SMTA International Conference Proceedings


Authors: Hugh Roberts, Sven Lamprecht, Elaine Bevan, John Coates and Steve Prosser
Company: Atotech USA Inc., Atotech Deutschland GmbH, and TRW Automotive
Date Published: 10/11/2007   Conference: SMTA International

Abstract: Few events in the electronics manufacturing industry have caused such a widespread impact as the recent adoption of Pb-free initiatives. Nearly every aspect of PWB and IC substrate fabrication and assembly has been affected, either directly or indirectly. One of the most widely studied areas is that of the PWB and package substrate surface finish. Exemptions within the automotive electronics segment have not stopped major OEMs from considering and preparing for the eventual transition to the Pb-free world. As such, alternative surface finishes are being examined by some Tier-1 suppliers to meet the extreme requirements for automotive PWB and substrate applications.

Immersion tin is one surface finish receiving considerable attention, particularly for automotive applications. Unlike the traditional hot air solder leveling (HASL) process, immersion tin provides an extremely planar finish on the copper pads, making it very suitable for SMT applications. Use of immersion tin does not compromise the solder joint by introducing (or increasing) ternary metals that may affect solder joint reliability. Furthermore, unlike organic coatings, immersion tin provides a surface that is easily detectable by common AOI techniques, has excellent wettability for both reflow and wave applications and is readily suited for in-circuit testing methods.

This paper summarizes the results of the second phase of a comparative study of immersion tin and immersion silver as a Pb-free surface finish. The first phase characterized the two finishes in terms of solderability and solder joint reliability in response to various reflow conditions.[1] This paper summarizes the results of environmental testing performed under the supervision of the Tier 1 supplier.

Results of the investigations under this phase include:
-Mechanical shock and vibration testing
-Thermal cycling -40 to 125C
-Mixed Gas Testing

The test boards consisted of surface mount resistors and capacitors, as well as compliant pin components. The comparative performance of immersion silver and immersion tin in terms of reliability and press fit compatibility was assessed by measuring the shear forces, continuity and press fit retention forces. In addition to examining the selected Sn-Ag-Cu solder alloy, traditional eutectic Sn/Pb was also tested as a control.

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