Tech Briefs

Fasteners for Printed Circuit Board Attachment

Surface-mount fasteners reduce damage to PC boards during attachment.

No matter how sophisticated or advanced, electronic components must be attached reliably and securely if they are to deliver optimum performance. Beyond this universal requirement, particular attachment demands often will emerge. Printed circuit (PC) boards carry their own set of challenges, based on function and location in an assembly.

Incorporating PC boards may require component-to-board, board-to-board, or board-to-chassis attachment; some boards may need to be stacked or spaced; and others may have to be removed and reattached from time to time. As a result, strides have been made in fastener products for use in or with PC boards and in the methods used to attach them.

In the electronics industry, concerns about potential (and expensive) damage to PC boards during fastener installation in the final stages of manufacture have recently prompted an innovative solution: surface-mount fasteners. These can be mounted on PC boards in the same manner and at the same time as other surface-mount components utilizing standard automated surface-mount techniques. The fasteners simply become another board component.

Panel fasteners for surface mounting are already on the market (with standoff and right-angle fastener types to come). The panel fasteners serve well in applications requiring easy removal and reinstallation of boards. Hybrid metal/plastic panel fasteners feature an electro-tin-plated steel retainer and a separate metal Phillips drive screw captivated in an ABS cap. The metal retainer adheres to the board for mating with the cap and screw. The fastener assembly is completed when the screw is snapped into the soldered retainer. (A spring action of the cap’s plastic “fingers” holds the screw in the retracted position. When tightened, the plastic cap completely covers the retainer.) To keep pace with automated PC board assembly lines, retainers for these panel fasteners are packaged on tape-and-reel (each reel holding 465 retainers). Screws are packaged separately in bags.

ImageAs fasteners join the lineup of other soldered surface-mounted components, users find they can reduce the risk of damage to boards (and resulting scrap) that may occur when improperly installing fasteners with offline equipment, reduce handling of loose parts and realize labor savings, and boost production. Installation is fast and secondary operations are eliminated.

The electronics industry increasingly seeks fastener attachment that will reduce hardware requirements and promote specific functions, and broaching fasteners offer practical alternatives to traditional “loose” hardware. They install permanently in all types of PC boards and reduce excess parts.

A broaching fastener is a knurled-shank fastening device that can be pressed into a hole to provide a strong threaded or unthreaded attachment point in non-ductile (non-metal) materials. Specially formed axial grooves around the shank of the fastener “broach,” or cut into, the material, creating a firm, interference-type fit resistant to rotation.

Among these fastener types and their functions are:

  • Broaching Nuts. Steel or stainless steel nuts for pressing into PC boards offer permanent threads for board mounting or components. Only a mating screw is required for final component attachment.
  • Broaching Standoffs. Threaded or unthreaded fasteners in steel or stainless are designed for stacking or spacing boards. A specialized brass-type fastener features a spring action to hold a PC board securely without screws or threaded hardware, and allows for the board to be snapped into place and easily removed (then replaced) without tools. A brass type with a flare-mounted design delivers even greater pullout performance.
  • Broaching Threaded Studs. These phosphor bronze fasteners can be used as solderable connectors or as permanently mounted mechanical fasteners with external threads.
  • Broaching Board-Mount Assemblies. With captive screws, these one-piece, stainless board-mount screw assemblies enable easy mounting and removal of boards.

All types install simply, quickly, and permanently. For nuts, standard standoffs, and one-piece board-mount assemblies, the fastener is placed in the anvil hole and the mounting hole in the board is placed over the shank of the fastener. Squeezing force is then applied until the fastener’s shoulder contacts the board. Flare-mounted standoffs utilize a punch flaring tool and a recessed anvil to apply squeezing force until the shoulder of the fastener contacts the board. As the fastener seats itself in proper position, the punch tool flares the extended portion of the shank outward to complete installation. For threaded studs and snap-action standoffs, the fastener is placed into a mounting hole and squeezing force is applied until the fastener head contacts to the board.

Evolving applications have paved the way for a variety of other fastener types for board-related attachment applications. Ideal for boards with plated thru-holes, stainless self-expanding fasteners feature a self-expanding shank, which ensures positive contact with the plated thru-holes and eliminates any risk of shaving the plating out of a hole. A flat punch and anvil are engaged to squeeze the fastener with sufficient force so that the tips of the projecting knurl teeth are embedded and the inside shoulder of the knurl contacts the board. As the fastener seats itself in the proper position, the shank expands outward to complete installation.

Grounding standoffs install into steel or aluminum chassis to ground PC boards. Their barrel end is placed into a punched or drilled round mounting hole in the chassis and a squeezing force is then applied until the head is embedded and flush with the surface.

Because every board-related application will be governed by distinct parameters, the experienced hardware manufacturer will be in the best position to help arrive at the best-suited fastening and assembly solution.

This article was written by Jay McKenna, Special Products Manager for Penn Engineering® Fastening Technologies of Danboro, PA. For Free Info Visit http://info.ims.ca/5081-123.