1. Introduction
Demand is growing for thermally controlled materials in microelectronics. Semiconductors stimulated the development of sophisticated metal–ceramic composites (MMC) with high thermal conductivity (TC) for efficient heat dissipation, and adjustable coefficient of thermal expansivity (CTE) to minimize thermal stresses. This is of principal significance for enhancing the power efficiency, life cycle, and reliability of electronic equipment.
Because the specific TC (TC divided by density) of Al-based composites is high, they are desirable for applications, such as in the aviation industry, where low mass is also desirable [
1,
2]. Materials used for packaging underwent immense development, owing to many key factors: an increase in the density of packages, greater demand for reliability, growing use of large semiconductor materials with phase arrays, and strict weight limitations for onboard components and many other systems [
3].
From the viewpoint of packaging technology, it is essential that electronic devices are capable of operating faster with smaller dimensions and with lower weight.
For this reason, along with the composite materials, new technologies for packaging with high-temperature solders are also applied. This concerns technologies such as ball grid arrays (BGAs), flip-chip technology (C4), chip-scale packaging (CSP), and/or multi-chip modules (MCMs) [
4].
The development of high-temperature solders is a major challenge, mainly due to the ban imposed on the use of lead and other elements that are detrimental to health and the environment. At present, there are only a limited number of lead-free soldering alloys available that could, at least partially, substitute for high-lead solders [
5]. The alternative alloy systems are hypo-eutectic Bi–Ag, Sb–Sn, or Au–Sn alloys [
6,
7]. Other alloy systems comprise Zn–Al alloys (alloyed with Mg, Ge, Ga, Sn, and Bi).
These alloys are very attractive from the perspective viewpoint, because Zn is even less expensive than Pb. A small addition of Mg or Ga to this alloy lowers the melting point and forms a ternary alloy that preserves the desired solidification criteria for soldering at higher application temperatures [
8,
9,
10]. Zn–Al-based alloys intended for the electronic industry offer a suitable melting range, as well as good TC and electrical conductivity.
This is why it was selected by the authors of Reference [
11] as the basis for their study. They added the Mg and Ga elements to that alloy and, thus, developed the Zn4Al3Mg3Ga alloy. Copper and silicon were used as substrate materials, which were metallized with a Ti/Ni/Ag layer.
Soldering was performed at 370 to 400 °C. The ZnAlMgGa/Cu joints attained a shear strength between 21.8 and 29.4 MPa at soldering temperatures between 370 and 400 °C. This is a comparable joint strength value to that of the Pb5Sn/Cu joint, which attained a value of 28.2 MPa.
The research in Reference [
12] dealt with four Zn-based alloys (Zn6Al, Zn6Al1Ga, Zn3Al3Mg, and Zn4Al3Ga3Mg), which could be good substitutes for a classic high-lead solder. Their microstructure and mechanical properties were studied. The authors found that the main cause of solder embrittlement was the thermal dependence of Ga solubility in the hexagonal lattice (hcp) of zinc, in combination with the affinity of Ga for the surface of phases enriched by Al and Zn. This embrittlement may be suppressed by lowering the solubility at higher temperatures. Studies [
12,
13] dealt with similar issues, where Sn, In, and Ga elements were added to solder types Zn–Al–Mg and Zn–Al–Ge. However, improvement of properties by this alloying was not proven.
Other authors also studied the soldering of ceramic materials with Zn-based solders. For example, in Reference [
14], the direct soldering of SiC ceramic by ultrasound assistance was studied. Ceramic SiC substrates were soldered with Zn8.5Al1Mg solder in air at 420 °C. Shear strength increased with prolonged ultrasound assistance. The highest strength (148.1 MPa) was achieved with an ultrasound assistance of 8 s.
In Reference [
15], the authors studied the soldering of sapphire (crystalline form of Al
2O
3) with ultrasound assistance. Zn4Al solder was used.
A composite interlayer of SiCp/A356 was applied in order to reduce the coefficient of thermal expansion of the soldered joint. The application of this interlayer also resulted in a significant increase in joint shear strength to approximately 155 MPa. This represented an increase of about 250% compared to joints that used only Zn–Al alloy.
Issues related to soldering SiC/Al composite with Zn–Al-based solders were studied by the authors of Reference [
15]. In order to assure wetting, they employed ultrasound with a 20 kHz frequency. After ultrasound activation, the transfer of SiC particles to the molten solder was observed. Shear strength increased with increasing soldering temperatures. The authors attributed this to an increasing content of Al and volume proportion of reinforcing particles in the joint. Research on application of the same composite material (SiC) was also carried out by the authors of Reference [
16]. In order to improve the strength of joints, an Ni layer was deposited on the composite, ensuring better wetting of Zn-based solder (Zn–Cd–Ag–Cu) with melting at approximately 400 °C.
A number of authors [
17,
18,
19,
20] showed that solder reactions with metal ceramic composites are preferably oriented to the matrix, which was aluminum in those cases. The selection of soldering alloy is, therefore, aimed toward materials used for Al soldering. Vacuum soldering technology may also be suitable for the fluxless soldering of composites. This issue was studied by the authors of References [
21,
22,
23,
24,
25]. However, the time required for soldered joint fabrication was on the order of several tens of minutes and, in some cases, hours. By comparison, joints can be fabricated by ultrasound in just a few minutes (often in less than 5 min).
The research in this work is devoted to the characterization of Zn6Al6Ag solder for soldering at high application temperatures. Microstructure, phase composition, temperature of phase transformations, and tensile strength were all studied. This work deals with the application of Zn6Al6Ag solder for ultrasonic soldering of Al2O3/composite (Al/Al2O3) combinations. This study is oriented toward an assessment of the characteristics of soldered joints by analysis performed at the solder/substrate boundary. The strength of fabricated joints was also assessed by the shear strength test, and then by observation of the fractured surfaces. This research brings a new understanding to the field of the ultrasonic soldering of metal–composite combinations through the application of Zn–Al–Ag-based solder.
2. Experimental
Zn-based solder was used in the experiments. This was a three-component solder with the chemical composition shown in
Table 1. Atomic emission spectrometry using the induction-coupled plasma (ICP-AES) method was applied for analysis of the chemical composition of the chosen solder. The analysis was performed on a SPECTRO VISION EOP instrument (SPECTRO Analytical Instruments GmbH, Kleve, Germany). The specimens of alloys for ICP-AES analysis were dissolved in suitable chemical solutions of acids and bases. The analysis proper was performed on an emission atomic spectrometer (BAS Rudince Ltd., Blansko, Czech Republic) with a pneumatic atomizer and a Scott-type atomizing chamber.
This solder is characterized as a solder suitable for high application temperatures intended for the electrotechnical industry. The solder was manufactured by casting in the form of an ingot. Weighing of single solder components was done after setting the weight ratio of the prepared alloys. Components with high purity from 3N to 5N were used for solder fabrication. The manufacture was performed in a horizontal tube vacuum furnace with resistance heating. The working temperature used during manufacture was 900 °C, at a vacuum of 10−4 Pa. At this temperature, held for 20 min, the homogenization of soldering alloy took place. The cooling in the vacuum furnace was slow, occurring at the rate of 14 °C/min.
Substrates of the following materials were used in the experiment:
A composite substrate from Fraunhofer Ltd. (Dresden, Germany) composed of an Al matrix, reinforced with ceramic particles of Al2O3 with an average particle size of 30 μm in the form of square plates, with dimensions of 10 × 10 × 3 mm. The volume fraction of Al2O3 particles in the composite was 50%;
Al2O3 ceramic substrate from Flocculus Ltd. (Libina, Czech Republic) in the form of discs Ø15 × 3 mm and in a shape of square plates with dimensions of 10 mm × 10 mm × 3 mm with 3N purity;
4N purity metallic Cu substrate in the form of discs Ø15 × 3 mm;
Al alloy Al7075 substrate in the form of discs Ø15 × 3 mm.
Figure 1 shows the schematic orientation of soldered joint substrates. The carrying material in this study was the metal–ceramic composite, which was also combined with other materials.
Soldering was performed on Hanuz UT2 ultrasonic equipment (HANUZ Ltd., Nove Mesto nad Vahom, Slovak Republic) with the parameters given in
Table 2. Solder activation was realized using an encapsulated ultrasonic transducer consisting of a piezo-electric oscillation system and a titanium sonotrode with a tip diameter of Ø3 mm. The soldering temperature was 20 °C above the liquid temperature of the solder. The soldering temperature was controlled by continuous temperature measurement on a hot NiCr/NiSi plate by a thermocouple.
The soldering process on a hot plate in the presence of ultrasonic vibrations consisted of several steps as follows:
Cleaning and degreasing soldered materials;
Laying the substrate on a hot plate;
Heating the hot plate to 430 °C;
Deposition of a small amount of solder with thickness of 600 μm on the surfaces of the substrates;
Placing the ultrasonic device sonotrode into the molten solder for 5 s;
After ultrasound activation, removing the surface oxides on the molten solder by use of a stainless-steel plate;
Bringing the prepared substrates with the solder on their surface into contact, and leaving them for approximately 1 min.
A schematic representation of the soldering procedure is shown in
Figure 2.
The metallographic preparation consisted of grinding, polishing, and etching the embedded specimens. The specimens were inserted into a jig grinder. Grinding was performed on SiC paper with granularities of 600, 1200, and 2400. During the grinding process, water was supplied to the grinding paper in order to wash away grinding debris. The grinding process on each paper lasted 3 min. After grinding, the polishing process continued on polishing discs with diamond emulsions with particle sizes of 9 μm, 6 μm, 3 μm, and 1 μm. The entire polishing process with all emulsions lasted 4 min.
After polishing, the etching process continued. Nital etchant with a chemical composition of 2 mL of HNO3 and 98 mL of ethanol was applied for 4 s. X-ray diffraction (XRD) analysis was used for the identification of phase composition of the solder and composition on the fractured surfaces after the shear strength test. The X-ray diffraction measurements were carried out using a PANalytical Empyrean diffractometer with Bragg–Brentano geometry (Malvern Panalytical Ltd., Malvern, UK). Characteristic CuKα1,2 (CuKα1 = 1.540598 × 10–10 m, CuKα2 = 1.544426 × 10–10 m) was emitted at an accelerating voltage of 40 kV with a beam current of 40 mA and was collimated using fixed slits. Diffracted radiation was collected using area-sensitive detectors operating in one-dimensional (1D) scanning mode. XRD data were analyzed using the ICSD Inorganic Crystal Structure Database and ICDD PDF2 powder diffraction and crystal structure database.
The differential scanning calorimetry (DSC) analysis was performed on Setaram SETSYS 18TM equipment with a type E DSC sensor. Analysis of alloy specimens was performed in the corundum crucibles with lids in a shielding atmosphere of Ar (6N). The rate of heating and cooling was 5 °C/min. The weight of the analyzed specimens was 40–60 mg. Specimens were, prior to analysis, properly ground and cleaned in acetone with the simultaneous assistance of ultrasound. Prior to analysis proper, the oven space was washed with Ar (6N), and the space around the specimen was subsequently vacuum-pumped and then filled with Ar. A constant dynamic atmosphere of Ar (6N, 2 L/h) was maintained during the entire analysis.
The microstructure of soldering alloy and soldered joints was observed by SEM on an FEI (Field Electron and Ion) Quanta 200 FEG microscope (Scientific and Technical Instruments, Hillsboro, OR, USA).
The solder and soldered joints were studied with the aid of qualitative and quantitative chemical analysis on JOEL 7600 F equipment with a Microspec WDX-3PC microanalyzer (Microspec Corporation, Peterborough, NH, USA).
For the strength measurement of soldered joints, shear strength testing was performed on LabTest 5.250SP1-VM equipment (Labortech Ltd., Prague, Czech Republic). The strength was measured by use of a special jig, for which a schematic representation is shown in
Figure 3. The measurement was performed until complete joint failure. The fractured surfaces obtained were assessed by observation of the microstructure, and chemical analysis was performed on JOEL 7600 F equipment (Microspec Corporation, Peterborough, NH, USA).