Wetting and spreading behavior of borosilicate glass on Kovar

doi:10.1016/j.jallcom.2008.10.028

Journal of Alloys and Compounds

Volume 477, Issues 1–2, 27 May 2009, Pages 407-413

https://doi.org/10.1016/j.jallcom.2008.10.028 Get rights and content

Abstract

The wetting and spreading behavior of borosilicate glass on Kovar with oxides of FeO + Fe₃O₄ was investigated as a function of wetting time by the sessile drop method. The contact angle and drop base diameter were measured by the VHX-100K digital microscopy. The top view of spreading and cross-sectional microstructure of glass/Kovar was investigated by SEM, and elements distribution of Fe, Co, Ni and Si in interfacial regions was characterized by EDS line scanning. The results showed two halos were formed at the fringe of glass base in the wetting process. By analyzing the change of contact angle and drop base diameter, wetting process could be divided into three stages: incubation period, reaction period and equilibrium period. Chemical reaction is occurred between glass and oxides, and the reaction product is Fe₂SiO₄. Chemical and mechanical bonding effects are all present in the wetting of glass to Kovar.

Introduction

Kovar alloy (ASTM F-15 alloy) is widely utilized for matched glass-to-metal sealing in microelectronic packages, as its thermal expansion curve is accordant with that of hard borosilicate glass between 0 °C and 400 °C [1]. But the glass does not wet on the Kovar alloy, hermetic sealing is realized by bonding of molten glass with oxide film on the Kovar alloy [2]. Thus it is necessary to investigate the wetting and spreading behavior of glass on oxides of Kovar alloy.

As all kinds of oxides could be formed on the Kovar alloy, and double layer oxides of FeO and Fe₃O₄ are chosen in this paper. The author's results [3] show that mixed oxides of FeO and Fe₃O₄ are generated when Kovar alloy is oxidized in N₂–2.31% H₂O at 1000 °C, and FeO is closely attached to Kovar base and Fe₃O₄ is formed above FeO, intergranular oxide is also obtained underneath FeO along grain boundary in the Kovar alloy. The sessile drop technique is used in the investigation of spreading behavior of borosilicate glass on Kovar alloy. According to oxidation thermodynamics, the composition of oxides may change if the wetting atmosphere is different from the oxidation atmosphere of oxides. For example, if the oxidation partial pressure of wetting atmosphere is lower than that of decomposition pressure of oxides, the oxides on Kovar alloy would be decomposed even evaporated completely, on the contrary, a new oxide may form on the Kovar alloy. Air, high-purity nitrogen, or Ti-purified argon was employed as wetting atmosphere by most scholars [4], [5], [6], [7] in the analogous experiment, and then the composition of oxides to be researched would be changed, it was also be proved by the authors. As a result, the wetting atmosphere in this experiment is in N₂–2.31% H₂O at 1000 °C, which just like oxidation atmosphere.

As a matter of fact, the subject of glass to Kovar sealing is a question of wetting behavior of glass on oxides of Kovar. Unfortunately only a few papers [8], [9] are available on this topic and no interaction mechanism is given. In order to shed some light on the mechanism of glass to Kovar sealing, this paper presents the results of wetting and spreading of borosilicate glass on Kovar with oxides of FeO + Fe₃O₄ as a function of wetting time by the sessile drop method. The results will be discussed in terms of wetting kinetics, top view and cross-sectional microstructure. The aims of this work were to gain basic knowledge about the characteristic wetting behavior of glass on Kovar, and to know the role of glass and oxides during sealing.

Section snippets

Materials

The Kovar specimens are cut into coupons with dimensions of 20 mm × 10 mm × 1.5 mm, and the composition of Kovar alloy is Ni 29.0 wt%, Co 17.3 wt%, Mn 0.4 wt%, Si 0.2 wt% and C 0.02 wt%, balanced with Fe. The specimens were abraded through 1200 grade metallographic paper, and then were ultrasonically cleaned in acetone followed by ethanol. After being dried, the specimens were stored in a desiccator.

The glass used in this investigation is borosilicate glass. And its composition is Na₂O 3.7, K₂O 3.8, Al₂O₃

Contact angle and drop base diameter

Wetting time dependence of contact angle measured at 1000 °C based on the cross-section image is shown in Fig. 1. The feature of the curve is the rapid decrease in contact angle during the first 1 min, followed by much slower spreading, leading to the final contact angle of 23° in about 20 min.

Fig. 2 shows the top view of glass on Kovar wetting at 1000 °C for different time. At the initial of wetting for 1 min (Fig. 2(a)), the glass begins to soften. When processed for 2 min, one halo was found at

Wetting kinetics

According to the changes of contact angle and glass base diameter, the wetting process could be divided into three stages.

The first stage is incubation period for wetting time lower than 1 min. Glass drop quickly begins to soften when arriving at high temperature zone of furnace and the viscosity of glass rapidly decreases. In order to reduce the surface energy, the glass base forms a spherical cap, and the contact angle is higher than 90° (such as wetting time for 30 s). With the extending of

Summary

The characteristic wetting and spreading behavior of borosilicate glass on Kovar with oxides of FeO + Fe₃O₄ was investigated as a function of wetting time in N₂–2.31% H₂O at 1000 °C by the sessile drop method. Reactive wetting was observed, according to contact angle and drop base diameter, wetting process could be divided into three stages: incubation period, reaction period and equilibrium period. Two halos were formed at the fringe of glass base in the wetting process. The upper glass in the

Acknowledgement

This work was supported by the National Natural Science Foundation of China (No. 50671014).

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Wetting and spreading behavior of borosilicate glass on Kovar

Abstract

Introduction

Section snippets

Materials

Contact angle and drop base diameter

Wetting kinetics

Summary

Acknowledgement

J. Alloys Compd.

Mater. Sci. Eng. A

J. Eur. Ceram. Soc.

Mater. Sci. Eng. A

J. Solid State Chem.

Acta Metall.

J. Alloys Compd.