SMTA International Conference Proceedings


Author: Srinivasa Aravamudhan et al.
Company: Speedline Technologies
Date Published: 9/25/2005   Conference: SMTA International

Abstract: The move towards Pb-free electronic assemblies continues to be fueled by the combination of proposed legislative restrictions on Pb usage and disposal in Europe and Asia and increasing commercial and marketing activities of Japanese and European OEMs. The impact of the transition from SnPb eutectic solder to Pb-free alloys is being felt by the entire electronics manufacturing supply chain, from component suppliers to equipment manufacturers. While industry has identified possible alternatives to the SnPb solder, there is no drop-in replacement, and much work remains to be done to get our understanding of Pb-free on par with SnPb eutectic.

It is well known that chip components (primarily 0402, 0603, 0805, and 1206 resistors and capacitors) that have been placed slightly off center from the printed circuit board (PCB) pads will center onto the component pads during reflow soldering process using the standard SnPb solder alloys. By contrast, it is not clear if Pb-free alloys will exhibit similar self-centering characteristics to the SnPb alloys, due to the different wetting characteristics of Pb-free alloys.

A formal experiment was conducted to understand the self-centering capability of offset chip components in a Pb-free assembly. However the study could not identify the performance of alloys to a satisfactory statistical confidence1. This project will allow us to understand the ability of Pb-free alloys to self-center as a function of component/paste print offsets and compare their performance to SnPb alloys.

In this study chip components (i.e., R0402, R0603 and R0805) were deliberately placed off center from their pads and an automated optical inspection (AOI) system was used to determine whether the component centered onto the printed circuit board pad. The performance of the Pb-free alloy was compared to the SnPb alloy. The Pb-free and SnPb solder pastes used in this experiment were printed as accurately as possible on the printed circuit board pads.

The second part of this study was to compare the selfcentering ability of the chip components when print deposits were deliberately printed offset from the PCB pads. The solder paste deposits were offset from the pads by several pre-determined distances. The components were placed centered onto the pads.

Additionally, the impact of reflow atmospheres (Air and Nitrogen) on the self-centering ability of Pb-free alloys was evaluated. The performance of the alloys are evaluated with their ability to self-center the offset components/ offset print deposits and the results are discussed in the following sections of this paper.

Key words: Pb-free, Self-Centering Phenomenon, Stencil Printing, Reflow Soldering and Process Control

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