Journal of SMT Article


Authors: G. Subbarayan and R. Kinyanjui, J. Deutsch and M.
Company: Technology Development Center, Sanmina-SCI Corpora
Date Published: 10/1/2007   Volume: 20-4

Abstract: In this study, the termination finish of Small-Outline Integrated Circuit (SOIC) chip components were converted from lead-free to tin-lead (backward conversion) and from tin-lead to lead-free (forward conversion). The motivation for these conversions is due to a combination of factors such as the supply chain constraints on component availability and the European Union’s (EU) legislation on “restriction of certain hazardous substances” (or RoHS). Additionally, component leads with 100 weight percent matte-tin (100wt.%Sn) finish were converted to lead-free Sn-3.5wt.%Ag-0.5wt.%Cu (commonly referred to SAC305) finish. This type of conversion would be necessary to mitigate against the tin whiskers issue common in matte tin-plated components. All the conversions were performed by E-Certa, Inc using their “Robotic Stripping and Solder Dipping Process”.

The converted parts were examined using x-ray fluorescence (XRF) technique to test for RoHS compliance for the metallic species. Electrical and die visual inspection utilizing optical microscopy, direct current (DC) pin-to-pin electrical testing and acid de-encapsulation were performed to inspect for damage on die, after the conversion process. There were no opens or short circuits found during the electrical test. The die was undamaged after the conversion process.

As the final evaluation method of the robotic conversion process, the converted parts were assembled on a printed circuit board (PCB) using both tin-lead and lead-free processes. Three PCB surface finishes namely immersion silver (Im-Ag), organic solderability preservative (OSP), and electroless nickel/immersion gold (ENIG) were used. The microstructures of the converted components were typical in appearance and had well defined intermetallic compound (IMC) both along the bulk solder/PCB and bulk solder/component interfaces. Further, lead pull testing was performed on ‘as-assembled condition’ (AA) and ‘after thermal aging’ (ATA) components to determine the mechanical integrity of converted component termination finishes with respect to the original termination finish. The results indicate that the converted components have similar pull strength as that of the original components.

Key words: Robotic Termination finish Conversion process, leadfree microstructure, XRF

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