This document provides users of underfill material with guidance in selecting and evaluating underfill material for assembly solder joints second-level interconnects. Underfill material is used to increase reliability of electronic devices by two methods: alleviate coefficient of thermal expansion (CTE) mismatch (between the electronic package and the assembly substrate) and/or increase mechanical strength. Underfill materials are also used for environmental protection, mechanical shock or vibration, and anti-tampering uses. Materials used in underfill applications should not adversely affect device reliability nor degrade electrical performance (e.g., ionic impurities). When correctly selected and applied, underfill material should increase the life of the assembly solder joints. In addition, Revision A contains acceptability criteria for underfill on completed assemblies.