The use of chip scale packages (CSPs) is rapidly expanding, particularly in portable electronic products. Many CSP designs will meet the thermal cycle or thermal shock requirements for these applications. However, mechanical shock and bending requirements often necessitate the use of underfills to increase the mechanical strength of the CSP-to-board connection. This paper examines the assembly process with capillary and fluxing underfills. Issues of solder paste versus flux only, solder flux residue cleaning and reworkability are investigated with the capillary flow underfills. Fluxing underfills eliminate the issues of flux-underfill compatibility, but require placement into a predispensed underfill. Voiding during placement is discussed. To evaluate the relative performance of the underfills, a drop test was performed and the results are presented. All of the underfills significantly (5-6x) improved the reliability in the drop test compared to nonunderfilled parts. Test vehicles were also subjected to liquid-to-liquid thermal shock testing. The use of underfill improved the thermal shock performance by /spl ges/5x.