Indium Thermal Interface Material Assembly Manufacturability and Reliability

Bonding silicon to an external copper lid with a solder requires extensive process development. This includes silicon metallization development, metallization of the lid cavity, extensive assembly process development to ensure good control in thermal interface material (TIM) bond line thickness (BLT), minimal voids at the interface, and the metallic bonding meeting all relevant reliability tests. Among many reliability tests performed to ensure product field life, high-temperature storage (HTS) test is critical. Therefore, a comprehensive HTS test was conducted to study its impact on thermal performance, inter-metallic compound (IMC) formation and growth, voids, and stability of BLT.

Two indium-TIM attachment methods were studied: pre-attached indium approach and indium pre-form approach. Pre-attach refers to pre-attaching indium-TIM on a lid cavity prior to assembling lid/indium with the microprocessor package. Indium pre-form refers to bonding indium pre-form to lid and silicon concurrently. HTS was conducted on thermal test vehicles (TTVs) built with indium to measure thermal resistance at each read point of HTS. Research found no significant change of thermal resistance up to 1,000 hrs of HTS at 125°C. C-SAM was used to monitor structural changes in indium such as void size. There is no significant change in voids after 1,000 hrs of HTS.

SEM and TEM were used to study IMC formation at each HTS read point. Two types of IMC were identified: AuIn2 and In27Ni10. This research found that IMC thickness at the indium/silicon interface is very stable and does not increase with HTS time. However, significant IMC thickness increase is observed at the lid/indium interface. This observation can be explained by the nickel layer thickness used in the lid cavity and silicon backside. However, the increase of IMC at the lid/indium interface does not result in any thermal degradation or reliability degradation. This study concludes that indium- TIM is stable under HTS test.