SMTA International Conference Proceedings


ANALYSIS OF SOLDER PASTE RELEASE IN FINE PITCH STENCIL PRINTING PROCESSES

Author: Paul Houston
Company: Siemens Dematic EAS
Date Published: 9/22/2002   Conference: SMTA International


Abstract: Advanced electronics packaging technologies such as chip scale packages, fine pitch ball grid arrays, 0201’s, and flip chip are pushing solder paste stencil printing to the limit. In order to achieve solder print deposits of the sizes required for emerging electronic packaging technology, a rigorous understanding of the process is required. Stencil printing is a critical step in surface mount assembly processing and becomes increasingly challenging as packages shrink in size, increase in lead count and require closer lead spacing (finer pitch).

It is well documented that stencil printing can account for more than 50 percent of the defects generated during surface mount assembly processing. Although several investigations have been attempted in order to better understand and analyze the fine pitch stencil printing process, its sheer complexity, the large number of process variables, the complex nature of the solder paste suspension flow, and the exceedingly small printed volume make this task difficult. This work seeks to expand our understanding of the physical characteristics of stencil printing; specifically focusing on the solder paste release process based on experimental and analytical approaches.

This paper seeks to expand our understanding of the physical characteristics of stencil printing specifically focusing on the solder paste release process based on experimental and analytical approaches. First, designed experiments were conducted to identify the main process variables affecting final print quality. An in-situ measurement system using a high speed imaging system monitored the solder paste release process.

Based on experimental observations, different modes of solder paste release and their corresponding mechanisms were identified and a model was developed to predict print quality for fine pitch applications. The proposed model was experimentally verified showing good agreement with measured values for fine pitch and very fine pitch printing.

It was found that the cohesive and adhesive forces acting on the paste tend to govern the release process rather than the viscous and inertial forces. Finally, a case study of stencil printing fine pitch 0201 components is presented. The results confirm the finding of the overall analysis.

Key words: stencil printing, fine pitch, paste release, 0201 processing.



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