of contamination before soldering occurs, as well as to avoid corrosion over the life of the connector. Uneven, porous plating increases the likelihood of contact failure. Plating should always be applied after blanking and forming so that no bare edges are exposed to the environment. Electrostatic or galvanic plating is a superior plating application process. In the clamping bodies where wire guards are employed, or where the clamping mechanisms have direct contact to the conductors, superior designs incorporate ridges. This serves two purposes: the ridges grip the conductors more firmly (mechanical), thereby stabilizing contact resistance (electrical).
In two-piece pluggable PCB connectors, the mating area between the plug and socket is also a design issue, and again both mechanical and electrical functions must be considered. Firm contact must be maintained between the male and female contacts via normal forces to create stable resistance, maintain gastight connections and prevent corrosion via fretting, the vibration inherent in a mated contact (constant rubbing at the mate zone; see Figure 3). If contact is not tight enough and normal forces are too low, then fretting will result in wearing away of the plating surface, possibly permitting corrosion and increasing resistance (both of which are detrimental to the transmission of the signals or power).
Normal forces that are too high will create a situation where plating can be rubbed away during mating/unmating or inhibit wiping action. Wiping action will remove surface contaminants, and occurs when contacts are mated/unmated. Normal forces are also affected by the environment, as heat and humidity cycling over time can result in stress relaxation of contacts. An environment that has strong vibration characteristics can also lead to increased fretting corrosion.
Conclusion
Contact manufacture and design are as important as material selection in producing superior connectors. Major pitfalls to avoid are contamination, stress relaxation, and wear and fretting corrosion. These can be avoided through proper design, material selection and manufacturing processes. High-quality connectors eliminate many problems in the field and prevent costly downtime and troubleshooting or product replacement. Choosing quality metal parts of a PCB terminal block assures a high-quality, effective PCB-to-terminal connection. By specifying terminal blocks using these guidelines, design engineers can improve the functionality and reliability of their products.
BRUCE BURROUGHS is Group Product Manager, Wieland Electric Inc., 49 International Rd., Burgaw, NC 28425-0759; (910) 259-5050; Fax: (910) 259-3691; E-mail: bburr@wielandinc.com; Web site: www.wielandinc.com.
SPEC SHEET
End Applications:
PCB production and applications
Related Products:
Terminal blocks, PCBs, IDCs
Main Point:
Choosing a PCB terminal block that will ensure a reliable, effective, gastight connection is no small task. Design engineers must identify and select the appropriate PCB terminal block clamping body for the application, while not overlooking the importance of high-quality metal parts, such as screws and current-carrying parts, or the platings and base metals that give these components their durability and conductivity.
