A Fast, Precise and Reproducible QC-Rheometry Routine for Solder Paste

摘要

SPC data has shown that the solder paste printing process is the primary source of soldering defects in SMT assembly.rnConsequently, verification of the specified printing properties of solder paste is of paramount importance in the pursuit ofrnhigher quality goals, and higher overall yields. Process variations such as temperature fluctuations in the printing area,rnchanging printing speeds and varying stencil life have been recognized as important parameters in the characterization ofrnsolder paste, but until today, they never actually have been resolved in a reproducible rheometric methodology.rnMoreover, printing developments in recent years, such as the introduction of closed print heads, the apparent diversity in flowrnbehavior of solder paste on electroform versus laser-cut stencils, and specific adhesion to Au-pads, apparently cannot berncharacterized by the traditional single point viscosity measurement that is still used in many facilities today.rnDiscrepancies in the flow dynamics of different types of solder pastes used in closed print heads clearly show that the verticalrnpressure imposed by these systems exerts a major impact on material performance. This phenomenon supports the idea ofrncharacterizing the flow properties of solder paste by controlled shear-stress mode methods rather than by traditionalrncontrolled shear-rate methods.rnThis paper describes a rheometric QC-routine for solder paste that takes approximately 16 minutes, and is based on therncombination and automation of two different methods. The first method is run in oscillation mode and provides a rheometricrncharacterization of slumping and tackiness. This procedure is automatically followed by a method that is run in rotationalrnmode. The latter provides an index (iv) of shear-rate at different settings of controlled stress. The second index (it) providesrnviscosity versus different temperature settings, also measured in controlled stress mode. The combined index (ivt) provides arnfull indication of the printing properties of a solder paste in one single number. In order to ensure compatibility withrntraditional methods, this routine also provides a single point viscosity determination.rnThe new QC-routine described herein is a fairly quick and cost-effective testing method that yields precise and reproduciblernresults usable in an SPC program. Moreover, it provides a complete overall picture of the printing properties of solder paste,rnincluding its numeric classification regarding slumping, tackiness, its performance at different speeds and its sensitivity torntemperature variation.