Inline flux volume measurement for CSP process control

For many years the flux dispensing process used in ball grid array (BGA) and chip scale package (CSP) manufacturing has been increasing in importance with requirements to hold smaller solder balls in place, adequately clean solder surfaces, and leave no residues. Previous work established basic guidelines relating insufficient flux to missing and misallocated solder balls and excessive flux to a "flux barrier effect" and excess flux residue. However, a noncontact process control tool capable of assisting in proper set-up and monitoring in production has not been available. Technically, it is difficult to measure a non-Newtonian, temperature sensitive, transparent, viscous fluid in-situ. Crude sampling measurements based on microscopes or weighing scales are in use on the manufacturing floor. This paper discusses a novel process measurement tool that allows high-speed location, distribution, and volume measurement of flux which could be used inline or offline in a EGA or CSP manufacturing line The paper reviews the techniques evaluated for flux measurement, and details the novel method chosen; the measurement of naturally occurring fluorescent emissions of the flux which occur when it is excited by ultraviolet light. These emissions have been calibrated using machine vision techniques to provide micron level measurements of location, height, and volume measurements of flux deposits, as well as identification of common shape defects. This paper includes results on 16 fluxes commonly used in BGA/CSP manufacture including no-clean, water and solvent cleaned fluxes. Images and Scanning Electron Microscope (SEM) photos show the nature of the 3D representations. Experimental results on actual packages showing gage reproducibility and repeatability (GR&R), correlation of data to defects and use of the data for statistical process control (SPC) control are also provided.