Stacked thin dice packaging

Abstract

This paper reports on a developed stacking method to produce a small volume three-dimensional package. The first part of the 3/sup rd/ dimension is tackled by reducing package thickness and also the stand-off height. The steps came through thinning dice, using a thin interposer, and to stack the components. The thickness of the used ICs was 90 /spl mu/m, whereas typically thicknesses are around 250-300 /spl mu/m. Thin dice were connected through eutectic solder bumps on thin aramid epoxy substrates. The package was studied with the finite element method (FEM) using three-dimensional (3-D) models and the Ansys program. The average plastic work in the solder bump was used to define the reliability of the structure. Structures with one to four layers are compared. In current flip-chip assemblies, rigidity assists good electrical performance and reliability. Reducing the IC thickness below 100 /spl mu/m creates new challenges for handling, interconnecting, reliability and design. These tasks have been addressed in this study. The designed circuits for the above tests have been characterized and more details of the results are presented. Further progress in density increase has been achieved by stacking layers of flexible substrate and thin die on top of each other. For this work, the first level connection has been flip-chip bonding. The goal was to develop a method to produce modules on a small scale to verify the feasibility of various System-in-Package (SiP) solutions. The method has been tested using thin dice, mainly daisy chain. Devices are miniaturized to be more comfortable to carry; this size reduction desire, together with increased functionality, have become drivers, especially for wireless devices. Size reduction of electronics has set a challenge for packaging and provided the motivation to verify emerging technologies.