CHARACTERIZING AND CONTROLLING THE SMT SCREEN PRINTING PROCESS
Author: Michael R. Beck Company: General Scanning Date Published: 4/28/1997
Surface Mount International
Abstract: Screen printing solder paste is the foundation of the SMT assembly process. Maintaining a consistent print is the key to avoiding solder joint defects that result in lower than optimal yields, increased rework costs, and customer dissatisfaction due to increased levels of field failures. Many SMT manufacturing operations are now discovering that controlling the screen printing process is the most cost effective way to realize high-yield and high-quality SMT products. The nature of the screen printing process is dynamic due to the large number of interactive process variables. Maintaining a consistent printing process is made dramatically more difficult as the lead pitch decreases and the board complexity increases. This paper will focus on some of the methods used to characterize and then control the screen printing process. The design of experiment (DOE) methodology used by a major SMT manufacturer will be presented. The type of tools available to measure solder paste and the methods for evaluating the performance of various technologies will be illustrated. Practical considerations that should be considered when selecting measurement equipment will be discussed. Various printing control strategies ranging from sampling schemes using SPC software to full 100% in-line paste verification will be illustrated and compared.