Designing inside the box

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

Broaching fasteners install permanently in all types of printed-circuit boards, as well as in components made from acrylics or polycarbonates.

Clinch fasteners for right-angle attachment are an alternative to bent tabs and flanges, brackets, tack welds, and loose hardware. Eliminating tab cutouts improves EMI/RFI shielding and keeps out dirt and dust. Mating screws can be inserted from each end of the fastener to mount a pair of PC boards. Open-top types support one edge of a PC board and help build mini enclosures in box corners.

Enclosures present special challenges for fastening and joining methods. Choices are typically limited to those that work in tight spaces and give access to enclosed components that might need service or replacement.

Conventional "permanent" fastening methods such as adhesives or welding don't permit product disassembly. Welding further causes undesirable fumes and burnouts and often needs complicated electrodes and pilots that can stall production. Some types of conventional mechanical fasteners have drawbacks as well. For example, sheet-metal screws may lack adequate holding power and aren't always reusable.

Self-clinching fasteners don't suffer such limitations. They install quickly and permanently, eliminate extra hardware, and ease access and serviceability. The use of captive screws keeps loose hardware from falling into power supplies or circuitry. Self-clinching fasteners become part of an assembly in which they are installed and thereby reduce inventory and the amount of hardware to handle. They can install with automatic equipment, making them suitable for high-volume production. And they fully satisfy both DFMA (Design for Manufacturing and Assembly) and DFS (Design for Serviceability) objectives. Self-clinching fasteners come in dozens of types in more than 12,000 variations.

Here are just a few:

Self-clinching threaded fasteners work in sheet metal too thin to tap and outperform extruded/tapped or stamped threads. They are typically steel, stainless steel, or aluminum and install permanently in metal sheets as thin as 0.020 in.

Installation involves pressing the fastener into place in a properly sized drilled or punched hole. This displaces sheet material, causing it to cold flow into a specially designed annular recess in the shank or pilot of the fastener, permanently locking it in place. A serrated clinching ring, knurl, ribs, or hex head stops the fastener from rotating in the metal when applying tightening torque to mating hardware.

Blind-threaded inserts work when there is access from only one side, as with tubing and extrusions. They install in workpieces as thin as 0.010 in. They let several thin sheets of dissimilar materials join at once before final components are attached. The approach eliminates spot welding or other secondary operations, saving production steps, time, and cost.

Broaching fasteners are good for component-to-board, board-to-board, and board-to-chassis fastening. These install permanently in all types of printed-circuit boards, as well as in components made from acrylics or polycarbonates. Common styles include broaching nuts with permanent threads for board mounting or component attachment.

All-metal inserts (brass, stainless steel, or aluminum) attach plastic components and permit repeated disassembly and reassembly. Types include ultra-sonic/heat-staking threaded inserts, which are installed with ultrasonic welders or with a thermal press. Molded-in threaded inserts install during the molding process, while press-in threaded inserts go in premolded or drilled holes.

Self-clinching snap-top standoffs work without screws or other threaded fasteners. They attach and accurately space PC boards and subassemblies. Upon installation, self-clinching standoffs use spring force to securely hold components while permitting quick attachment and removal of boards or panels with a tool-less snap-on/snap-off action. Threaded or unthreaded standoffs also permit the stacking or spacing of attached boards, while flare-mounted standoffs offer greater pull-out strength. Threaded studs serve as solderable connectors or as permanently mounted mechanical fasteners with external threads.

And one-piece board-mount screw assemblies with captive screws let PC boards easily mount and be removed.

Self-clinching keyhole standoffs let PC boards or panels slip into place and be easily removed (without tools) by sliding the board side-ways and lifting it off. They work especially well for spacing or hanging replaceable components. Typically, several keyhole standoffs are used along with a standard threaded standoff, which accepts a screw to fix components.

Self-clinching cable tie-mounts and hooks provide secure attachment points for wires within enclosures. They install permanently without screws and adhesives that can fail over time with temperature cycling.

Regardless of the type of self-clinching fastener used, be sure to test new designs before starting production.

MAKE CONTACT
PennEngineering Fastening Technologies, www.pennfast.com