Elsevier

Microelectronics Reliability

Volumes 76–77, September 2017, Pages 460-464
Microelectronics Reliability

Reliability investigation of the copper-zinc system for solid diffusion bonding in power modules

https://doi.org/10.1016/j.microrel.2017.07.023Get rights and content

Highlights

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    Development of a process for the solid state diffusion of Copper-Zinc as bonding solution for power modules.

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    Based on standard processing equipment, as it is employed for instance for Silver sintering.

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    Use of test vehicles consisting in a stack of Copper platelets and Zinc foils.

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    Samples are submitted to thermal storage.

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    Time-dependency of the shear-strength of the bond as a function of the processing temperature.

Abstract

Solid Diffusion Bonding of the Copper-Zinc system is preliminarily investigated as a possible bonding solution for power modules. The idea behind the study is to use standard processing equipment, as it is employed for instance for Silver sintering. The investigation is carried out on test vehicles consisting in a stack of Copper platelets and Zinc foils. After Solid Diffusion Bonding under different pressure and temperature conditions, the samples are submitted to thermal storage. Analysis is performed by non-destructive and destructive techniques. Particular attention has been paid to the time-dependency of the shear-strength of the bond as a function of the processing temperature.

Section snippets

Problem definition

In power modules, the common binary system used for soldering is based on copper and tin. During the soldering process, the Cu-Sn system normally forms two intermetallic phases: Cu6Sn5 and Cu3Sn. Since the Cu-Sn soldering has been already largely in use for decades, its limitations are well known.

Nowadays, this system is already overwhelmed because of the stringent requirements imposed to power assemblies, especially in terms of power density and operating temperature. In power modules, bonding

The investigated alternative

Solid Diffusion Bonding is a process, by which two flat interfaces are joined at elevated temperature and at high pressure. The main advantage of this process is that high quality joints are produced so that neither metallurgical discontinuities nor porosity exist across the interface. In order to obtain reliable joints, great is required in surface preparation, in particular excessive oxidation or contamination would decrease the joint strength drastically [12].

In present paper, the

The experimental procedure

In this work, the feasibility of Solid Diffusion Bonding by Cu-Zn is investigated on the base of a representative model, where a thin film of Zinc is bonded to two copper platelets. The processing parameters are inspired by two existing industrial processes, soldering and silver sintering. The bonding temperature is imposed to stay below 350 °C. However, since under these conditions, the melting temperature of Zinc is not reached, the solid diffusion is activated by the application of 10 MPa

Experimental: sample preparation and bonding

The experiments have been carried out with stacks of copper platelets and zinc foils. The area of the square Cu platelets is 100 mm2 and the thickness 2 mm. The copper platelets have been prepared by an external supplier from a standard copper, Cu-ETP, DIN 2.0060 material. The maximum deviation of the flatness of both faces does not exceed 20 μm and the roughness is specified according to Ra 2.0–3.2. Commercial 0.1 mm thick Zn foils (Goodfellow) with an area of 625 mm2 and rolled with a purity of

Thermal aging

After the characterization of the samples as bonded by SAM, high temperature storage has been performed at 150 °C under reduced nitrogen atmosphere (100 mbar) for 200, 400, 600, 800 and 1000 h, respectively. A lot of five samples has been chosen for each dwelling time batch. After thermal aging, one sample has been selected for cross sectioning and the bonding force of the residual samples has been investigated by shear test.

Results and discussion

As predicted by the theory, solid diffusion occurred at the Cu-Zn interface, such that all 96 samples have been properly bonded. To verify some literature data, which identified oxidation before and during the bonding process as a main issue independently of the chosen metallic system, the copper platelets and the Zn films have been cleaned prior to bonding but no additional countermeasure has been undertaken to prevent oxidation. Therefore, because the process occurred in air, surface

Summary and conclusions

This investigation shows that even without taking major precautions to avoid oxidation, solid diffusion between copper and zinc occurs properly resulting in in initial strong bonds. This result has been achieved by the use of standard equipment for silver sintering. Thermal aging of the obtained bonds at 150 °C did show a noticeable reduction of the bond strength with time. This effect can be traced back to the oxidation of Zn during the processing phase and thermal aging. The formation of the

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