Development of Large Size CPU Package Structure Using Embedded Thin Film Capacitor Package Substrate

This paper reports on a large-size CPU package for UNIX servers which employs embedded thin film capacitor layers. The substrate of this package has two thin film capacitor layers in the surface of the core layer, which has a capacitance of 25 uF in total. In order to adopt this package substrate, we confirmed the effect of the thin film capacitor layers on the package assembly process. We actually measured mechanical characteristics and the coefficient of thermal expansion (CTE), and confirmed that we can use it like conventional package substrates. This package has passed the preprocessing and subsequent environmental test on JEDEC Level 4, and we think that a high-quality package has been developed this time. Fujitsu adopted a low melting point solder BGA from a previous-generation package. Its composition is Sn-57Bi-1.0Ag. However, this solder has an issue with deformation after a low-temperature reflow profile for the solder ball attachment process on the package. In this work, we were able to find the cause of this phenomenon, and by changing the Ag content from 1.0% to 0.4%, we achieved a complete low-temperature assembly process. In addition, BGA connection reliability was confirmed.