Sticky films develop

St. Paul, MN -Not so hot and really strong. What may sound like a recipe for a bad cup of coffee instead decribes the latest adhesive bonding films from 3M.

One new film employs a moisture-cure urethane and targets some of the electronics applications currently served by thermal bonding films as well as general industrial applications. According to market development manager Jamie McCormick, the new adhesive film cures in the presence of environmental moisture and can do away with the need for heat bonding equipment. Several versions of the film will be available to provide a range of bond strengths and to vary the amount of moisture needed for curing. McCormick says these new load-bearing adhesives will have a shear strength of roughly 800 psi.

Another new product will be an extra-strong version of its Thermo-Bond thermal bonding film. Currently under evaluation as an alternative for liquid epoxy as a way to bond LED screens, this new structural adhesive film will offer overlap shear strengths greater than 3,000 psi, McCormick reports. The added strength goes hand in hand with what may prove to be even more important attributes-heat and chemical resistance. McCormick says the high-strength product has a high enough glass-transition temperature (226F) to offer "at least double"

the heat resistance of earlier thermal bonding films. The only downside to the new adhesive film is longer cure times. At 250F, for example, it would take about 60 minutes to cure.

Slated for full commercial introduction by the middle of this year, both adhesive films are now available for sampling.

The sound of bonding

There's a new way to turn up the heat on thermal bonding films. 3M engineers recently discovered that the company's thermal bonding films work in conjunction with standard ultrasonic welding equipment. "We've found that ultrasonics can provide the quick, localized heat needed for the thermal bonding film to flow," McCormick says. The resulting bonds typically offer shear strengths between 800 and 1,000 psi, he reports.

Ultrasonic welding already creates reliable joints in small plastic assemblies and does so quickly. But the use of adhesive films may make the welding process even better by allowing it to join materials as unalike as metals and plastics. "Ultrasonic welding is normally unable to handle dissimilar materials," McCormick says.

The ultrasonic-adhesives hybrid may also help create more forgiving joints than ultrasonic welding alone. McCormick explains that adhesives can overcome molding defects and loose tolerances by flowing into gaps in the joint. "We're seeing a more complete seal than with conventional ultrasonic welding," he says. And thermal bonding films, whether ultrasonically welded or not, can be reworked, he adds.

So far, 3M and an undisclosed electronics OEM have evaluated this hybrid joining technology as a way to bond radio and cell phone battery covers.