Masters Abstracts (1993)

CHUNG, CHAO-WEN
(September 1993), Sr. Package Development Engineer, LSI Logic, Milpitas, CA

Email: cwchung@lsil.com

Closed Loop Process Control for Solder Paste Stencil Printing

The importance of solder paste has increased along with the move towards fine and ultra- fine pitch Surface Mount Components (SMCs). Increasing component lead counts and decreasing lead pitches mandate the need to thoroughly understand surface mount technology and solder paste related issues. For fine pitch applications, solder paste is best applied to the PCB by using a stencil, a stencil printer, and a squeegee. The print quality depends on several domain related parameters such as the printing equipment used, the PCB characteristics, the stencil, and the solder paste used. These factors are often inter-related. Studying the general relationship among the domain parameters and their effects on a print will help achieve good print quality. Knowledge based expert systems can be used to provide decision support, design closed loop control systems, diagnose defects, and monitor processes in every facet of the surface mount PCB assembly domain. The heuristic nature of this domain and the lack of widespread domain knowledge makes it an ideal field for expert system development and use.

The global objective of this research was to design and develop a prototype real-time closed-loop process control system to monitor the solder paste stencil printing process for the assembly of surface mount PCBS. This prototype knowledge based stencil printing process control system was implemented on a personal computer and connected to the stencil printer and a laser based solder paste inspection system to form a closed-loop. Validation of this system reveals that this prototype is capable of recommending the expected printing parameters for either a 'new' PCB application or an 'existing' PCB application. Furthermore, this stencil printing process control system can communicate with the DEK 265 stencil printer to transmit the print commands and to precisely adjust the print parameter values. This prototype process control system achieves 'optimal' print conditions through an iterative approach.