A Cu-Sn based intermetallic compound solder joint and preparation method thereof

Abstract

The present invention provides a Cu-Sn-based intermetallic compound solder joint and a preparation method thereof, and relates to 3D packaged chip stacking interconnection manufacturing and high-power device packaging manufacturing. Cu-xNi alloy is selected as the first metal matrix, pure Cu or Cu-xNi alloy is the second metal matrix, pure Sn or Sn-based solder is used as the intermediate solder layer, and Cu-xNi / solder / Cu or Cu-xNi / solder / Cu-xNi structural combination is formed, and then the above combination is subjected to soldering reflow reaction under certain temperature conditions, so that the solder is completely reacted and converted into an intermetallic compound solder joint with (Cu, Ni)6Sn5 as the main body, (Cu, Ni)6Sn5 has fine grains, and the grain orientation is disordered and random, and the thermal stability is good. The present invention has the advantages of low cost, simple process flow, good compatibility with prior art equipment, and significantly shortening the bonding time.

Description

Technical Field

[0001] The present invention relates to the technical field of 3D packaged chip stacking interconnection manufacturing or high-power device packaging manufacturing, and in particular to a Cu-Sn-based intermetallic compound solder joint and a rapid preparation method thereof. Background Art

[0002] With the development trend of miniaturization, high performance, low power consumption and high integration of electronic devices, 3D integrated circuit packaging is considered to be the most promising packaging technology in the semiconductor industry, and its application prospects are very broad. 3D packaging is based on 2D packaging and stacks chips layer by layer in the vertical direction, with higher interconnection density, smaller interconnection size and faster data transmission speed. In 2D packaging, the solder joint is mainly composed of metal matrix-intermetallic compound layer-Sn-based solder-intermetallic compound layer-metal matrix. Usually, during the soldering reflow process, a metallurgical reaction occurs between the common Cu metal matrix and the Sn-based solder, forming an interfacial Cu-Sn-based intermetallic compound layer with Cu6Sn5 as the main phase and a small amount of Cu3Sn as the auxiliary phase on the Cu6Sn5 / Cu interface. However, the interfacial intermetallic compound layer grows slowly, and its thickness after soldering reflow is much smaller than that of the Sn-based solder.

[0003] In 3D packaging, the size of interconnect structures continues to shrink, and the height of interconnect solder joints between chips has been shortened to tens of microns or even just a few microns. At this time, the proportion of the interface intermetallic compound layer to the solder joint increases significantly after bonding (the effect is equivalent to solder reflow). Under subsequent service conditions, the interface Cu6Sn5 intermetallic compound will transform into Cu3Sn, and Cu3Sn will become the main body of the interface intermetallic compound, which will cause Kirkendall voids in the Cu-Sn-based intermetallic compound layer and on the intermetallic compound / Cu interface, seriously reducing the performance and reliability of the solder joint; at the same time, the residual Sn-based solder is easy to melt again during the bonding of the next level, affecting the bonding effect. In addition, in high-power devices or high-temperature service devices based on 2D packaging technology, the melting point of the Sn-based solder joint is low, and it is easy to soften or even melt, which cannot meet the requirements of long-term high-temperature service.

[0004] In order to solve the above technical problems, a technical solution for intermetallic compound solder joints has emerged, that is, to allow the interface reaction to proceed fully under specific brazing reflow or bonding conditions, so that the Sn-based solder is completely consumed and converted into a solder joint with an intermetallic compound as the connecting medium and a higher melting point, namely, an intermetallic compound solder joint.

[0005] At present, the common methods for preparing intermetallic compound solder joints mainly include high-temperature brazing reflow, hot pressing bonding, external field assisted (such as ultrasound, current or temperature gradient) brazing reflow or bonding, nanoparticle sintering, etc. However, the existing technology usually has the following problems: 1) high process temperature, low efficiency, complex process, high cost, and poor compatibility with existing semiconductor or packaging technology processes; 2) Cu6Sn5 phase grains are coarse and easily make the solder joints have preferential orientation, resulting in anisotropy of physical and mechanical properties; 3) Cu6Sn5 phase will inevitably transform into Cu3Sn phase, accompanied by the formation of Kirkendall holes.

[0006] Therefore, in view of the problems existing in the existing technology for preparing intermetallic compound solder joints, it is urgent to develop a new intermetallic compound solder joint structure and a fast, simple and low-cost preparation method thereof. Summary of the invention

[0007] According to the above-mentioned technical problems that the existing preparation of intermetallic compound solder joints is time-consuming, high in temperature, complicated in process, and the prepared solder joints have coarse grains and are prone to form holes, a Cu-Sn-based intermetallic compound solder joint and a rapid preparation method thereof are provided. The present invention mainly adopts Cu-xNi alloy as a metal substrate. During the reflow process, the alloy matrix is ​​rapidly melted, and a large number of Cu and Ni metal atoms enter the liquid Sn-based solder to cause Cu-Ni interaction, so that the concentration of Cu and Ni in the liquid solder is always in a saturated state, and the growth of intermetallic compounds at the interface is continuously accelerated, and the generation of Cu3Sn-type intermetallic compounds at the interface is effectively inhibited, and the formation of Kirkendall voids during service is avoided, and the reliability of the solder joint is improved. It has the advantages of low cost, simple process flow, and good thermal stability.

[0008] The technical means adopted by the present invention are as follows:

[0009] A Cu-Sn based intermetallic compound solder joint, characterized by comprising:

[0010] The first metal matrix is ​​a Cu-xNi alloy, the second metal matrix is ​​pure Cu or a Cu-xNi alloy, and a Cu-Sn-based intermetallic compound connecting the first metal matrix and the second metal matrix; the Cu-Sn-based intermetallic compound is mainly composed of a (Cu, Ni)6Sn5 phase and contains a small amount of Cu3Sn phase located at the interface of (Cu, Ni)6Sn5 / first metal matrix and (Cu, Ni)6Sn5 / second metal matrix; wherein the mass percentage of Ni element in the Cu-xNi alloy is 2-20%.

[0011] Furthermore, when the first metal matrix and the second metal matrix are both Cu-xNi alloys, the content of Ni element is the same.

[0012] Furthermore, in the (Cu,Ni)6Sn5 phase, the atomic percentage of Ni is less than 20%.

[0013] Furthermore, one or more of the elements Ag, In, Bi, Au, Pd, and Zn are solid-dissolved in the (Cu, Ni)6Sn5 phase.

[0014] Furthermore, the (Cu, Ni)6Sn5 phase has the characteristics of fine grains, disordered and random grain orientation, and no preferred orientation, and the average grain size of the (Cu, Ni)6Sn5 phase is less than 10 μm.

[0015] The present invention also discloses a method for preparing the above-mentioned Cu-Sn-based intermetallic compound solder joint, which is characterized by comprising the following steps:

[0016] Step 1: providing a first metal matrix Cu-xNi alloy, and making a solder layer or solder bump on the first metal matrix, wherein the solder layer or solder bump is a Sn-based solder composed of pure Sn or Sn and one or more of Ag, Cu, In, Bi, Zn, and Ni elements, and the mass percentage of Sn in the Sn-based solder is not less than 90%; the height of the solder layer or solder bump is less than 300 μm;

[0017] Step 2: providing a second metal substrate of pure Cu or Cu-xNi alloy, aligning and contacting the solder layer or solder bump with the second metal substrate to form a combination;

[0018] Step 3: heating the assembly in step 2 to a set temperature to melt the solder layer or solder bump, performing soldering reflow, and applying a certain pressure to the assembly until the molten solder is completely reacted and converted into a Cu-Sn-based intermetallic compound to form a Cu-Sn-based intermetallic compound solder joint;

[0019] The Cu-Sn based intermetallic compound is grown simultaneously on the first and second metal substrates;

[0020] The first metal matrix and the second metal matrix both remain after the soldering reflow is completed.

[0021] Furthermore, before forming the assembly in step 2, a solderable layer is made on the second metal substrate; in step 2, the solder layer or solder bump is aligned and contacted with the solderable layer to form an assembly; the solderable layer is composed of one or more of Sn, Ag, Au, Pd, Ni, and OSP.

[0022] Furthermore, before forming the assembly in step 2, a soldering flux is coated on the second metal substrate or the solderable layer.

[0023] Furthermore, during the soldering reflow in step three, the pressure is set to be less than 1 MPa; the highest temperature in the molten solder is less than 300° C.; and an electric current is applied to the assembly, with the direction of the electric current being from the second metal substrate to the first metal substrate.

[0024] Preferably, a physical field such as ultrasound, electric current, electromagnetic field, temperature gradient, etc. can be applied to the assembly during the reflow process. Under the condition of applying the physical field, the material of the metal matrix and the solder, the height of the solder joint, the physical field, and the reflow temperature are the most important factors affecting the growth rate and performance of the intermetallic compound, and other conditions have less influence. Therefore, the present invention is not limited to the structure in the above technical solution.

[0025] Furthermore, during the soldering reflow in step three, the temperature of the second metal substrate is made lower than that of the first metal substrate, thereby forming a temperature gradient in the molten solder, and the temperature gradient is less than 500° C. / cm.

[0026] Compared with the prior art, the present invention has the following advantages:

[0027] The present invention can solve the technical problems of low preparation efficiency, inability to serve in high temperature environment, coarse grains of Cu6Sn5 phase with prominent anisotropy and poor thermal stability in intermetallic compound solder joints or bumps in the preparation of 3D packaging and high-power device packaging using existing process technologies.

[0028] The solution of the present invention is to select Cu-xNi alloy as the first metal matrix, pure Cu or Cu-xNi alloy as the second metal matrix, and pure Sn or Sn-based solder as the intermediate solder layer to form a Cu-xNi / solder / Cu or Cu-xNi / solder / Cu-xNi structural combination, and then subject the above combination to a soldering reflux reaction under certain temperature conditions so that the solder is completely reacted and converted into an intermetallic compound solder joint with (Cu, Ni)6Sn5 as the main body.

[0029] The present invention uses Cu-xNi alloy as a metal substrate. During the reflow process, the Cu-xNi alloy matrix melts rapidly, and a large number of Cu and Ni metal atoms enter the liquid Sn-based solder to cause Cu-Ni interaction, so that the concentration of Cu and Ni in the liquid solder is always in a saturated state, and the growth of intermetallic compounds at the interface is continuously accelerated. The Cu-xNi alloy is selected as the first metal matrix and the Sn-based solder to form a (Cu, Ni)6Sn5 ternary compound, which inhibits the generation of Cu3Sn-type intermetallic compounds at the interface and avoids the formation of Kirkendall voids during service, thereby improving the reliability of the solder joint. In addition, the generated (Cu, Ni)6Sn5 is a fine grain with a disordered, random, and non-preferred grain orientation, thereby improving the mechanical properties and reliability of the solder joint. The melting temperature of the formed all-metallic compound solder joint exceeds 415°C and has high thermal stability.

[0030] In summary, the technical solution of the present invention has the advantages of low cost, using traditional reflow methods and temperatures for bonding, avoiding thermal damage to the device, simple process flow, good compatibility with existing bonding equipment, significantly shortening the brazing reflow time, good thermal stability, high production efficiency, and being able to achieve low-temperature bonding and high-temperature service, etc. It can be widely used in the preparation of intermetallic compound solder joints or bumps in 3D packaging and high-power device packaging. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0032] Figure 1 It is a schematic diagram of preparing solder bumps on the first metal substrate of the present invention.

[0033] Figure 2 It is a schematic diagram of preparing a solder layer on the first metal substrate of the present invention.

[0034] Figure 3 This is a schematic diagram of the structure of the Cu-Sn based intermetallic compound solder joint prepared by the present invention.

[0035] Figure 4 This is the EBSD image of the Cu-8Ni / Sn / Cu solder joint obtained after brazing reflow in Example 3.

[0036] Figure 5 This is the EBSD image of the Cu-14Ni / Sn / Cu solder joint obtained after brazing reflow in Example 5.

[0037] Figure 6 This is the backscattered electron image obtained after the Cu-5Ni / Sn / Cu-5Ni solder joint is reflowed in Example 7.

[0038] In the figure: 10, first metal substrate; 20, Sn-based solder bump; 22, Sn-based solder layer; 30, second metal substrate; 31, solderable layer; 32, soldering flux; 40, intermetallic compound layer. DETAILED DESCRIPTION

[0039] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0040] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is by no means intended to limit the present invention and its application or use. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0041] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0042] Example 1

[0043] Step 1: If Figure 1 As shown, a first metal substrate 10 is provided as a Cu-4Ni alloy, pure Sn is electroplated on the first metal substrate 10 and then reflowed to obtain a Sn-based solder bump 20 with a height of 20 μm; a second metal substrate 30 is provided as pure Cu, an OSP solderable layer 31 is chemically deposited on the second metal substrate 30, and a soldering flux 32 is coated on the surface of the solderable layer 31;

[0044] Step 2: Align and contact the Sn-based solder bump 20 and the solderable layer 31 together to form a soldering structure assembly;

[0045] Step 3: The assembly in step 2 is heated to 250°C for soldering reflow, and a pressure of 0.2 MPa is applied to the assembly until the molten Sn-based solder layer is completely consumed to generate an interface Cu-Sn-based intermetallic compound 40 (such as Figure 3 As shown), a Cu-Sn based intermetallic compound solder joint is obtained.

[0046] Example 2

[0047] Step 1: If Figure 2 As shown, the first metal substrate 10 is provided to be a Cu-6Ni alloy, a Sn-based solder layer 22 with a thickness of 10 μm is electroplated on the first metal substrate 10, and the Sn-based solder layer 22 is a SnAgCu solder layer; the second metal substrate 30 is provided to be pure Cu, an Au solderable layer 31 is electroplated on the second metal substrate 30, and a soldering flux 32 is coated on the surface of the Au solderable layer 31;

[0048] Step 2: Align and contact the SnAgCu solder layer 22 and the Au solderable layer 31 together to form a soldering structure assembly;

[0049] Step 3: The first metal matrix Cu-6Ni alloy in the assembly in step 2 is set as the hot end, so that the temperature of the second metal matrix 30Cu is lower than that of the first metal matrix 10, and then a temperature gradient of 300°C / cm is formed in the molten SnAgCu solder, and the average temperature is 260°C. Reflow is performed with the assistance of the temperature gradient, and the Sn solder layer is gradually consumed completely to form a Cu-Sn-based intermetallic compound solder joint.

[0050] Example 3

[0051] Step 1: Provide a first metal substrate 10 of Cu-8Ni alloy, electroplate pure Sn on the first metal substrate 10 and then reflow to obtain a Sn solder layer 22 with a height of 35 μm; provide a second metal substrate 30 of pure Cu, and coat the surface of the pure Cu of the second metal substrate 30 with a flux 32;

[0052] Step 2: aligning and contacting the pure Sn solder layer 22 and the second metal substrate 30 pure Cu together to form a brazing structure assembly;

[0053] Step 3: Heat the assembly in step 2 to 240°C for soldering reflow, and apply a pressure of 0.5 MPa to the assembly until the molten Sn solder layer is completely consumed and all the interface Cu-Sn-based intermetallic compounds are generated to obtain Cu-Sn-based intermetallic compound solder joints. Figure 4 As shown, a (Cu,Ni)6Sn5 full intermetallic compound solder joint with fine grain size and no preferred orientation is generated between the first metal matrix Cu-8Ni alloy and the second metal matrix pure Cu.

[0054] Example 4

[0055] Step 1: Provide a first metal substrate 10 of Cu-10Ni alloy, electroplating a SnAg solder layer 22 with a thickness of 15 μm on the first metal substrate 10; provide a second metal substrate 30 of pure Cu, and coat a soldering flux 32 on the surface of the pure Cu of the second metal substrate 30;

[0056] Step 2: Align and contact the SnAg solder layer 22 and the second metal substrate 30 pure Cu together to form a soldering structure assembly;

[0057] Step 3: The first metal matrix Cu-10Ni alloy in the assembly in step 2 is set as the cathode, and the second metal matrix Cu is set as the anode, and then a 5×10 3 A / cm 2The current density is 200°C and the average temperature is 270°C. Under the action of electromigration, the Sn solder layer is gradually consumed and a Cu-Sn based intermetallic compound solder joint is formed.

[0058] Example 5

[0059] Step 1: Provide a first metal substrate 10 of Cu-14Ni alloy, electroplate pure Sn solder on the first metal substrate 10 and then reflow to obtain a Sn solder bump 20 with a height of 40 μm; Provide a second metal substrate 30 of pure Cu, electroplate a Ni / Pd / Au solderable layer 31 on the second metal substrate 30, and coat a soldering flux 32 on the surface of the Ni / Pd / Au solderable layer 31;

[0060] Step 2: Align and contact the Sn solder bump 20 and the Ni / Pd / Au solderable layer 31 together to form a soldering structure assembly;

[0061] Step 3: Heat the assembly in step 2 to 280°C for soldering reflow, and apply a pressure of 0.6 MPa to the assembly until the molten pure Sn solder layer is completely consumed and all the interface Cu-Sn-based intermetallic compounds are generated to obtain Cu-Sn-based intermetallic compound solder joints. Figure 5 As shown, a (Cu,Ni)6Sn5 full intermetallic compound solder joint with fine grain size and no preferred orientation is also generated between the first metal matrix Cu-14Ni alloy and the second metal matrix pure Cu. Changing the Ni content in the first metal matrix does not significantly change the generated (Cu,Ni)6Sn5 grain size and preferred orientation.

[0062] Example 6

[0063] Step 1: Provide a first metal substrate 10 of Cu-20Ni alloy, on which a 20 μm thick Sn solder layer 22 is electroplated; provide a second metal substrate 30 of pure Cu;

[0064] Step 2: Align and contact the Sn solder layer 22 and the second metal substrate 30 which is pure Cu together to form a soldering structure assembly;

[0065] Step 3: Heat the assembly in step 2 to 260°C for soldering reflow, and apply a pressure of 0.05 MPa to the assembly until the molten Sn solder layer is completely consumed and all interfacial Cu-Sn-based intermetallic compounds are generated to obtain Cu-Sn-based intermetallic compound solder joints.

[0066] Example 7

[0067] Step 1: Provide a first metal substrate 10 of Cu-5Ni alloy, electroplate a pure Sn solder layer 22 with a thickness of 20 μm on the first metal substrate 10; Provide a second metal substrate 30 of Cu-5Ni alloy, chemically deposit an OSP solderable layer 31 on the second metal substrate, and coat a solder flux 32 on the surface of the OSP solderable layer 31;

[0068] Step 2: Align and contact the Sn solder layer 22 and the OSP solderable layer 31 together to form a soldering structure assembly;

[0069] Step 3: Heat the assembly in step 2 to 240°C for soldering reflow, and apply a pressure of 0.25 MPa to the assembly until the molten Sn solder layer is completely consumed and all the interface Cu-Sn-based intermetallic compounds are generated. Figure 6 As shown, the Cu-Sn based intermetallic compound solder joints are all composed of dense (Cu, Ni)6Sn5 intermetallic compounds.

[0070] Example 8

[0071] Step 1: Provide a first metal substrate 10 of Cu-10Ni alloy, electroplate a pure SnAg solder layer 22 with a thickness of 25 μm on the first metal substrate; provide a second metal substrate 30 of Cu-10Ni alloy, electroplate an Au solderable layer 31 on the second metal substrate 30, and coat a solder flux 32 on the surface of the electroplated Au solderable layer 31;

[0072] Step 2: Align and contact the SnAg solder layer 22 and the electroplated Au solderable layer 31 together to form a soldering structure assembly;

[0073] Step 3: The first metal matrix Cu-10Ni alloy in the assembly in step 2 is set as the hot end, and the temperature of the second metal matrix Cu-10Ni is lower than that of the first metal matrix, thereby forming a temperature gradient of 500°C / cm in the molten SnAg solder, with an average temperature of 270°C. Reflow is performed with the assistance of the temperature gradient, and the SnAg solder layer is gradually consumed to form a Cu-Sn-based intermetallic compound solder joint.

[0074] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A Cu-Sn based intermetallic compound solder joint, characterized in that: include: The first metal matrix is ​​a Cu-xNi alloy, the second metal matrix is ​​pure Cu, and a Cu-Sn-based intermetallic compound connecting the first metal matrix and the second metal matrix; the Cu-Sn-based intermetallic compound is mainly composed of a (Cu, Ni)6Sn5 phase and contains a small amount of Cu3Sn phase located at the interface of (Cu, Ni)6Sn5 / first metal matrix and (Cu, Ni)6Sn5 / second metal matrix; wherein the mass percentage of Ni element in the Cu-xNi alloy is 2-20%; Prepared based on the following method: Step 1: providing a first metal matrix Cu-xNi alloy, and making a solder layer or solder bump on the first metal matrix, wherein the solder layer or solder bump is a Sn-based solder composed of pure Sn or Sn and one or more of Ag, Cu, In, Bi, Zn, and Ni elements, and the mass percentage of Sn in the Sn-based solder is not less than 90%; the height of the solder layer or solder bump is less than 300 μm; Step 2: providing a second metal substrate of pure Cu, aligning and contacting the solder layer or solder bump with the second metal substrate to form a combination; Step 3: heating the assembly in step 2 to a set temperature to melt the solder layer or solder bump, performing soldering reflow, and applying a certain pressure to the assembly until the molten solder is completely reacted and converted into a Cu-Sn-based intermetallic compound to form a Cu-Sn-based intermetallic compound solder joint; The Cu-Sn based intermetallic compound is grown simultaneously on the first and second metal substrates; The first metal matrix and the second metal matrix both have a residue after the brazing reflow is completed; A (Cu, Ni)6Sn5 full intermetallic compound solder joint without preferential orientation is generated between the first metal matrix Cu-xNi alloy and the second metal matrix pure Cu, and the average grain size of the (Cu, Ni)6Sn5 phase is less than 10 μm; Step 3: During soldering reflow, the pressure is set to be less than 1 MPa; the highest temperature in the molten solder is less than 300° C.; a current is applied to the assembly, the direction of the current is from the second metal substrate to the first metal substrate, and an ultrasonic or electromagnetic field is applied to the assembly; During the soldering reflow in step three, the temperature of the second metal substrate is made lower than that of the first metal substrate, thereby forming a temperature gradient in the molten solder, and the temperature gradient is less than 500° C. / cm.

2. The Cu-Sn based intermetallic compound solder joint according to claim 1, characterized in that: In the (Cu,Ni)6Sn5 phase, the atomic percentage of Ni is less than 20%.

3. The Cu-Sn based intermetallic compound solder joint according to claim 1, characterized in that: One or more of the elements Ag, In, Bi, Au, Pd, and Zn are solid-dissolved in the (Cu, Ni)6Sn5 phase.

4. The Cu-Sn based intermetallic compound solder joint according to claim 1, characterized in that: Before forming the assembly in step 2, a solderable layer is made on the second metal substrate; in step 2, the solder layer or solder bump is aligned and contacted with the solderable layer to form an assembly; the solderable layer is composed of one or more of Sn, Ag, Au, Pd, Ni, and OSP.

5. The Cu-Sn based intermetallic compound solder joint according to claim 4, characterized in that: Before forming the assembly in step 2, a soldering flux is coated on the second metal substrate or solderable layer.

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