Liquid Crystal Polymers and Packaging

Think liquid crystal polymers (LCPs) and you think flat-panel displays. Think again. Think new packaging and interconnect substrate alternatives.

LCP refers to the broad class of thermoplastic polymers that typically have long, rigid backbones with flexible ends. Polyester is the most common polymer backbone used in LCPs. When extruded, the rigid rods align in the flow direction and the polymer takes on some crystalline properties. This is both good and bad. The good part is that the alignment provides high strength in the crystal axis direction. The bad part is that the strength in the transverse direction is almost non-existent. An extruded sheet can easily be pulled apart in the transverse direction.

More than 10 years ago, an innovative extrusion process using counter-rotating die was developed by Foster-Miller Inc. (Waltham, Mass.). It applied a controlled shear to both surfaces of the extruded film. The resulting LCP films were biaxial crystal sheets, having excellent mechanical strength and dimensional stability in both the direction of the extrusion and transverse to it. LCP film's high temperature stability, low water uptake and low permeability to water and oxygen have opened the door for food packaging and medical products applications.

LCP films have seen limited use in microelectronics applications because of poor metalization processes. The traditional process for metalizing is to laminate a roughened copper foil to the LCP film at high temperature. When the film cools, the copper roots are mechanically locked in the film. The copper foil can be patterned and subtractively etched. Unfortunately, small copper nodules are left behind in the LCP surface. This has limited the use of LCP films to course pitches only.

Dual-processor multichip module with a six-layer LCP substrate. (Source: Foster-Miller Inc.)

Recently, the Microinterconnect Systems Division (MSD) of 3M (St. Paul, Minn.) developed a sputtering process using chromium or nickel as a tie layer followed by copper. After blanket metalization, the copper is patterned and etched conventionally. Features as fine as 30 µm pitch have been fabricated on LCP films with this process. The most likely applications for LCP films in microelectronics will be as flex substrates, both for packaged components and for bare die as chip-on-flex (COF) and TAB or tape BGA packages.

The most common film material for flexible circuits is polyimide. This is because of its high strength for thin films (typically 1 mil thick), roll-to-roll processing ability and high temperature stability. Polyimide flex substrates are in wide-scale use in consumer products such as cell phones, cameras, laptops and under the hood.

The biggest downside to polyimide is the high water moisture pickup. Typical films have 1-3% water absorption by weight. With water pickup also comes curling, blistering under the metalization, shift in high-frequency electrical properties, and the requirement for bake-out before some assembly operations.

Enter LCP materials. With biaxially oriented films, mechanical strength and stability in all directions is as good or better than polyimide, but the water pickup is less than one-tenth that of polyimide, typically 0.04-0.1%. The dielectric constant of 3.0 and dissipation factor of 0.003 is stable as humidity changes. Temperature stability is excellent up to 300°C so it is compatible with high-temperature, lead-free solders.

Bill Balliette, new business development manager with 3M, suggests the cost premium might be about 50% higher for LCP over polyimide. This will drive the early applications to those that can't be done with polyimide, where the water absorption limits performance.

Balliette suggests two possible applications: for flex antennas used at frequencies >20 GHz where the absorbed water losses in polyimide are too high, and in under-the-hood applications. As volume increases, the cost differential will decrease and the application spectrum for LCP will grow.

This expectation is reflected in recent announcements from LCP flex film suppliers. Sanmina-SCI (San Jose) announced its capability to fabricate 1-mil-thick LCP flex cores with chromium/copper sputtered metalization and laser-drilled holes. W.L. Gore (Newark, Del.) announced its BIAC line of flex film dielectric. Rogers (Rogers, Conn.) recently introduced its Zyvex family of LCP substrates.

With a low dielectric constant and low loss and electrical properties stable over humidity, low moisture pickup, high temperature stability and high mechanical strength, LCP films may see a bright future in microelectronic applications.

For additional information on assembly and packaging, go to www.semiconductor.net/assembly.