New plastics for electronics.

New grades of PPS and silicone molding materials, and a completely new polymer for circuit boards, were announced at two recent electronics shows--NEPCON West '92 in Anaheim, Calif., and the Micro Electronic Packaging and Processing Engineers Association (MEPPE) exposition in Santa Clara, Calif.

* GE Plastics, Pittsfield, Mass., has expanded its Supec PPS line with the introduction of high-ductility and low-flash grades for surface-mount technology (SMT) and high-performance connectors. The two new high-ductility grades, Supec G301T and G401T (30% and 40% glass, respectively), are said to have better tensile elongation and weld-line strength than conventional PPS. This reportedly improves press-fit pin insertion/retention and enhances torque retention after heat aging.

The two low-flash grades, Supec 301M and 401M (also 30% and 40% glass), are said to reduce costly deflashing operations and offer enhanced physical properties and greater leg-tab design flexibility for printed circuit-board mounting. (CIRCLE 94)

* GE Silicones, Waterford, N.Y., introduced two new silicone molding compounds for electronics and electronics packaging. MC580 is a transfer molding grade based on a flexible polymer system that permits the material to handle both thermal shock and frequent thermal cycling. Because of its resistance to a variety of automotive chemicals, GE sees opportunities in under-the-hood electronic applications. MC580 is available as a powder or preform, has a low CLTE and high tensile and flexural strengths.

GE Silicones also introduced MC573 silicone molding resin for a wide variety of electromechanical applications. MC573 exhibits hard, "plastic-like" properties and a max. continuous-use temperature over 500 F. It's also available as powder or preform. (CIRCLE 95)

* Hercules Inc., Wilmington, Del., announced a brand-new resin chemistry, Sycar silicon-carbon thermosets. They're currently available in a neat resin grade for multilayer circuit boards and a formulated liquid resin for potting and encapsulating. Both are one-component systems including a proprietary catalyst. Sycar resins reportedly feature the lowest dielectric constant and lowest moisture absorption (by nearly an order of magnitude) of any resin currently used in circuit boards. The latter results from the absence of polar groups in the cured polymer. In addition, Sycar has a glass-transition temperature of 356-392 F, similar to bismaleimide-triazine (BT) epoxy, and better thermal-aging characteristics at 356 F than typical FR4 epoxy or BT epoxy. By TGA, it shows only 5% weight loss at 878 F. Sycar has one-third lower flex modulus of epoxy, but this should result in lower thermal stresses from plated-through holes. All these properties are said to be particularly valuable for high-speed, high-frequency circuit boards. A UL 94V-0 grade is not currently available but is in development. Prices are said to be competitive with cyanate esters and polyimides--generally in the $15-30/lb range.

Sycar circuit-board grade is formulated for conventional prepregging and for moldability similar to epoxies at the same press cycles. Laminate grades have a minimum shelf life of six months. Encapsulant resins last 4-5 hr at 176 F and retain constant viscosity for several weeks at room temperature. Prepreg made with Sycar is much less brittle than typical circuit-board material and thus results in less resin flaking and "prepreg dust."

Hercules has entered into a multi-year joint development, supply and marketing agreement with Polyclad Laminates of Franklin, N.H., which will market Sycar circuit-board prepreg and copper-clad laminates. (CIRCLE 96)