A Novel Package-on-Package Technology Using Coreless Substrate with Cu Posts

Abstract

We have successfully developed a novel Package-on-Package (PoP) technology for achieving small package thickness and low package warpage. The novel package structure is based on an ultra-thin coreless substrate, called a Multi-Layer Thin Substrate (MLTS) . The MLTS is fabricated by completely removing the Cu base plate after forming a high-density buildup structure on the base plate. In this paper, however, the Cu posts are formed by selective wet etching of the Cu base plate. Since the novel package has built-in Cu posts as external terminals, the overall substrate can be molded for the peripheral area as well as the chip mounting area. This paper also describes how two types of novel wet etching processes have been developed for obtaining narrower pitch and higher aspect ratio. Through the novel wet etching process, we achieve Cu posts of 0.5 mm pitch and high aspect ratio of 6.0, which cannot be realized by wet etching. A prototype package has 361 pads arranged in 5 rows with 0.5 mm pitch, and the thickness is only 0.23 mm.