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Making a Big Difference with Micro Clinch Fastners

Tue, 06/15/2010 - 3:33pm
Brian Bentrim, PennEngineering

Today’s wireless electronic devices from cell phones to e-readers exemplify ideal marriages of user-friendly form and function integrated into ever-smaller packages. The trend toward highly compact designs, however, presents an array of issues relative to component attachment, parts count and costs, and the overall efficiency of product assembly.

Making a Big Difference with Micro Clinch Fasteners
Example: While miniature threaded screws can be found as standard hardware for many compact electronic assemblies, challenges persist. Difficulties can arise when tapping extremely small host holes into thin metal sheets; the necessary handling of potentially dozens of tiny screws in one unit without the benefit of automation can become both time-consuming and tedious; and threads or drives may easily strip if screws are not properly driven. In addition, the locking patches typically applied over screw threads to provide a tight interference fit inherently prevent screw removal and subsequent product serviceability.

Non-hardware attachment methods carry their own sets of tradeoffs. Among conventional “permanent” technologies (preventing subsequent access), laser welding is “messy” and an impractical option for aluminum, plastic, or stainless steel assemblies (with 300 Series stainless having emerged as a preferred material of choice due to rigidity requirements). Adhesives can fail over time.

These issues have given rise to an entirely new generation of self-clinching “micro” fasteners (some threaded and some not), which offer performance, production, and serviceability advantages that other fastening and joining methods cannot. They install permanently where designed, can minimize the need for additional hardware, and can streamline the production process, especially when installed automatically.

Making a Big Difference with Micro Clinch Fasteners
Several self-clinching micro fastener types and styles have been introduced (and more are on the horizon) that fit especially well in the diminutive design envelopes encountered in hand-held and other wireless devices.

Threaded versions, such as self-clinching micro standoffs (to stack or space internal components) can provide strong, permanent, and reusable load-bearing threads in metal sheets as thin as 0.40 mm. Thread sizes have been developed as small as M1.0 and fastener lengths range between 2 mm and 3 mm. Non-threaded fastener types, such as self-clinching micro pilot pins, can serve as posts or pilots for locating, positioning, and alignment applications and will install in panels as thin as 0.5 mm.

All become integral parts of an assembly by clinching permanently in place, will not loosen or fall out, and never have to be handled again (if at all).

Self-Clinching by Design
Clinch fasteners become permanently retained in metal sheets via a displacer and an undercut in their design. When the fastener is pressed into a properly sized hole, metal from the edge of the hole is forced (by the displacer) to flow into the undercut around the circumference of the part beneath the fastener’s head. The fastener is then securely locked in place.

Making a Big Difference with Micro Clinch Fasteners
The specialty design and engineering behind micro fasteners underscores that they are not simply scaled-down versions of their larger self-clinching counterparts. As fasteners are “downsized,” issues relating to tight tolerances and performance values, among others, become magnified. Unique features engineered into the fastener often become both critical and essential.

Example: Self-clinching micro tack pins with diameters as small as 1mm can enable permanent attachment of two panels for such applications as anchoring internal divider plates, among others. Advantages over miniature screws include the elimination of time-consuming and costly tapping and screw-driving operations and the capability to auto-feed the fasteners for quick installation in high-volume applications. These factors become important considerations as manufacturers compete to increase productivity and profitability without adversely affecting end-product integrity and quality.

In addition to displacer and undercut, the tack pins incorporate a tapered point and tangential interference band. They work together both to guide the part into the small-diameter (and shallow) hole in the panel and create a slight interference fit between the hole and the fastener at the interference band. This creates minimal stresses in an aluminum host sheet (which can be as thin as 0.77mm) and absorbs any tolerance between the diameter of the hole and the diameter of the clinching pin.

Among other key criteria for this tack pin design, engineers use proportions and ratios to determine the proper clinch profiles and apply state-of-the-art software modeling and custom programs to achieve maximum pullout performance.

Considerations with Stainless
Some types of self-clinching micro fasteners now rolling out into the marketplace are made from either precipitation hardened stainless steel or 400 Series stainless steel. This allows them to install successfully into stainless steel panels, which have become a widespread choice for electronics assemblies that must be both thin and rigid.

Designers should understand, however, that the self-clinching process requires that the fastener always be harder than its host sheet in order for the sheet material to displace and flow into the fastener’s undercut. For this reason self-clinching fasteners (no matter how small) made from 300 Series stainless steel cannot be expected to perform in 300 Series stainless sheets, because the sheets are as hard or harder than the fastener.

Examples: Self-clinching micro pins made from precipitation hardened stainless can be installed reliably in stainless sheets whose maximum hardness is 90 or less on the Rockwell “B” scale (also designated HRB 92). Self-clinching micro standoffs made from 400 Series stainless will install in 300 Series stainless panels (most widely used in the industry) with maximum sheet hardness HRB 88. They can also be specified for other metal assemblies, such as aluminum or steel, as long as the host panel is “softer” than the fastener to allow for the displaced sheet material to flow as intended during installation.

Looking ahead at micro fastener technology, more types (self-clinching and otherwise) are in development, including brass or stainless micro inserts with thread sizes as small as M1.0. These are designed to provide strong and reusable metal threads to attach plastic assemblies and will be offered in three styles: ultrasonic, molded-in, and press-in types.

The production of quality fasteners begins with good engineering research, design, development, and testing. Precision is necessary in all facets of fastener production, especially true with micro versions. One of the more promising trends has been a growing partnership between end-product designer and fastener manufacturer to develop the best solution for an application. Knowledge that a hardware design team is equipped to deliver solutions based on experience can make all the difference as computer electronics evolve into even smaller, thinner, and lighter packages.

Brian Bentrim, P.E., is manager of Global New Product Development at PennEngineering®, 800-237-4736 and 215-766-8853; Email: bbentrim@pemnet.com; www.pemnet.com

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