A novel robust and low cost stack chips package and its thermal performance

In an attempt to provide a high density memory solution, especially for wor kstation and PC server, a stack chips package (referred to as "SCP" hereaft er) has been developed. The major characteristics of SCP are as follows: 1) SCP contains a plurality of both memory chips and lead frames within a mol ded plastic package; 2) chip selection is made through the wire bonding opt ion, resulting in the package with a memory capacity twice or four times th at of monolithic chip; 3) plural lead frames are electrically interconnecte d all at once, using metal solders electroplated on the lead frame surface; and 4) SCP is found reliable and cost competitive when compared to other s tack packages because it basically adopts the molded plastic packaging tech nology as well as the metal solder interconnection method. As electrical in terconnection methods, both fluxless soldering joint of Ag/Sn and high-pres sure mechanical joint of Ag were evaluated extensively and they successfull y provided a reliable electrical conduction path without any signal degrada tion, Temperature cycle test and pressure cooker test were proved not to pr oduce any micro cracks across the joint. The thermal performance of SCP was simulated by a thermal model based on finite element method (FEM) [1] and also experimentally verified, showing good agreement within 10% deviation f rom simulated value. 128M SCP showed better thermal performance than stacke d two TSOP's because one chip could serve as a heat sink while the other ch ip Is activated and thermal conduction path through the lead frame is short .