This document provides users of underfill material with guidance in selecting and evaluating underfill material. Underfill material is used to increase reliability of electronic devices by two methods: alleviate CTE mismatch (between the electronic package and the assembly substrate) and/or increase mechanical strength. Materials used in underfill applications should not adversely affect device reliability (e.g. ionic impurities, alpha emitters) nor degrade electrical performance. When correctly selected and applied, underfill material should increase the life of the assembled solder joints. The following three types of currently available, underfill materials addressed in this document are: Capillary Flow Underfill, No-Flow/fluxing Underfill and Removable/Reworkable Underfill.