Sn and Bi whisker growth from SAC0307-Mn07 and SAC0307-Bi1-Mn07 ultra-thin film layers

Balázs Illés,Tamás Hurtony,Bálint Medgyes,Olivér Krammer,Karel Dusek,David Busek

doi:10.1016/j.vacuum.2021.110121

Balázs Illés, Tamás Hurtony + Show 4 more

Open Access

https://doi.org/10.1016/j.vacuum.2021.110121

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Abstract

In the past years, the use of low silver content SAC0307 (99Sn0.3Ag0.7Cu wt%) solder alloy grew considerably due to its good soldering quality. In the latest version of this alloy, manganese, and bismuth are used to improve further its soldering parameters. In this study, a reliability issue, the whisker growth from SAC0307-Mn07 and SAC0307-Bi1-Mn07 solder alloys was investigated. Ultra-thin films (in 100–150 nm thickness) were vacuum evaporated from the investigated alloys onto Cu substrates. The samples were stored at laboratory conditions for 28 days. Whisker growth was followed by a scanning electron microscope, and the ultra-thin film layers structures were investigated in focused ion beam cuts. In the case of the SAC0307-Mn07, the mechanical stress due to the intermetallic layer growth resulted in whisker formation right after the layer deposition. However, Bi addition could increase the stress relaxation ability of the ultra-thin film layer; the whisker formation started only three days after the evaporation in the case of the SAC0307-Bi1-Mn07 layer. Besides, the SAC0307-Bi1-Mn07 alloy could entirely suppress the formation of filament-type whiskers that might cause reliability issues in microelectronics. Furthermore, a unique phenomenon was observed that the SAC0307-Bi1-Mn07 layer produced mostly Bi–Sn whisker couples.

Highlights

Sn whisker formation became highlighted after the transition to leadfree soldering, and it is still a current reliability topic in the microelec tronics industry
The investigation of the dots with SEMBSE detector and with larger magnification revealed that these grains are different in composition, but they have the same size as the average grain size of the layer (Fig. 2a)
It was found that vacuum evaporation resulted in mono-layers, and in the case of the SAC0307-Bi1-Mn07 alloy, evenly distributed Bi grains formed in the ultra-thin film layer

Summary

Introduction

Sn whisker formation became highlighted after the transition to leadfree soldering, and it is still a current reliability topic in the microelec tronics industry. Sn whiskers are spontaneously forming at the surface defects on the pure or high tin content solder joints or surface finishes. Their dimensions are usually between 0.5 and 3000 μm [1], so they can cause reliability risks in microelectronics via short circuit formation. The driving mechanism of the whisker growth is always a kind of mechanical stress in the Sn grains. It could originate from a direct mechanical load, thermo-mechanical effect, residual stress, or from volumetric changes in the Sn layer by oxidation or intermetallic (IMC) layer formation [2]. There are two growing research fields of Sn whiskers: whisker growth from nano laminate ternary ceramics (MAX phase [4]) and whisker growth from high Sn–Ag–Cu (SAC) solder alloys

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