SMTA International Conference Proceedings


Authors: Sven Lamprecht, Dr. H-J Schreier, and Dr. R. Vogel
Company: Atotech Deutschland GmbH
Date Published: 9/22/2002   Conference: SMTA International

Abstract: The technology of surface finishes for printed circuit boards is seeing a dramatic shift from the Hot Air Solder Levelling (HASL) towards alternative finishes like electroless Nickel - immersion gold (ENIG), immersion tin, immersion silver and organic solder preservatives (OSP's).

This trend is mainly caused by the worldwide environmental pressure to ban the use of lead for electronic assemblies as well as the demands of modern assembly technology, which require a higher coplanarity of the surface finish for surface mount assembly.

One of the main benefits of HASL is the extremely good resistance of the surface against ageing under high temperature conditions. Alternative finishes, due to their limited thickness, offer excellent coplanarity, but inferior surface protection.

A detailed understanding of the ageing processes is thus a prerequisite for the implementation of alternative surface finishes in high yield PCB production. In this paper the ageing characteristics of immersion tin surface finishes are investigated and correlated with the reliability during soldering operations.

Ageing of tin surface finishes takes place by intermetallic compound (IMC) formation at the Sn/Cu interface due to solid state diffusion. Hence the kinetics of the process is assumed to follow a simple law. The kinetics of IMC formation was investigated experimentally and correlated with data from literature.

The results show that the simple kinetic model leads to an overestimation of the kinetics in the temperature regime below 60°C. Changes in the reaction mechanism with temperature as well as with time must be taken into account for a conclusive understanding of the ageing characteristics.

The only parameter with relevance for practical solderability is the thickness of the immersion tin layer.

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