CN112501568B

CN112501568B - Micro-nano multilayer structure composite material and preparation method and application thereof

Info

Publication number: CN112501568B
Application number: CN202011376101.0A
Authority: CN
Inventors: 朱家俊; 周灵平; 孙强; 杨武霖; 符立才; 李德意; 张超
Original assignee: Hunan University; CETC 18 Research Institute
Current assignee: Hunan University; CETC 18 Research Institute
Priority date: 2020-11-30
Filing date: 2020-11-30
Publication date: 2022-06-24
Anticipated expiration: 2040-11-30
Also published as: CN112501568A

Abstract

The invention provides a micro-nano multilayer structure composite material and a preparation method and application thereof. First, Ar generated by a sputtering ion source⁺Sputtering an Ag target by using an ion beam, bombarding the surface of a base material by using a high-energy ion beam generated by an auxiliary ion source capable of generating medium energy, and forming an Ag nanocrystalline transition layer on the surface of the metal foil; and then, continuously depositing an Ag film on the basis of the Ag transition layer by utilizing magnetron sputtering to form the composite material with the micro-nano multilayer structure. According to the invention, the bonding performance of the interface of the nano-crystalline film and the substrate in the micro-nano multilayer structure composite material is enhanced, the conductivity of the composite material is improved, and the surface Ag film melting point is reduced, so that the welding current is reduced during resistance welding, the welding time is shortened, the damage of the substrate and the embrittlement and stress concentration of the welding layer tissue caused by solid solution chemical precipitation are avoided, and meanwhile, the preparation process conditions of the material are simplified to meet the requirements of industrial production.

Description

Micro-nano multilayer structure composite material and preparation method and application thereof

Technical Field

The invention relates to the technical field of thin film materials, in particular to a composite material with a micro-nano multilayer structure, and a preparation method and application thereof.

Background

Metal Ag has excellent optical, thermal and electrical characteristics, and thus has wide applications in electrical interconnections, mirrors, transparent conductive films, and the like. Silver is preferred as an interconnect material for low earth orbit spacecraft solar cell arrays because of its excellent electrical conductivity and good solderability. However, in a Low Earth Orbit (LEO) environment, due to the existence of atomic oxygen, Ag is extremely aggressive (the atomic oxygen reactivity of silver can reach 10.5 × 10)^-24cm³·atom^-1) Therefore, the capability of resisting the space environment of the low earth orbit is insufficient, and the requirement of long-term on-orbit service of the aerospace craft cannot be met.

Corresponding research work is carried out by some scholars at home and abroad on the aspects of the process, the performance, the related mechanism and the like of plating Ag films on the surfaces of substrates such as metal Mo, Pd, Fe-Ni alloy and the like. Relevant researches have been started in Europe and America and other countries in the earliest 60 years in the last century, wherein STS-8, STS-7, STS-41G and the like are flight missions for specially researching the atomic oxygen effect, and in the flight missions, NASA makes comparison tests on the atomic oxygen corrosion resistance of materials such as pure silver foil, gold-plated silver foil, palladium-plated silver foil, silver-plated molybdenum foil and the like, and according to test results, the pure silver foil is found to be completely corroded by atomic oxygen quickly; the gold-plated silver foil eroded at a rate of about 1/100 that of pure silver foil, mainly due to pinhole pitting caused by the lack of density of the surface plating; although the palladium-plated silver foil passes through the experiment, some oxides still exist on the surface of palladium, and the phenomenon that palladium does not completely cover the silver foil exists in partial areas is found, while the silver-plated molybdenum foil has the stripping phenomenon of the silver layer although the molybdenum foil is not corroded by atomic oxygen. According to the test results, the pure silver foil is not suitable for the low orbit flying task with high atomic oxygen concentration, the gold-plated silver foil and the palladium-plated silver foil can slow down the corrosion of atomic oxygen to a certain extent, but the test results are not ideal, and according to the atomic oxygen test of the European Bureau on the gold-plated silver foil, the gold-plated silver foil is further found to be easily influenced by temperature impact, and generate 'pinhole defects' at the stress reduction ring, and the atomic oxygen can corrode the silver below the plating layer through the defects to cause the final failure of the interconnection piece. The test result of the silver-plated molybdenum foil shows that metal molybdenum has a strong erosion-resistant effect on atomic oxygen, but Mo and Ag belong to an immiscible binary alloy system, so that the difficulty of directly plating Ag on the surface of Mo is high, the bonding force between the prepared composite material layers is not high, the stripping of an Ag plating layer can be caused under a certain temperature impact environment, in order to improve the bonding strength between the layers of the silver-plated molybdenum foil, a metal transition layer is generally required to be added to improve the bonding strength of an Ag/Mo interface, the process is complex, and the cost is relatively high; the Chinese invention patent (CN106048534A) discloses a surface treatment process of a molybdenum foil for an aerospace interconnection sheet, wherein the bonding force between a silver plating layer and a Mo base material is improved by performing the processes of degreasing, acid washing, alkali washing, nickel-chromium alloy plating, silver plating and the like on a metal molybdenum foil, and introducing a nickel-chromium alloy layer between the silver plating layer and the metal Mo base material and performing annealing treatment on the nickel-chromium alloy layer.

The kovar alloy has excellent conductivity, good heat resistance and low thermal expansion coefficient (about 5 multiplied by 10)^-6K^-1) And good thermal conductivity, and has strong resistance to atomic oxygen attack and lower density (e.g., 4J29 Kovar alloy density of about 8.17g cm)^-3) Compared with the silver-plated molybdenum foil interconnection material used for the spacecraft, the weight of the spacecraft can be reduced by about 20%, and the requirement of the spacecraft on light weight is met.

Some work is also carried out in the research aspect of silver/Kovar composite materials in China, for example, a rolling composite process adopted by Shanghai space power supply research institute is used for compounding two layers of silver foils and one layer of Kovar alloy foil to form an Ag/Kovar/Ag laminated sandwich structure, annealing is carried out for 0.5-2 hours under the protection condition of 400 plus 750 ℃/atmosphere, the thickness range of a rollable Kovar foil in the prepared silver/Kovar metal laminated composite material is 10-100 mu m, the thickness range of a pure silver foil is 8-50 mu m, and the tensile strength of a welding spot after the welding of the Kovar/silver metal laminated composite material and a gallium arsenide solar cell is carried out through parallel resistance welding can reach 6-8N. However, according to previous research by the applicant, the problem that the thickness uniformity of the laminated composite material prepared by the rolling composite method is difficult to control accurately is found, fig. 9 shows a cross section SEM of the Al/Cu/Al laminated composite material prepared by the rolling method and having a total thickness of 250 μm, and it can be obviously seen that the thicknesses of the metal Al layers at the upper layer and the lower layer are obviously different, and the thicknesses of the metal layers at different positions of the same sample piece are also different to a certain extent. And the total thickness of the interconnecting material adopted at present is generally controlled below 30 mu m, and the requirement of the rolling process on the reduction of the thickness of the rolled material is further improved.

In the aspect of Kovar alloy surface silver plating research, the eighteenth institute of electronic technology and Tianjin university of China adopt an electroplating process to plate an Ag film on the surface of a Kovar alloy foil to prepare a silver-plated Kovar composite material, a Ni/Cu transition layer is introduced at an Ag/Kovar interface in a nickel flash plating and copper preplating mode to enhance the bonding strength between a silver layer and the Kovar alloy layer, and a tensile test of the obtained Kovar alloy silver-plated interconnection sheet and a single solar cell after welding shows that the average strength of welding spots is 1.5-2 N.mm^-20.83 N.mm higher than the space product standard^-2The technical requirements of (1) but the Ag electroplating needs to adopt cyanide plating solution, which is not beneficial to environmental protection; in addition, according to the previous research of the applicant, impurity residues contained in the plating solution in the electroplating product are often difficult to completely remove, and cannot generate important influence on the product quality in a short time, but along with the service life extension, the residues enter the interface layer of the plating layer and the substrate in a diffusion mode, so that the bonding strength of the interface between layers is weakened, and the service life of the composite material is further influenced.

Disclosure of Invention

The invention provides a micro-nano multilayer structure composite material and a preparation method and application thereof, and aims to enhance the bonding performance of a nano-crystalline film and a substrate interface in the micro-nano multilayer structure composite material, simultaneously improve the conductivity of the composite material and reduce the melting point of a surface Ag film so as to reduce welding current during resistance welding, shorten welding time, avoid damage to the substrate and embrittlement and stress concentration of a welding layer tissue due to solid solution chemical precipitation, and simplify the preparation process conditions of the material to adapt to the requirements of industrial production.

In order to achieve the aim, the invention provides a preparation method of a composite material with a micro-nano multilayer structure, which comprises the following steps:

s1: mounting a metal base material on a roller sample table and placing the metal base material in a vacuum chamber, respectively mounting pure Ag metal target materials on an ion beam sputtering target table and a magnetron sputtering target table, closing the vacuum chamber, and pumping the vacuum chamber to be in a background vacuum; then introducing a proper amount of argon into the vacuum chamber, starting a cleaning ion source after the air pressure in the vacuum chamber is stable, and cleaning the surface of the metal substrate by using an ion beam;

s2: after the ion beam cleaning is finished, starting a sputtering ion source to sputter the Ag target material, so that Ag is deposited on the surface of the metal base material, and simultaneously starting an auxiliary ion beam source capable of assisting the ion beam source to bombard the surface of the base material to form an Ag nanocrystalline transition layer on the surface of the metal base material;

wherein the energy of the bombardment ion source is below 35keV, and the bombardment ion beam is vertical to the surface of the base material;

s3: and after the Ag nano-crystal transition layer is obtained, turning on a magnetron sputtering target power supply, adjusting the output power of the sputtering power supply, and forming an Ag nano-crystal film on the surface of the Ag nano-crystal transition layer obtained in the step S2 by utilizing magnetron sputtering.

Preferably, the metal substrate is placed into a NaOH solution for boiling before being cleaned by ion beam cleaning, and then is dried after being cleaned by ultrasonic cleaning sequentially by acetone and alcohol solution.

Preferably, the concentration of the NaOH solution is 5-15%, and the boiling time is 0.5-1 h; the ultrasonic cleaning time is 10-30 min.

Preferably, the metal substrate is one or more of Kovar (Kovar) alloy, molybdenum (Mo) metal, Invar steel (Invar).

Preferably, the thickness of the metal substrate is less than 100 microns.

Preferably, the sputter ion source energy is 2.5keV or less; the deposition time is 20-60 min.

Preferably, the purity of the Ag nanocrystalline film is more than 99%, the thickness of the Ag nanocrystalline film is 1-10 μm, the melting point of the Ag nanocrystalline film is lower than 960 ℃, and the size of Ag grains is less than 100 nm.

The invention also provides a silver nanocrystalline film composite material, which is obtained by forming Ag nanocrystalline films on the upper surface and the lower surface of a metal substrate by the method.

Preferably, the total thickness of the composite material is less than 100 micrometers, wherein the thickness ratio of the Ag nanocrystalline thin film to the metal substrate is 0.1-1: 1.

The invention also provides an application of the composite material, and the composite material is applied to the electrical interconnection of the solar cell array single cells of the space low-orbit aircraft or applied to electronic equipment which adopts a resistance welding technology to realize electrical connection.

The scheme of the invention has the following beneficial effects:

(1) compared with the conventional Ag foil, the Ag nano-crystalline film on the surface of the micro-nano multilayer structure composite material has a low melting point, so that the welding power is low during resistance welding, and the damage to the back electrode of the solar cell is reduced while the welding performance of welding spots is ensured.

(2) The Ag nanocrystalline transition layer prepared on the surface of the metal substrate by adopting the method improves the interface bonding and stress state of the Ag film and the metal substrate, obviously improves the adhesive property of the Ag film on the surface of the metal substrate, and can induce Ag crystals deposited by subsequent magnetron sputtering to realize Ag crystal grain refinement.

(3) The method of the invention does not introduce a heterogeneous transition layer when preparing the Ag film, thereby not only reducing the influence of impurities on the performance of the composite material, but also simplifying the preparation process.

(4) The Ag nanocrystalline film prepared by magnetron sputtering in the method can obtain Ag nanocrystalline films with different thicknesses according to requirements, and the process of preparing the Ag nanocrystalline transition layer in the process is carried out in the same vacuum chamber environment, so that secondary pollution is avoided.

Drawings

FIG. 1 is a tensile curve of the film/substrate bond strength test for sample 1 prepared in example 1.

Fig. 2 is SEM images of the surface (a) and the cross section (b) of sample 1 prepared in example 1.

Fig. 3 is an XRD diffraction pattern a and a film/base bond strength test tensile curve b of sample 2 prepared in example 2.

Fig. 4 is an XRD diffraction pattern a and a film/base bond strength test tensile curve b of sample 3 prepared in example 3.

Fig. 5 is a surface topography (a) of the silver-plated film kovar composite after the tensile test of the sample 4 prepared in example 4 and a surface topography (b) of the solar cell after the tensile test.

Fig. 6 is a surface topography (a) of the silver-plated film kovar composite after the tensile test of the sample 5 prepared in example 5 and a surface topography (b) of the solar cell after the tensile test.

Fig. 7 is an XRD diffraction pattern a and a film/base bond strength test tensile curve b of sample 6 prepared in comparative example 1.

Fig. 8 is a tensile curve of the film/substrate bond strength test of sample 7 prepared in comparative example 2.

FIG. 9 is SEM images of cross-sections of different regions of Al/Cu/Al laminated composite material prepared by rolling method.

Detailed Description

In order to make the technical problems, technical solutions and advantages of the present invention more apparent, the following detailed description is given with reference to the accompanying drawings and specific embodiments.

Example 1

Firstly, obtaining an Ag nanocrystalline transition layer on the surface of the Kovar alloy foil by adopting a sputtering ion source and an intermediate energy auxiliary ion beam source, and then plating an Ag nanocrystalline film by adopting magnetron sputtering.

And (3) boiling the Kovar alloy foil in a 5% NaOH solution for 30min, then ultrasonically cleaning the Kovar alloy foil for 15min by using acetone and alcohol solution in sequence, drying the Kovar alloy foil and filling the Kovar alloy foil into a vacuum chamber.

Selecting pure Ag with purity of 99.99% as target material, selecting high-purity Ar with purity of 99.99% as working gas, pumping vacuum chamber to background vacuum, charging appropriate amount of high-purity inert gas argon (Ar) into low-energy cleaning ion source, and maintaining vacuum chamber pressure at 8.0-9.0 × 10^-3Pa, starting low-energy ion source (screen voltage 600V, beam current 40mA), and adjusting KoCarrying out ion beam etching cleaning on the surface of the var alloy for 15 min; the low energy ion source is turned off.

After the ion beam etching cleaning process of the surface of the Kovar alloy substrate is finished, simultaneously starting a sputtering ion source and an intermediate energy auxiliary ion beam source to prepare an Ag transition layer on the surface of the Kovar alloy foil, wherein the screen voltage of the auxiliary ion source is 30kV and the beam current is 8mA, the screen voltage of the sputtering ion source is 2.5kV and the beam current is 60mA, and the deposition time is 40min, so that the Ag nanocrystalline transition layer is formed.

And after the Ag nanocrystalline transition layer is obtained, continuously preparing the Ag nanocrystalline film by adopting magnetron sputtering, wherein the power is 200W, and the deposition time is 60min, thus obtaining a sample 1.

The film/base interface bond strength of sample 1 was tested by tensile testing, and the tensile curve is shown in FIG. 1, with a maximum pull-off force of 195.6N, based on the area of the pull-off region (about 7.1 mm)²) The film-to-base bond strength of this sample was calculated to be about 27.55MPa (film-to-base bond strength P was calculated from the formula P ═ F/a, where F is the maximum pull-off force and a is the area of the pull-off region).

Fig. 2 is a surface and cross-sectional SEM image of sample 3. The result shows that the Ag film prepared on the surface of the Kovar alloy is flat and compact, no large particles are accumulated, the thickness of the Ag film is about 3.3 mu m according to the section SEM result, the film thickness is uniform and consistent, and the combination of the Ag film and the Kovar alloy substrate is good.

Example 2

Plating an Ag film on the surface of the metal Mo foil, obtaining a sample 2 by adopting the process and process parameters completely consistent with those of the example 1, wherein fig. 3(a) is an XRD diffraction pattern of the sample 2, so that the Ag nanocrystalline film presents the mixed orientation of (111) and (200), and the grain size of the Ag film is measured and calculated to be about 75nm according to the Sheer formula. The tensile test was conducted to test the film/base bond strength of the sample, and the tensile curve results are shown in fig. 3(b), and the maximum pull-off force was 113.1N, and the film/base bond strength was calculated to be about 16 MPa.

Example 3

The Ag film is plated on the surface of the Invor alloy foil, the sample 3 is obtained by adopting the process and the process parameters completely consistent with those of the example 1, the XRD diffraction pattern of the sample 3 is shown in figure 4(a), the Ag film shows a certain (111) preferred orientation, and the grain size of the Ag film on the surface is measured to be about 90nm according to the Sheer formula. The tensile test was conducted to test the film/base bond strength of the sample, and the tensile curve results are shown in fig. 4(b), whereby the maximum pull-off force was 153.3N, and the film/base bond strength was calculated to be about 21.6 MPa.

Example 4

And plating Ag films on the upper and lower surfaces of the Kovar alloy foil by adopting the same technological process and technological parameters as those in the embodiment 1 to form the micro-nano multilayer structure composite material, thereby obtaining a sample 4. And welding the sample 4 and the back electrode of the solar cell sheet by adopting a resistance spot welding method, wherein the welding power is 45W, the pulse time is 50ms, after the welding is finished, a 45-degree tensile test is adopted to perform a tensile test on the sample, the five welding spot strengths are cumulatively tested, the measured pull-off forces are 657gf, 528gf, 496gf, 442gf and 675gf respectively, the average value reaches 560gf and is higher than 160gf required by the space product standard.

The welding spot area after the pull-off test is observed under a microscope, as shown in fig. 5, it is obvious that the main failure mode of the welding spot is the welding spot failure, and the delamination or peeling phenomenon of the Ag film and the Kovar substrate does not occur, which further illustrates that the composite film layer prepared by the method of the present invention is well combined with the Kovar substrate.

Example 5

And plating Ag films on the upper and lower surfaces of the Kovar alloy foil by adopting the process and process parameters completely consistent with those of the embodiment 1 to form the micro-nano multilayer structure composite material, thereby obtaining the sample 5. The test sample 5 and the back electrode of the solar cell are welded by adopting a resistance spot welding method, the welding power is 50W, the pulse time is 50ms, after the welding is finished, a 45-degree tensile test is adopted to carry out a tensile test on the test sample, the measured pull-off forces are 668gf, 647gf, 454gf, 820gf and 416gf respectively, the average value reaches 601gf and is higher than that of the test sample 5, but a relatively obvious corrosion phenomenon appears at the welding point of the interconnection piece and the back electrode of the solar cell, as shown in figure 6.

By combining the results of the example 4 and the example 5, the silver-plated film composite material prepared by the invention can obtain excellent welding effect at the welding power of 45W compared with the power of 60W selected when the traditional pure silver foil material is subjected to resistance welding.

Comparative example 1

Preparing the Ag film on the surface of the Kovar alloy foil by magnetron sputtering. The difference from the embodiment is that: the Ag nanocrystalline transition layer is prepared without adopting a sputtering ion source and a neutral energy auxiliary ion beam source.

Adjusting the power of the magnetron sputtering target to 100W before preparing the Ag film, carrying out pre-sputtering on the Ag target for 5min to remove an oxide layer on the surface of the Ag target, adjusting the target power to 200W after the target current and the target voltage are stable, and beginning to deposit the Ag film for 60min to obtain a sample 6.

Fig. 7(a) is an XRD diffraction pattern of sample 6, and it can be seen that the Ag film exhibits a mixed orientation of (111) and (200), in which the intensity of the (111) diffraction peak is about twice that of the (200) diffraction peak, and the grain size of the surface Ag film is about 375nm as measured by scherrer's equation (D ═ K λ/(β cos θ)). The tensile test was conducted to test the film/base bond strength of the test piece, and the tensile curve results thereof are shown in FIG. 7(b), whereby the maximum pull-off force of 84.5N was obtained in terms of the area of the pulled-off region (about 7.1 mm)²) The film/base bond strength was calculated to be about 12 MPa.

Comparative example 2

Firstly preparing an Ag transition layer on the surface of a Kovar alloy foil by adopting a sputtering ion source, and then preparing an Ag film by adopting magnetron sputtering, wherein the difference from the embodiment 1 is as follows: the substrate was not bombarded with a source of intermediate energy assisting ions.

Sample 7 was obtained and subjected to a tensile test, and the film/base interface bonding strength of sample 7 was measured, and the tensile curve results are shown in fig. 8.

The test results showed a maximum pull-off force of 133.1N, based on the area of the pull-off region (about 7.1 mm)²) The calculated film/base bonding strength is about 18.73MPa, and compared with the sample 6 only adopting the magnetron sputtering to deposit the Ag film, the film/base bonding strength of the sample under the process is improved to a certain extent, but is far less than that of the sample1 film/substrate bond strength.

TABLE 1 test results of samples of examples 1 to 3 and comparative examples 1 to 2

	Base material	Maximum pull-off force/N	Film/base bond strength/MPa	Grain size/nm
					Example 1	Kovar	195.6	27.55	78
Example 2	Mo	113.1	16	75
					Example 3	Invor	153.3	21.6	90
Comparative example 1	Kovar	84.5	12	375
					Comparative example 2	Kovar	133.1	18.73	400

As can be seen from the example 1 and the comparative examples 1-2, the Ag crystal grain size obtained by the method is small, the interface bonding and stress state of the Ag film and the metal substrate are effectively improved, and the maximum pull-off force and the film/base bonding strength of the material are obviously improved. The film/base bonding strength of a sample obtained by preparing the Ag film on the surface of the metal Mo substrate is not high, mainly because Mo and Ag belong to a binary immiscible system which is not solid-dissolved and mutually combined, firm combination of the film layer and the substrate is difficult to realize at an Ag film/Mo substrate interface by a combination (alloy) interface and other methods, the difference of the thermal expansion coefficients of Ag and Mo is large, and when the sample obtained by directly adopting the Ag film deposited on the surface of the metal Mo through magnetron sputtering is taken out of a vacuum chamber, the film layer is automatically curled and peeled from the surface of the substrate. However, the method of the invention effectively improves the interfacial bonding property of the Ag film/Mo matrix and improves the film/matrix bonding strength. Provides possibility for preparing Ag film on the surface of Mo substrate.

While the foregoing is directed to the preferred embodiment of the present invention, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

1. A micro-nano multilayer structure composite material is characterized in that the composite material is obtained by compounding Ag nano-crystalline films deposited on the upper surface and the lower surface of a metal base material with micron-sized thickness;

the Ag nanocrystalline film has the purity of more than 99%, the thickness of 1-10 mu m, the melting point of less than 960 ℃, and the size of Ag grains of less than 100 nm;

the preparation method comprises the following steps:

s1: carrying out ion beam cleaning on the surface of the metal base material by using a low-energy ion source;

s2: using metal Ag as a target material, sputtering the Ag target material by adopting a sputtering ion source, and bombarding the surface of the base material by adopting an intermediate-energy auxiliary ion beam source to form an Ag nanocrystalline transition layer on the surface of the metal base material;

wherein the energy of the bombardment ion source is below 35keV, and the bombardment ion beam is vertical to the surface of the base material; the energy of the low-energy ion source is 0.5keV-1keV, and the cleaning time is 15-60 min; the energy of a sputtering ion source is 1.5keV-2.5keV, and the deposition time is 20-60 min;

s3: and forming an Ag nanocrystalline film on the surface of the Ag nanocrystalline transition layer obtained in the step S2 by utilizing magnetron sputtering.

2. The micro-nano multilayer structure composite material according to claim 1, wherein the metal substrate is one or more of kovar alloy, metal molybdenum and invar steel.

3. The micro-nano multilayer structure composite material according to claim 1, wherein the thickness of the metal substrate is less than 100 micrometers.

4. The micro-nano multilayer structure composite material according to claim 1, wherein the total thickness of the composite material is less than 100 micrometers, and the thickness ratio of the Ag nano-crystal film to the metal substrate is 0.1-1: 1.

5. The micro-nano multilayer structure composite material according to claim 1, wherein the metal substrate is boiled in NaOH solution before being cleaned by ion beams, and then is dried after being cleaned by ultrasonic waves with acetone and alcohol solution in sequence.

6. The micro-nano multilayer structure composite material according to claim 5, wherein the concentration of the NaOH solution is 5-15%, and the boiling time is 0.5-1 h; the ultrasonic cleaning time is 10-30 min.

7. Use of a composite material according to any one of claims 1 to 4, in the electrical interconnection of cells of a solar array of a spacecraft in a space low orbit, or in electronic devices electrically connected by resistance welding.