Originally Published MPMN March 2002

SPECIAL

Company develops viable method for fabrication of LCP flexible circuits

Flexible circuits based on liquid-crystal polymer (LCP) dielectric film feature properties superior to more commonly used polyimide-based components, but they have had limited acceptance. LCP is difficult to process, which has precluded its use as an alternative material in many cases. Now 3M Microinterconnect Systems Div. (Austin, TX; www.3m.com/microflex) has developed a fabrication process that will make LCP-based flexible circuits a feasible technology for a number of applications, according to the firm.

The LCP substrate has some uniquely desirable properties, according to marketing manager Bill Balliette. "The primary benefit is its low moisture-absorption and gas-transmission rates," he says. LCP has a water absorption rate of 0.1%, whereas polyimide has a 1 to 3% rate. Consequently, LCP flexible circuits are suited for applications where device electronics are in close proximity to a fluid or gas, raising concern about cross-contamination. "Bioanalytical equipment as well as electronic devices that undergo frequent sterilization with some kind of fluid or gas" are among key applications, says Balliette.

LCP offers dimensional stability with respect to changes in humidity and temperature, a property that benefits circuits to which numerous small elements are bonded. "As more and more elements are added to electronic components and they are made smaller and smaller, you end up with a potential manufacturing problem," says Balliette, who cites the example of multielement transducers used in ultrasound equipment. "When you're ready to bond that sensor element, the leads may not line up properly. The more dimensional stability you have in the material, the greater your ability to make that sensor array larger to cover more ground and capture more data."

3M processes its LCP flexible circuits in a manner similar to its polyimide-based components. "We build up the metal in a vacuumization process," says Balliette. This presents a number of advantages compared with laminating a copper foil over a substrate, which is how competing products are often fabricated. "When you etch away the laminated copper foil from the surface, it leaves the surface a lot rougher and you can't really get down to a fine pitch. The copper has a tendency to peel off," he says. "Vacuum metallization, on the other hand, lets you get to a very fine pitch and the copper traces still adhere to the substrate," Balliette adds. The chemical etching process, which is unique to 3M, and an absence of adhesives add value to the product.

LCP flexible circuits are designed to meet the needs of sophisticated electronic assemblies used in digital hearing aids, ultrasound transponders, and similar devices. The firm will present technical data on the product's reliability at the International Conference on Advanced Packaging and Systems (www.imaps.org) in Reno, Nevada, on March 10–13.

Benjamin Lichtman, Norbert Sparrow, Katherine Sweeny, Zachary Turke, and Susan Wallace

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