Circuit-board fasteners that won't let go

Aug. 4, 2005
The latest hardware is smaller, installs faster, and holds better.

Jay McKenna
Special Products Manager PennEngineering Fastening
Technologies Danboro, Pa.

Surface-mount fasteners are supplied on tape and reels, and install using standard automated surface-mount and solder-reflow processes.

Panel fasteners permit quick removal and reinstallation of boards. They feature a steel retainer soldered to the board and a separate metal drive screw held in an ABS cap.

A knurled shank on broaching fasteners provides a firm, interference fit that resists pullout and rotation.

Miniature self-clinching fasteners require minimal space and provide strong, reusable threads in metal sheets as thin as 0.019 in.

Self-expanding fasteners sink knurled teeth into the board while a shank expands to ensure contact.

No matter how sophisticated electronics get, circuit boards and other components must still be securely attached to their housings if they are to deliver optimum performance.

Incorporating printed-circuit (PC) boards into designs requires component-to-board, board-to-board, and board-to-chassis attachments. Some boards need to be stacked or spaced, and others have to be removed and reattached periodically. At the same time, board fabricators are under the gun to fit more into less space, do it faster, and with higher quality. To keep up with these demands, fastener manufacturers have improved their fasteners for PC boards, as well as the methods used to install them.

In the electronics industry, concerns about damaging PC boards when installing fasteners in the final stages of assembly have prompted an innovative solution: surface-mount fasteners. These fasteners mount on PC boards in the same manner and at the same time as other surface-mount components, prior to the solder-reflow process. They use standard automated surface-mount techniques, and simply become another board component.

Surface-mount nuts, spacers, and panel fasteners are already on the market, with right-angle fasteners and other variations slated for introduction in the near future.

Panel fasteners, for example, work well in applications requiring easy removal and reinstallation of boards. The hybrid metal/plastic devices feature an electro-tin-plated steel retainer and a metal Phillips drive screw held in an ABS cap. Installation involves soldering the retainer to the board. The mating cap-and-screw snaps onto the retainer, and spring action of the cap's plastic "fingers" holds the screw in a retracted position. Tightening the screw completes the assembly and, when secure, the plastic cap completely covers the retainer.

To keep pace with automated assembly lines, retainers and other surface-mount fasteners come on tape-and-reel packages. Depending on the type and size, each reel holds up to 1,500 fasteners. Screws are supplied separately.

Surface-mount technology reduces risk of damage to boards — and resulting scrap — due to improperly installing fasteners with offline equipment. It also reduces handling of loose parts and eliminates many secondary operations, leading to labor savings and faster production. Surface-mount technology can also be applied to other designs that use automated, high-volume, high-speed production.

TORQUE-OUT, lb-in.
SMT spacer/nut
Broaching nut
Threaded stud
Self-expanding fastener
The table shows typical test data for fasteners installed in 0.060-in. FR-4 fiberglass sheet. Variations in mounting-hole size, sheet material, and installation procedure, as well as the amount of solder, soldering time, and temperature, will affect actual results.

The electronics industry increasingly demands fasteners that reduce hardware, and broaching fasteners offer practical alternatives to traditional loose hardware. They install permanently and require fewer parts.

Broaching fasteners have a knurled shank that presses into punched or drilled holes, providing a strong threaded or unthreaded attachment point in nonductile materials. They are suitable for all types of printed-circuit boards, as well as acrylic, polycarbonate, and aluminum sheets. Specially formed axial grooves around the shank broach, or cut, into the base material, creating a firm, interference-type fit that resists pullout and rotation.

Parts designed for specific applications include:
Broaching nuts. Steel or stainless-steel nuts press into PC boards and offer permanent threads for mounting boards or components. Only a mating screw is required to attach parts.

Broaching standoffs. Threaded or unthreaded fasteners in steel or stainless are for stacking or spacing boards. A spring-action brass standoff secures PC boards without screws or threaded hardware, letting boards snap in place and remove easily without tools. A brass, flare-mounted design offers high pullout resistance.

Broaching threaded studs. The phosphor-bronze fasteners serve as solderable connectors or permanently mounted mechanical fasteners with external threads.

Broaching panel fasteners. With captive screws these one-piece stainless screw assemblies simplify board mounting and removal.

Broaching fasteners install quickly and permanently using punch and anvil tooling tailored to the fastener. For nuts, standard standoffs, and one-piece board-mount assemblies, users place one in an anvil and center the mounting hole in the board over the fastener shank. Applying a squeezing force embeds the fastener's shoulder in the board.

Installing flare-mounted standoffs involves setting a fastener in a drilled or punched hole in the board. A flaring punch and recessed anvil squeeze it until the shoulder of the fastener contacts the board. As the fastener seats, the punch tool flares the extended portion of the shank to complete installation. The combination of broaching and flaring provides high push-out resistance.

For threaded studs and snap-action standoffs, installation technicians place the fastener in a mounting hole and, with punch and anvil faces parallel, squeeze it until the fastener head contacts the board.

Other fasteners for board-related attachments include:
Self-expanding fasteners
are stainless-steel products suited for boards with plated through-holes. They feature a self-expanding shank that ensures positive contact with the plating and eliminates any risk of shaving the plating out of a hole. A flat punch and anvil squeeze the fastener until the tips of the projecting knurl teeth embed in the board as the inside shoulder of the knurl contacts the board. As the fastener seats itself, the shank expands to complete installation.

Grounding standoffs install in steel or aluminum chassis to ground PC boards. Assemblers place the barrel end into a round mounting hole in the chassis and apply a squeezing force until the head embeds flush with the surface.

Miniature fasteners address a significant trend in electronics assembly — downsizing. As manufacturers pack more components on smaller boards, there is less room to position and install fastening hardware. And thinner sheets can jeopardize holding power.

One solution is miniature self-clinching fasteners. Options include locking and nonlocking threaded nuts, threaded studs, and threaded standoffs. They require minimal space and provide strong, reusable threads in metal sheets as thin as 0.019 in. As with all self-clinching fasteners, they need fewer assembly operations than loose hardware, deliver more holding power than sheet-metal screws, and can mount closer to a surface edge than other fasteners.

Much like broaching fasteners, miniature self-clinching fasteners install by embedding a knurled collar into a sheet. It becomes a permanent part of an assembly and eliminates any risk from loose hardware, even when components are removed for service.

Despite their small size and precision manufacture, miniature self-clinching fasteners are highly reliable due to the knurled-collar. When installed, the embedded collar prevents fastener rotation.

The fasteners are typically made from 303 stainless steel and are used in aluminum and cold-rolled-steel sheets. Thread sizes range from #0-80 through 1 /4-28 and M2 through M6. For best performance, mating screws should be long enough so that at least two threads project through the fasteners when tightened.

PennEngineering Fastening Technologies
(800) 237-4736

About the Author

Kenneth Korane

Ken Korane holds a B.S. Mechanical Engineering from The Ohio State University. In addition to serving as an editor at Machine Design until August 2015, his prior work experience includes product engineer at Parker Hannifin Corp. and mechanical design engineer at Euclid Inc. 

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