Low viscosity, thermoplastic modified, low CTE thermosetting polyester encapsulants for microelectronics

A thermosetting injection moldable polyester resin formulation adapted for encapsulation of electronics devices with a low stress induced device failure rate, said formulation comprising

• about 25 to about 50 weight percent of a resin composition comprising about 35 to about 60 weight percent of unsaturated polyester, about 20 to about 30 weight percent of thermoplastic resin having an acid number less than 15, and about 10 to about 40 weight percent of unsaturated monomer capable of cross-linking the unsaturated polyester,
• about 50 to about 75 weight percent of an inert filler substantially free of mineral fibers and comprising low aspect ratio particulate mineral filler having an average particle size of less than 15 microns, and
• an organic peroxide curing agent in an amount effective to polymerize the resin composition, said resin formulation having a molding viscosity of less than 5.times.10.sup.6 centipoise at ambient temperature and said formulation being capable of thermosetting to a plastic matrix having a coefficient of thermal expansion of less than 30.times.10.sup.-6 cm/cm/.degree.C.

A thermosetting polyester resin formulation comprising about 20 to about 60 weight percent of a resin composition comprising about 30 to about 60 weight percent of unsaturated polyester, about 15 to about 35 weight percent of thermoplastic resin, and about 10 to about 40 weight percent of unsaturated monomer capable of cross-linking the unsaturated polyester,

• about 40 to about 80 weight percent of inert filler substantially free of mineral fibers and consisting essentially of mineral particulates having an aspect ratio of less than 3:1 and an average particle size of less than 20 microns in their largest dimension and
• a free radical initiating agent in an amount effective to polymerize the resin composition at elevated temperature, said formulation having a molding viscosity of less than 8.times.10.sup.6 centipoise at ambient temperature and capable of thermosetting to a plastic matrix having a coefficient of thermal expansion of less than 30.times.10.sup.-6 cm/cm/.degree.C.