Electrical products range in size from the microns of integrated circuits to mile-long automated control systems sitting in factories and processing lines. And no matter how far designers stretch their imaginations — whether they’re inserting selftapping screws into plastic housings or securing 500-MCM bus bars — they always find suitable fasteners to hold these creations together.
Mechanical fasteners for electrical products fall in three categories. One deals with threaded inserts for plastic. These hold plastic cases to metal panels or to other plastic parts. Another bites into metal sheets such as panels and chassis. Because the main job of these fasteners is to hold components together, they depend primarily on mechanical properties. But a third type of fastener depends also on its corrosion resistance and electrical properties, such as bolts and nuts that connect solid bus bars or screws to wires in terminal strips.
Plastics in a bind
Threaded inserts for plastics come in three styles. Ultrasonic tools install one type, another molds directly in the plastic, and the third press fits into an existing hole. Ultrasonic tools generate high-frequency vibrations that heat the plastic-to-insert interface and melt the plastic. The plastic flows into the insert’s knurled serrations, barbs, and undercuts, then resolidifies holding the insert in place. Ultrasonic installation provides higher torsional and pullout forces than cold press-fit methods.
Ultrasonic inserts for plastics are further classified into three types. One fits tapered holes for rapid and accurate alignment. Another fits straight-walled holes, where a special lead-in helps alignment. The third is a symmetrical insert which installs in either a straight or tapered hole and needs no special orientation.
Although these inserts are designed primarily for ultrasonic installation, heat from a thermal press is also sufficient to seat them. Metal pins transfer heat to the inside of the insert to soften the plastic. The press drives the insert into place, and the fastener locks into the plastic after cooling.
The second style, molded inserts, requires core pins. The plastic flows around the pins in the mold cavity, and when the pins release, the plastic encapsulates the inserts. Molded-in inserts adhere to the plastic better than other styles and provide higher torsional and pullout properties. The method also seals the top with plastic, a benefit lacking in press-fit or ultrasonic bonding.
Four types of inserts are made for molding, two with blind ends and two with threaded-through holes. Blind inserts prevent the plastic from finding the threads. One blind insert contains intentionally deformed threads to hold screws captive under high vibrations.
Threaded-through inserts have pilot diameters and undercuts for gripping the reflowed plastic, providing high pullout resistance. One threaded- through insert comes in various lengths for injection-molded assemblies and has a uniform knurled diameter to reduce the chance of pulling sink marks on the opposite side of the panel. The other threadedthrough insert press fits into drilled or premolded holes. An advantage of press-fits is they don’t require heat or ultrasonic bonding. The inserts drop into the holes and a simple press drives them in place.
Postmolded, press-fit inserts come in four different styles. Three have vertical slots which collapse during installation. Once in place, the insert expands under an assembly screw and broaches diamond knurls into the plastic walls. These inserts are ideal for plastics that can’t take high stresses — the inserts absorb most of the installation force.
One of these inserts contains a straight knurl on top and offers higher torsional resistance. Another provides a flange for assemblies where mating parts may not make direct contact. Yet another type of insert has a hexagonal barb which displaces plastic as it travels into the hole. In this insert, the plastic springs back to fill in the grooves between the barbs, providing high pullout resistance. Its hex shape also increases torsional resistance.
Design for assembly
Self-clinching fasteners are also widely used for electronics products. They are usually threaded and press into ductile metal or circuit boards. The circuit- board material displaces around the mounting hole, and the metal cold flows into a space around the fastener’s shank. A knurl, rib, or hex head prevents the fasteners from turning in place.
Self-clinching fasteners need less space and fewer assembly steps than traditional fasteners such as anchor nuts. They join sheet metal and various thin materials where other fasteners could pull out or turn in place under torque. Self-clinching fasteners are reusable, and they hold tighter than sheet-metal screws. Even when the base material is thick enough to be tapped, it’s often less expensive to use a self-clinching fastener with deep threads.
Self-clinching fasteners are favored in equipment where components are replaced occasionally, and where loose hardware such as nuts can’t be reached. The fasteners become an integral part of the panel, making assembly or disassembly easy in blind spots. Assemblers need only insert a screw in a hole and spin it in.
Self-clinching fasteners provide three advantages over loose fasteners. For one, they have fewer parts, eliminating hardware such as flat washers, lock washers, and loose nuts in final assembly. Second, the number of steps for final assembly decreases because hardware installation is done during fabrication. And third, fewer parts and less steps equal shorter assembly time.
Similar permanent fasteners come in a wide variety of styles for printed-circuit boards of all sizes. The fasteners hold electronic components to circuit boards, boards to boards, and boards to chassis, and fit all board materials such as acrylic, polycarbonate, and aluminum.
Several types of fasteners are available for different needs. One type, broaching nuts, press into PC boards, providing permanent mounts that could not otherwise be threaded into thin sheets. Others, standoffs and studs, flare mount and can be used as solder connectors, or they can simply hold stacked boards. Spring-loaded standoffs hold a PC board in place without screws or threaded hardware, while others hold screws captive while releasing the board.
Standoffs designed for fastening in steel and aluminum sheets also provide electrical ground connections for PC boards. Yet another fastener has a self-expanding shank that makes electrical contact with the inside of platedthrough holes in PC boards without shaving the plating from the hole.
Electrical fasteners typically use nonferrous components to prevent electrolytic corrosion and rust. Electrolytic corrosion develops between copper conductors and steel alloys in humid atmospheres, and rust lowers electrical conductivity and can seize screws. Consequently, high-quality clamping parts are usually made of brass while screws are high-tensile copper alloys. Likewise, threaded bars, thrust members, clamping part pockets, and terminal sleeves are copper alloys. These materials are also particularly resistant to stress-corrosion cracking. In addition, the materials provide high electrical conductivity which keeps temperatures down. Moreover, the screws are less likely to loosen because the thermal expansion between conductors and clamping parts is low.
For example, a new clamp from Phoenix Contact, Harrisburg, Pa., is based on a moving-cage design, called the Reakdyne principle. A threaded bar contains wide lugs which sit in a clamping part pocket. A groove on the upper side of the cage secures the screws. As the screw tightens and torque increases, the clamp walls move toward the screw and exert an increasing force. Progressive locking action generates increasing thread friction as the clamping part goes through elastic deformation. The terminal screw compresses the current bar in the cage while drawing the conductor tightly against it. The high force embeds the conductor in the soft tin layer of the current bar and provides a resistance of less than 0.3 mΩ.
Copper conductors can be clamped without pretreating. For example, although solder pretreating is a common operation, it should be avoided because tin in the solder alloy fractures under high pressure, increasing contact resistance and temperature rise. Also, corrosion can develop at soldered conductor ends, and notch fracture can develop at the transition point between rigid and flexible conductors. Copper ferrules, rather than solder, can contain and protect stranded conductors. They ensure high current transfer and reduce stress corrosion.
Aluminum conductors present a different problem. When aluminum is stripped, a thin non-conductive, oxide coating quickly forms on the surface of the conductor. Fasteners must penetrate the oxide to provide a conductive, gas-tight connection. To guarantee a reliable contact, observe the following minimum guidelines:
• Remove the oxide layer on the conductor with a clean wire brush.
• Dip the conductor end quickly in a nonacid or nonalkali material, such as petroleum jelly.
• Retighten the connection after a few days to ensure a secure connection.
• Keep the installation site free from damp and dirty atmospheres.