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An Investigation of Materials Selection and Manufacturing Process on Resulted Device Alpha Emissivity Levels
Keywords: low alpha, underfill, assembly process
For advanced silicon technologies, the decrease in feature size has resulted in the significant increase in SRAM and transistor density. However, this growth in circuit density, along with the continuous reduction in operating voltage, has also resulted in the reduction in the charge stored for SRAMs and transistors. Both of these changes may result in an un-intended increase of SER (Soft Error Rates) in IC devices. One of the major contributing factors for SER is alpha particles emitted from packaging materials. Left uncontrolled, alpha emission from the fillers and metals in underfill, molding compound, and solder materials can be as high as 10 count/hour/cm2, which is 10,000 times higher than the Ultra Low Alpha (ULA, 0.001 count/hour/cm2) level desired for long-term device reliability. Alpha emission in packaging materials mostly comes from the decay of Th and U, which have a concentration of just a few parts per trillion (PPB). As a result, even when a entirely ULA material set is adopted, contamination from the assembly process due to mix-use of equipment and tooling can result in high and unreliability alpha emissivity on the assembled device. In this work, we will investigate the key equipment and processes that contribute to this contamination. A flip chip device with an ULA package material set will be assembled first on ULA dedicated assembly line and a mix-use assembly line. Alpha emissivity data in the flip chip components from both lines will be reported. To ensure the accuracy of alpha emissivity, the impact of sample preparation and measurement procedures are investigated. Likely contamination from materials used during the sample preparation process such as grinding paper and pastes will be measured and reported. Procedures for emissivity measurements such as collection time and sample sizes will also be investigated and their impact will be discussed.
Peng Su, Packaging Technologist
Cisco Systems, Inc.
San Jose, CA

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