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Article
Publication date: 15 May 2009

Thomas Hetschel, Klaus‐Jürgen Wolter and Fritz Phillipp

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…

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.

Details

Circuit World, vol. 35 no. 2
Type: Research Article
ISSN: 0305-6120

Keywords

Article
Publication date: 1 February 2013

Xin Luo, Wenhui Du, Xiuzhen Lu, Toshikazu Yamaguchi, Gavin Jackson, Li lei Ye and Johan Liu

The composition and thickness of surface oxide of solder particles is extremely important to the quality of interconnect and reliability of packaged system. The purpose of this…

Abstract

Purpose

The composition and thickness of surface oxide of solder particles is extremely important to the quality of interconnect and reliability of packaged system. The purpose of this paper is to develop an observable measurement to research the issue.

Design/methodology/approach

AES (Auger electron spectroscopy), XPS (X‐ray photoelectron spectroscopy), TEM (transmission electron microscopy) and STEM (scanning transmission electron microscopy) were employed to examine the oxide layer on microscale solder powders. Conventional techniques and FIB (Focus Ion Beam) were employed for the TEM sample preparation. High angle annular dark field (HAADF) pattern was applied to distinguish the oxide layer and the solder matrix by the contrast of average atomic number. The results were confirmed by AES and XPS measurement.

Findings

The solder powders were exposed to air (70% relative humidity) at 150°C for 0, 120 and 240 h for the accelerated growth of oxide. The surface oxide thickness was 6 nm and 50 nm measured by TEM for 0 h and 120 h samples, respectively. It was found that the increase in surface oxide thickness of solder particles is proportional to the rooting of time. The elemental distribution along the oxide was quantified by line scanning using STEM and the atomic ratio of Sn to O in the oxide layer nearer to the outer, the middle, and the inner (adjacent to the solder matrix) was found to be 1:2, 2:3 and 1:1, respectively. The result was validated using XPS which gave Sn to O ratio of 1:2 at 5 nm depth of surface oxide.

Originality/value

This is the first time FIB technology has been used to prepare TEM specimens for solder particles and TEM pictures shown of their surface oxide layer. Though requiring more care in sample preparation, the measurements by TEM and STEM are believed to be more direct and precise.

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