The incredible shrinking fastener

DN Staff

January 18, 1999

5 Min Read
The incredible shrinking fastener

In the movie "The Incredible Shrinking Woman," Lily Tomlin slowly reduces in size from a full-sized woman to a miniature version of herself. Her clothes shrink along with her. Somewhat like that movie, electronic boards and components are shrinking, and the fasteners that hold them together are shrinking, too.

"We're seeing the downsizing clearly," says Dean Lamb, manager of marketing communications for Camcar Textron (Rockford, IL). But smaller fasteners don't necessarily have to have different features. "Engineers often don't realize that the features available on larger fasteners are available on smaller ones," he adds.

These features include self-tapping threads and anti-cam-out recesses. Within electronic-application fasteners themselves, stainless-steel and gold-plated materials for conductivity are becoming more prevalent, Lamb says.

"The fasteners are scaled down in proportion to what you find in the larger ones," says Bob Stotz, new product development engineer at Penn Engineering & Mfg. Corp. (Danboro, PA).

This miniaturization trend appears in several kinds of fasteners, including pins. Fasteners with a body diameter of 1/16 or less can be soldered or mechanically fastened to the board. Solderable pins, from Spirol Int'l Corp. (Danielson, CT), are available in coiled- and slotted-spring, knurled, and grooved versions, and are available as small as 1 mm diameter.

The soldered fastener can conduct current, which is its major advantage, according to Brian Hamilton, general manager of solid pins for Spirol. Electronic companies often attach hinges, heat sinks, and secondary daughter boards and use solder to connect the board and fastener.

Smaller fasteners at a glance

Here's a quick comparative look at the three smaller fasteners.





The user can also mechanically attach the pins to the board. Headed Twist-Lok pins from Spirol are available as small as 1.5 mm in diameter and replace screws in applications that can use a press-in fastener.

The advantage of the mechanically fastenable pins, Spirol's Hamilton says, is that they make a stronger connection than solderable pins. However, the mechanical pins do not conduct current as well as solderable pins, he adds.

The mechanically fastenable pin features a helical knurl on its end, which locks the pin in place for retention.

Losing weight. Electronic materials are also becoming thinner, which presents special problems, too. Holding power is sometimes compromised in thinner materials, so the fastener has to have strong threads and be assembled easily.

Penn Engineering and Mfg. Corp. has responded to the thinner material challenge with PEM(R) miniature self-clinching fasteners, which provide reusable threads in aluminum and cold-rolled steel sheets as thin as 0.020 inch.

Installation of these fasteners involves placing the shank into a properly sized punched or drilled mounting hole and applying squeezing force until the fastener's knurled collar is embedded in the sheet. The fastener becomes part of the sheet. Manufacturers install the miniature self-clinching fasteners using a PEMSERTER(R) press.

The fastener features a knurled collar similar in function to Spirol's knurled end that features pushout resistance up to 420 lb. This feature prevents the fastener from rotating in the sheet.

During installation, the knurled collar gets embedded into the parent material and forces the material into an undercut or relief, capturing the fastener in the sheet. As the collar embeds itself, it prevents the fastener from torquing out of the sheet. "Moving the sheet material is key to reliable clinching," Stotz says.

The miniature self-clinching fasteners address smaller-application accessibility issues.

"Once electronic cabinets are assembled and put in the field, some of the components become inaccessible, and if you have loose hardware, you're going to start losing nuts and such," Stotz says. "That's the advantage of the self-clinching fasteners."

When materials grow thin, another challenge is tapping them. Camcar's offering to the thin-material-holding arena, Perma-NutTM inserts, are permanent-threaded fittings that attach to the circuit board that create a nut member. The fastener will work in material as thin as 0.5 mm, Lamb says.

The fastener is available in thread sizes M2 to M10, and the user can install them with automatic feeding equipment, standard tooling, high-speed riveting machines, or presses. During installation, the fastener gets set by a roll-clinch action that requires two-sided access.

Smaller assembly. "Automating with smaller fasteners is difficult," Lamb says. "You can have problems between your drive bit and your fastener."

But if automation is difficult with smaller fasteners, trying to handle them manually is worse. "With the smaller fasteners, picture trying to pick up a tiny pin when it's half the size of your finger," says Phil Cyr, general manager of automation for Spirol. "As components get smaller, the need for automation becomes greater."

But automation has its challenges. One is the heat-treatment of miniature fasteners. Another is testing smaller fasteners. "There's not a lot of technology out there that allows the testing of small screws," Lamb says. Camcar has a torque-tension lathe to test the smaller screws.

Spirol's Hamilton says installation and automation are critical with small fasteners. "You can't just design the pin anymore and give it to the manufacturer in the next room."

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