Find the Best Fastener for PCB Assemblies

By Jay McKenna

Jay McKenna is the Special Products Manager, PennEngineering

Integrating a printed circuit board (PCB) into an assembly is often a custom job. Every application can require a different board location. In addition, part geometry and surface finish, solder paste, and surface mounting conditions change frequently. Some boards must be stacked or spaced, and others must be removed and re-attached periodically. These factors complicate board attachment, whether you specify component-to-board, board-to-board, or board-to-chassis. You may have a variety of hardware in the fastening toolbox, however, not all of it is practical, especially when you are working with sensitive electronics.

For example, a fastener that requires installation in a complex handling process can cause problems because of the fine work it takes to align all parts, often on both sides of a PCB. It is critical to remove any dropped hardware to prevent rattle and shorts, which could lead to product failure.

More permanent fastening methods and board-related hardware with fewer parts are now available. They simplify the production process, which helps increase product quality, and they reduce overall cost.

Cut down on parts
One choice to eliminate loose hardware problems is the broaching fastener. These fasteners install permanently in all types of PC boards with the fewest number of parts.

A broaching fastener is a knurled-shank device that you press into a hole for a strong threaded or unthreaded attachment point in non-ductile (non-metal) materials. Specially formed axial grooves around the shank of the fastener cut (or broach) into the material for a firm fit that resists rotation.

They come in many types, and each has a specific function and advantage. However, all types install simply, quickly, and permanently.

Broaching nuts make a permanent thread for board mounting when pressed into PCBs. They come in steel or stainless steel. A mating screw completes final component attachment.

Broaching standoffs are threaded or unthreaded fasteners in steel or stainless for stacking or spacing boards. A brass type with a flare-mounted design has a spring action that lets you snap the board into place, and remove and replace easily without tools.

Broaching threaded studs are phosphor bronze fasteners used as solderable connectors or as permanently mounted mechanical fasteners with external threads.

Broaching board-mount assemblies make it easy to mount and remove boards. They comprise one-piece stainless screw assemblies with a captive screw.

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More permanent fastening methods for PCB attachment have helped cut down on the number of loose parts in an assembly. Broaching fasteners, for example, comprise only a knurled-shank fastening device and, in some cases, a final captive screw for attachment.

Ease installation
Several PCB-mounting fastener types use the same soldering processes that mount other electronic components. With surface mount fastening, for example, the hardware is positioned while the board is being processed. Conventional surface mounting equipment then installs it along with the other electronic components.

Factors such as board cracking and misalignment are less likely to occur because the hardware installs automatically with the same pick-and-place robotic equipment used for a board’s electronic components. The process is not intrusive to the board, does not expose it to cracking, and lets you specify close-to-edge installations.

What’s more, unlike methods that require you to store fasteners in piles or bowls, surface mount types come on a tape and reel, so different parts will not inadvertently mix. Surface mount fastening can increase production rates because it takes less time and costs less money than parts’ handling, and it reduces quality failures.

Many standard types of surface mount fasteners are available. Spacers suit board-to-board stacking; nuts mount boards and attach components, and are an alternative to broaching nuts and loose hardware; panel fasteners suit applications requiring easy removal and reinstallation of boards; and right-angle fasteners join components at a 90º orientation.

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Several fastener types for mounting to a PC board already use the same soldering processes that mount other board components. (1) Since pneumatic pick-and-place fingers need a flat surface for suction, sometimes a patch can be used to cover the fasteners as they pass through the oven, and be removed later. (2) This panel fastener’s purple ABS plastic cap and metal Phillips drive screw are supplied separately from the soldered retainer so that the retainer can be placed on the board and run through the oven. After the re-flow, the cap and threads snap onto the retainer to complete the installation.

Spacers and nuts come with or without threads, and you can generally install them in boards as thin as 0.060 in. (1.53 mm). Reels carry up to 1,500 parts, depending on fastener size. Standard steel fasteners are plated with tin for soldering. A Kapton patch eases vacuum pickup.

Right-Angle Fasteners make reliable, permanent, and reusable right-angle attachment points on PCBs. They are good alternatives to conventional angle brackets or threaded right-angle blocks that attach board to chassis, chassis to board, or component to board. They install in PCBs as thin as 0.40 in. (1.0 mm).

Panel fasteners come in several variations. They incorporate a threaded screw or pin that retracts and advances to engage a nut or internally threaded piece of hardware.

Have you found your fit?
Several other fastener types are available for board-related attachment applications.
Stainless self-expanding fasteners are ideal for boards with plated thru-holes. They incorporate a self-expanding shank, which ensures positive contact with the hole. They also eliminate any risk of shaving the plating out of a hole.

To install this type, squeeze the fastener with a flat punch and anvil. Sufficient force is needed so that the tips of the projecting knurl teeth embed to where 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. Place their barrel end into a punched and drilled round mounted hole in the chassis, then apply a squeezing force until the head is embedded and flush with the surface.

Critical considerations
In surface mount fastening, the ability of a fastener to adhere to a PC board is determined by the part’s geometry and surface finish, the solder paste, and the conditions of the surface
mounting.

Three factors are critical to the proper function of a surface mount fastener: plating, feeding, and stencil design. For example, the fastening industry considers pure tin standard plating for surface mount fasteners. In applications where tin whiskers may be a concern, the International Electronics Manufacturing Initiative (iNEMI) recommends using matte tin plating and a bake/anneal cycle to relieve embrittlement.

Pneumatic pick-and-place fingers require an exposed flat surface that functions as a suction point. However, some surface mount fasteners, such as threaded or thru-hole spacers and nuts, do not have a convenient surface for this task. In these situations, a patch can cover the part. The patch then travels with fastener into the oven and is removed later.

All types of these fasteners can be soldered directly to the surface of a PCB. However, integrating a pilot stabilizes the assembly and assists in placement. When you expose the land to solder, you must use a stencil to mask the hole in which the pilot sits. A spoke design works well.

Distinct parameters govern every board-related application. Because each situation is unique and so many fastener types are available, it is best to consult with an experienced hardware manufacturer up-front. You can cut costs and production time, as well as ensure the quality of the product.

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Because surface mount fasteners are a board component, they eliminate the manual placement of the hardware on a populated PC board. The hardware is positioned during board processing and then installed with the other electronic components using conventional surface mounting equipment.


Installation tips

Broaching fastener types
For nuts, standard standoffs, and one-piece board-mount assemblies, place the fastener in the anvil hole, then place the mounting hole in the board over the shank of the fastener. Apply
squeezing force until the fastener’s shoulder contacts the board.

Use a punch flaring tool and a recessed anvil for flare-mounted standoffs. Apply squeezing force until the shoulder of the fastener contacts the board. As the fastener seats itself in the proper position, the punch tool flares the extended portion of the shank outward to complete installation.

For threaded studs and snap-action standoffs, place the fastener into a mounting hole and apply squeezing force until the fastener head contacts the board.

Surface mount fasteners
Because of the right angle fastener’s shape, the pick-and-place machine readily picks up the part without a patch. The fastener, supplied in the tape and reel, has two small pins that point downward, away from the pneumatic finger. The flat portion of the fastener’s head faces it. The two pins act as small pilots for stability and location accuracy during placement.

The solder fillet forms along this edge because of a step along the bottom. This happens even while the face of the right angle fastener is flush against an edge of the PCB. The fastener’s rectangular hole reduces the mass, which promotes faster heating. This lowers the potential for surrounding components on the board to overheat.

Panel fastener assemblies are mounted by snapping a screw into a soldered retainer. The screw thread and plastic cap initially are supplied separately so the retainer can be placed on the board and run through the oven. After the re-flow, the cap and threads snap onto the retainer, completing the assembly.


Surface mount fastening cuts costs

The fastening method you specify can determine production rates, the quality of the product, and ultimately the cost of the assembly. Surface mount fasteners can significantly reduce costs.

Put an end to loose hardware
A manufacturer required hardware for a PCB to mount a daughter board in a line card. The original method to fasten the boards used loose hardware, which included four standard hexagonal pillars, as well as eight sets of screws and washers. Four surface mount spacers were specified to secure the boards. They reduced hardware and cost, and improved unit quality and reliability.

Custom parts cannot compete
Another manufacturer needed to attach a PCB to the inside housing of an external wireless modem. Because of the limited space within the modem’s housing, the initial specification called for a custom-made bracket, with a high installation cost. A surface mount right-angle fastener was specified to replace the part because it suits applications with space constraints, and its installed cost is much lower. What’s more, it helped streamline the production process by eliminating secondary operations.

PennEngineering
www.pemnet.com 

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