Reflow ageing influences and wettability effects of immersion tin final finishes with lead‐free solder

Abstract

Purpose

The purpose of this paper is to investigate the oxidation behaviour of an immersion tin final finish after multiple reflow ageing under air and nitrogen atmospheres and to study their influence on the wetting behaviour with lead‐free solder. To design a model that describes the degradation of wetting behaviour after reflow‐cycling of the immersion tin final finish.

Design/methodology/approach

A special printed circuit boards (PCB) demonstrator was created to investigate the immersion tin final finish with surface analysis methods and wetting tests. The PCB samples were aged by multiple reflow‐cycling under air and nitrogen atmospheres. The tin oxide formation behaviour of immersion tin was characterised using X‐ray photoelectron spectroscopy (XPS), transmission electron microscope (TEM) and SERA analysis.

Findings

The native oxide layer of the investigated immersion tin final finishes was approximately 7 nm on average. The TEM and XPS investigations indicated an amorphous structure of SnO and SnO₂. The solder spread test showed significantly different results for PCBs in “as received” condition compared to those after one and two times reflow ageing under a nitrogen solder atmosphere. The analysis methods revealed a slight increase in the tin oxide layer thickness and small areas with semi‐crystalline structure. Reflow ageing under an ambient solder atmosphere induced considerably thicker oxide layers, which could be observed by a yellow discoloration of the surface.

Research limitations/implications

Measures to improve the wetting behaviour can be derived from the described model (i.e. use of higher tin layer thickness or protective films to reduce the tin oxidation).

Originality/value

A functional model for the solderability process of lead‐free solder on immersion tin PCB final finishes was derived and verified. By this, interactions between the state of the final finish and the solder can be described and potential solderability failures can be predicted.

Citation

Hetschel, T., Wolter, K. and Phillipp, F. (2009), "Reflow ageing influences and wettability effects of immersion tin final finishes with lead‐free solder", Circuit World, Vol. 35 No. 2, pp. 37-44. https://doi.org/10.1108/03056120910953303