O-ring package

A transfer molded plastic package having a cavity for accommodating a semiconductor chip is disclosed. A leadframe assembly process is shown wherein the leadframe finger pattern is provided with a resilient or elastic O-ring bead. Top and bottom housing plates which have dimensions that are larger than the bead form the upper and lower surfaces of the package. These plates can be formed of any suitably rigid material. They may be composed of ceramic in low power devices. For high power operation at least one metal plate can be employed. The chip or chips are connected to the lead frame and, along with the top and bottom plates, is located in a transfer mold. The plates are in registy and located so that their outer edges extend beyond the O-ring bead. The mold cavities include faces which press against the plates which are held apart by the O-ring bead so that the bead is compressed by the mold closure. A plastic encapsulant is molded around the periphery of the plates so that the leadframe fingers are secured therein to become package pins. The mold faces being pressed against the two plates precludes encapsulant from their faces which then become the outer faces of the package. The O-ring bead precludes the encapsulant from coming into contact with the semiconductor chip during the molding operation thereby creating a cavity package. In an improvement an elastic O-ring can be applied to a leadframe in registry with the outer package dimensions and the leadframe dam bars eliminated. When the transfer mold is closed the cavities are completed by the elastic ring. Therefore, the mold flash is eliminated and a dambarless leadframe assembly process is available. Finally, if desired, the dambarless process can employ a plastic that is soluble in a suitable solvent. Then, after the molding is completed, the bead is removed by means of the solvent. Desirably, a water solube plastic is employed.