Semiconductor dice in a so-called fan out package (FOP) configuration are becoming a popular packaging option, particularly for use in mobile devices such as smart phones and tablets, as well as automotive applications, as the assemblies enable ultra-thin, high-density packages. The technique eliminates the use of a conventional interposer and is particularly suitable when a semiconductor die used in a chip scale package undergoes one or more shrinks, resulting in lack of space for the associated ball grid array (BGA). In FOP technology, a redistribution layer (RDL) comprising at least one level of conductive traces, is employed to provide external connection points, such as a BGA from the fine pitch of bond pads on a semiconductor die to a larger footprint with much larger pitch between the external connection points.
Conventionally, FOP fabrication is implemented from either a die-first or a die-last wafer level approach. In a die-first approach, an array of redistribution layers is fabricated on an array of previously singulated semiconductor dice encapsulated on a carrier substrate, and the encapsulated dice and each associated redistribution layer (RDL) are singulated from the array. In a die-last approach, an array of RDLs is fabricated on a carrier substrate, semiconductor dice are connected to the RDLs, the dice are encapsulated, and each semiconductor die and associated RDL are then singulated from the array. In some instances, multiple semiconductor dice are connected to the same RDL, but the fabrication process is the same.
For a conventional die-first process, die shift is a challenge, as are non-uniform edges and handing after RDL fabrication. The as-formed array of semiconductor dice and RDLs is undersized compared to conventional wafers, presenting handling problems and requiring modifications of the tools used for wafer handling. Conventional die-first fabrication techniques involve placing an array of semiconductor dice active-surface down on a carrier substrate having an adhesive thereon. The semiconductor dice are encapsulated by a dielectric molding material, after which the carrier substrate and adhesive are removed from the molded semiconductor dice array characterized as a reconstituted wafer, and residual adhesive is removed. The reconstituted wafer is then inverted and an RDL is formed over the active surfaces of the semiconductor dice, solder balls or other external connections are attached or formed, and the reconstituted wafer is then singulated.
The above-described die-first approach requires expensive adhesives and solvents to remove the adhesive, warping of the reconstituted wafer may occur due to shrinkage of the molding material during cure, and the reconstituted wafer presents handling problems due to being undersized compared to standard wafers for which handling equipment is designed.