Evaluating the printability of solder paste from paste roll characteristics

The printability of the solder paste is critical to product yield in a SMT (Surface Mount Technology) line. Both the material and the process must be controlled to minimize variations in solder paste printability and reduce soldering defects. In this letter we describe a newly developed print tester that measures characteristics of the squeegee/drum-stencil interaction to monitor the printability of solder paste. The device includes an optoelectronic monitoring system to detect the paste roll velocity, allows precise adjustment of the printing parameters and imposes a continuous squeezing action on the solder paste. We use the measurements of roll velocity as function of time and distance to propose a new measurement standard, named “print life”, which offers a quantitative and cost-effective evaluation of the printability of solder paste.