Solder paste, solder joint, and method for manufacturing a solder joint

Abstract

The solder paste contains solder powder and a spacer, the content of the spacer is 1 mass% or less, the melting temperature of the spacer is higher than the melting temperature of the solder powder, the spacer does not dissolve in solder even when heated at a temperature of the melting point of the solder powder + 20°C for 5 minutes, the spacer is cylindrical or tubular, and the diameter deviation σ / Da of the spacer calculated from the average diameter Da and the standard deviation σ of the spacer is 6.0% or less.

Description

Technical Field

[0001] The present invention relates to a solder paste containing solder powder, a solder joint formed by bonding a first component and a second component via a solder layer, and a method for manufacturing the solder joint.

[0002] This application claims priority based on Japanese Patent Application No. 2024-055332 filed on March 29, 2024, the contents of which are incorporated herein by reference. Background Technology

[0003] For example, various devices such as LEDs or power modules are structures in which electronic components such as semiconductor elements are bonded on a circuit board.

[0004] When bonding electronic components such as semiconductor elements to a circuit board, methods using solder materials are widely used, as shown in Patent Documents 1 and 2.

[0005] In the case of a flip-chip mounted chip with a large area of ​​semiconductor chip, a power semiconductor that generates high heat, and multiple electrical connections, the thickness of the solder layer between the electronic component and the substrate varies, making it difficult to keep the distance between the electronic component and the substrate constant.

[0006] If the solder layer thickness deviates, causing the electronic component to be bonded to the substrate at an angle, a change in thermal resistance occurs within the chip, resulting in a failure to achieve the desired heat dissipation. Furthermore, in the case of a high-brightness LED as the electronic component, the emission angle from the LED may not be the expected angle.

[0007] Therefore, for example, Patent Document 3 discloses a solder paste using Ni balls that do not melt at the soldering temperature, have a diameter of 1 to 300 μm, and a sphericity of 0.90 or higher. Patent Document 1 describes a solder paste that, since self-alignment can be ensured when Ni balls are mounted on electrodes of semiconductor devices, and since the Ni balls do not melt at the soldering temperature, height deviation of the solder joint can be suppressed.

[0008] Patent Document 1: Japanese Patent Application Publication No. 2000-271782

[0009] Patent Document 2: Japanese Patent Application Publication No. 2021-010152

[0010] Patent Document 3: Japanese Patent No. 5585752

[0011] However, while Patent Document 3 describes a solder paste using Ni balls that do not melt at the soldering temperature, have a diameter of 1 to 300 μm, and a sphericity of 0.90 or higher, it does not disclose details such as the proportion of Ni balls contained in the solder paste. In particular, it does not disclose the proportion of Ni powder contained in the solder paste when the Ni balls are aggregates, i.e., Ni powder. Therefore, it may not be possible to sufficiently achieve the effect of suppressing the tilting of electronic components through the Ni balls in the solder layer.

[0012] Furthermore, when the Ni ball content is increased in order to suppress the tilting of electronic components, pores are easily formed in the solder layer, which may prevent the heat generated from the electronic components from being efficiently transferred to the substrate side through the solder layer. Summary of the Invention

[0013] The present invention was made in view of the foregoing circumstances, and its object is to provide a solder paste that can make the thickness of the solder layer formed when components are joined together uniform and can suppress the generation of pores in the solder layer, a solder joint formed by joining a first component and a second component through a solder layer with uniform thickness and few pores, and a method for manufacturing the solder joint.

[0014] To address the aforementioned issues, the solder paste of aspect 1 of the present invention is characterized by containing solder powder and spacers, wherein the content of the spacers is 1% by mass or less, the melting temperature of the spacers is higher than the melting temperature of the solder powder, and the spacers will not dissolve in the solder even when heated for 5 minutes at a temperature of +20°C to the melting point of the solder powder, the spacers are cylindrical or tubular, and the diameter deviation σ / Da of the spacers, calculated from the average diameter Da and the standard deviation σ, is 6.0% or less.

[0015] According to aspect 1 of the invention, the solder paste contains spacers whose melting temperature is higher than that of the solder powder. Even when heated for 5 minutes at a temperature 20°C above the melting point of the solder powder, the spacers do not dissolve in the solder. The diameter deviation σ / Da of the spacers, calculated from the average diameter Da and the standard deviation σ of the diameter, is less than 6.0%. Therefore, it is possible to make the thickness of the solder layer formed by applying solder paste and heating it uniform.

[0016] Furthermore, since the content of the spacer is less than 1% by mass, it is possible to suppress the formation of pores in the solder layer.

[0017] Furthermore, since the spacer is cylindrical or tubular, by cutting cylindrical or tubular materials with small diameter deviations along the length direction, the diameter deviation σ / Da of the spacer, calculated from its average diameter Da and standard diameter deviation σ, can be kept small. Moreover, the amount of spacer can be kept low, thereby suppressing the formation of porosity in the solder layer.

[0018] The solder paste of aspect 2 of the present invention is characterized in that, in the solder paste of aspect 1 of the present invention, the average diameter Da of the spacers is in the range of 1 μm or more and 300 μm or less.

[0019] According to aspect 2 of the invention, since the average diameter Da of the spacers is in the range of 1 μm or more and 300 μm or less, it is possible to uniformly form a solder layer with a thickness in the range of 1 μm or more and 300 μm or less.

[0020] The solder joint of aspect 3 of the present invention is characterized by having a first component, a second component, and a solder layer, wherein the first component and the second component are joined via the solder layer, wherein spacers are dispersed in the solder layer, the melting temperature of the spacers is higher than the melting temperature of the solder layer, and the spacers will not dissolve even when heated for 5 minutes at a temperature of +20°C to the melting point of the solder layer, the spacers are cylindrical or tubular, the diameter deviation σ / Da of the spacers calculated from the average diameter Da and the standard deviation σ of the spacers is 6.0% or less, the content of the spacers in the solder layer is 1% by mass or less, the inclination angle of the joint surface of the second component relative to the joint surface of the first component is less than 1.0°, and the porosity of the solder layer is 20% or less.

[0021] According to aspect 3 of the present invention, since spacers are dispersed in the solder layer, and the melting temperature of the spacers is higher than that of the solder layer, the spacers will not dissolve even when heated for 5 minutes at a temperature 20°C above the melting point of the solder layer. Therefore, the solder layer thickness can be uniformly formed, and the tilt angle of the bonding surface of the second component relative to the bonding surface of the first component can be less than 1.0°. Furthermore, since the porosity of the solder layer is less than 20%, stable heat transfer between the first and second components can be achieved through the solder layer.

[0022] The solder joint of aspect 4 of the present invention is characterized in that, in the solder joint of aspect 3 of the present invention, the average height of the spacer is in the range of 1 μm or more and 300 μm or less.

[0023] According to aspect 4 of the invention, since the average height of the spacers is in the range of 1 μm or more and 300 μm or less, the thickness of the solder layer is in the range of 1 μm or more and 300 μm or less, and the distance between the first component and the second component can be kept constant.

[0024] The solder joint of aspect 5 of the present invention is characterized in that, in the solder joint of aspect 3 or aspect 4 of the present invention, the first component is a circuit board and the second component is a semiconductor element.

[0025] According to aspect 5 of the present invention, since the first component is a circuit board and the second component is a semiconductor element, the tilted bonding of the semiconductor element and the circuit board can be suppressed, and the heat generated in the semiconductor element can be efficiently transferred to the circuit board side via the solder layer, thereby enabling the formation of a semiconductor device with stable performance.

[0026] The method for manufacturing a solder joint of aspect 6 of the present invention is a method for manufacturing a solder joint formed by bonding a first component and a second component via a solder layer, characterized by comprising: a paste application step, wherein solder paste of aspect 1 or aspect 2 is applied to either or both of the bonding surfaces of the first component and the second component; a lamination step, wherein the first component and the second component are laminated via the applied solder paste; and a solder bonding step, wherein the first component and the second component laminated via the solder paste are heat-treated to form a solder layer, and the first component and the second component are bonded via the solder layer, wherein the heating temperature in the solder bonding step is above the melting temperature of the solder powder and below the melting temperature of the spacer.

[0027] According to the method for manufacturing a solder joint according to aspect 6 of the present invention, since the solder paste of aspect 1 or aspect 2 of the present invention is used, the spacer is dispersed in the molten solder powder during the solder bonding process, and the thickness of the solder layer can be determined by using the spacer. Furthermore, since a solder paste with a low content of spacer is used, the formation of porosity in the solder layer can be suppressed.

[0028] According to an aspect of the present invention, it is possible to provide a solder paste that can make the thickness of the solder layer formed when components are joined together uniform and can suppress the generation of pores in the solder layer, a solder joint formed by joining a first component and a second component through a solder layer with uniform thickness and few pores, and a method for manufacturing the solder joint. Attached Figure Description

[0029] Figure 1 This is a schematic diagram illustrating a solder joint according to an embodiment of the present invention.

[0030] Figure 2This is an enlarged illustration of the solder layer according to one embodiment of the present invention.

[0031] Figure 3 This is a schematic diagram illustrating the spacers contained in the solder layer of a solder paste and solder joint according to an embodiment of the present invention.

[0032] Figure 4 This is a flowchart illustrating a method for manufacturing a solder joint according to an embodiment of the present invention. Detailed Implementation

[0033] Hereinafter, with reference to the accompanying drawings, the solder paste, solder joint, and method for manufacturing the solder joint according to embodiments of the present invention will be described.

[0034] like Figure 1 As shown, the solder joint 10 involved in this embodiment has a first component 11, a second component 12, and a solder layer 20, and the first component 11 and the second component 12 are bonded via the solder layer 20. In this embodiment, the solder joint 10 is a semiconductor device formed by bonding a circuit board (first component 11) and a semiconductor element (second component 12) via the solder layer 20.

[0035] Here, the circuit board (first component 11) is made of a material with excellent thermal conductivity, such as copper or copper alloy, aluminum or aluminum alloy, iron or iron alloy, or aluminum nitride. In this embodiment, it is made of an iron alloy (Kovar). In addition, a film of Au or the like is preferably formed on the bonding surface of the circuit board (first component 11) to ensure wetting with the solder.

[0036] Furthermore, in order to ensure wetting with solder, a film of Au or the like is preferably formed on the bonding surface of the semiconductor element (second component 12).

[0037] Solder layer 20 is formed from solder paste of this embodiment, such as Figure 2 As shown, spacers 21 are dispersed inside the solder layer 20.

[0038] In this embodiment, as Figure 3 As shown, the spacer 21 is cylindrical or tubular.

[0039] The solder paste of this embodiment will now be described. The solder paste of this embodiment contains solder powder and spacer 21.

[0040] In this embodiment, the content of spacer 21 in the solder paste is less than 1% by mass.

[0041] Furthermore, the upper limit of the content of spacer 21 is preferably 0.5% by mass or less. On the other hand, the lower limit of the content of spacer 21 is preferably 0.01% by mass or more, and more preferably 0.1% by mass or more.

[0042] The solder powder can be appropriately selected according to the material of the joint surfaces of the first component 11 and the second component 12 to be joined.

[0043] In this embodiment, when the bonding surface of the circuit board (first component 11) and the bonding surface of the semiconductor element (second component 12) are made of noble metal (Au), Sn-Ag-Cu solder, Au-Sn solder, Sn-Cu solder, etc. can be used as solder powder.

[0044] In this embodiment, the solder powder is an Au-Sn solder with a Sn content of 10% by mass or more and 80% by mass or less, and the remainder being Au and unavoidable impurities.

[0045] In addition, the average particle size of the solder powder is preferably in the range of 1 μm or more and 300 μm or less.

[0046] The spacer 21 is made of a material whose melting temperature is higher than that of the solder powder and which will not dissolve even when heated for 5 minutes at a temperature 20°C above the melting point of the solder powder. Examples of materials that constitute the spacer 21 include Cu, Ag, Au, Ni, Pt, Pd, W, and composites formed by coating these metals onto the surface of dissimilar materials.

[0047] In this embodiment, since the solder powder is the aforementioned Au-Sn solder, the spacer 21 is a spacer with a melting temperature higher than the melting temperature (280°C) of the solder powder (Au-Sn solder).

[0048] Moreover, such as Figure 3 As shown, in this embodiment, the spacer 21 is cylindrical or tubular, and the diameter deviation σ / Da of the spacer 21, calculated from the average diameter Da (average value of diameter D) and the standard deviation σ of diameter D, is 6.0% or less. Furthermore, the diameter deviation σ / Da is more preferably 2.0% or less. Moreover, the diameter deviation σ / Da can be 0% or more, 1.0% or more, or 1.5% or more.

[0049] Here, the diameter D of the cylindrical or tubular spacer 21 is the distance between the two furthest points within a spacer on a cut surface orthogonal to the length direction of the cylinder or tube.

[0050] Furthermore, in this embodiment, the average diameter Da of the spacer 21 is preferably in the range of 1 μm or more and 300 μm or less, and more preferably in the range of 1 μm or more and 100 μm or less.

[0051] Furthermore, in this embodiment, the aspect ratio (the ratio of average diameter Da to average length L) L / Da of the spacer 21 is preferably 1 or more and 50 or less.

[0052] Furthermore, the spacer 21 is preferably made of a material whose surface has good wettability with the molten solder powder.

[0053] In this embodiment, since the solder powder is the Au-Sn solder described above, at least the surface of the spacer 21 is preferably made of a metal material selected from one or both of Ni and Cu.

[0054] Next, refer to Figure 4 The flowchart illustrates the manufacturing method of the semiconductor device (solder bond 10) according to this embodiment.

[0055] (Paste coating process S01)

[0056] First, the solder paste of this embodiment is applied to either or both of the bonding surfaces of the circuit board (first component 11) and the semiconductor element (second component 12). Furthermore, the method of applying the solder paste is not particularly limited; for example, metal masking, screen printing, or dispensing can be used.

[0057] (Lamination process S02)

[0058] The circuit board (first component 11) and the semiconductor element (second component 12) are laminated by applying solder paste as described above.

[0059] (Solder bonding process S03)

[0060] The circuit board (first component 11) and semiconductor element (second component 12) stacked with solder paste are subjected to heat treatment. In addition, the heating temperature during this heat treatment is set to be above the melting temperature of the solder powder and below the melting temperature of the spacer 21.

[0061] Solder layer 20 is formed by solidifying molten solder paste, and circuit board (first component 11) and semiconductor element (second component 12) are bonded via solder layer 20.

[0062] Here, since spacers 21 are dispersed in the molten solder powder, the thickness of the solder layer 20 (the distance between the circuit board (first component 11) and the semiconductor element (second component 12)) remains constant.

[0063] In the solder paste of this embodiment as described above, since it contains spacer 21, the melting temperature of which is higher than that of the solder powder, and even if heated for 5 minutes at a temperature of +20°C the melting point of the solder powder, the spacer 21 will not dissolve in the solder. The diameter deviation σ / Da of the spacer 21, calculated from the average diameter Da and the standard deviation σ of the diameter D, is less than 6.0%, so the thickness of the solder layer 20 formed by applying solder paste and heating can be made uniform.

[0064] Furthermore, since the content of spacer 21 in the solder paste is less than 1% by mass, it is possible to suppress the formation of pores in the formed solder layer 20.

[0065] Furthermore, in this embodiment, when the average diameter Da of the spacer 21 is in the range of 1 μm or more and 300 μm or less, the thickness of the solder layer 20 formed can be uniformly formed in the range of 1 μm or more and 300 μm or less.

[0066] Furthermore, in this embodiment, when the spacer 21 is cylindrical or tubular, by cutting cylindrical or tubular materials with small diameter deviations in the length direction, the diameter deviation σ / Da of the spacer 21, calculated from the average diameter Da and the standard deviation σ of the diameter D, can be kept small. Moreover, the content of the spacer 21 can be kept low, thereby further suppressing the formation of porosity in the formed solder layer 20.

[0067] In the solder joint 10 of this embodiment, since spacers 21 are dispersed in the solder layer 20, and the melting temperature of the spacers 21 is higher than that of the solder layer 20, and the spacers 21 will not dissolve even when heated for 5 minutes at the melting point of the solder layer +20°C, and the diameter deviation σ / Da of the spacers, calculated from the average diameter Da and the standard deviation σ of the spacers 21, is set to 6.0% or less, the thickness of the solder layer 20 can be uniformly formed, and the tilted bonding of the circuit board (first component 11) and the semiconductor element (second component 12) can be suppressed. The tilt angle of the bonding surface of the semiconductor element (second component 12) relative to the bonding surface of the circuit board (first component 11) is less than 1.0°. The tilt angle of the bonding surface of the semiconductor element (second component 12) relative to the bonding surface of the circuit board (first component 11) is preferably 0.5° or less, and more preferably 0.3° or less. Furthermore, the tilt angle between the bonding surface of the semiconductor element (second component 12) and the bonding surface of the circuit board (first component 11) can be 0° or more, 0.1° or more, or 0.2° or more.

[0068] Furthermore, since the content of spacer 21 relative to solder paste is 1% by mass or less, the porosity of solder layer 20 is 20% or less. The porosity of solder layer 20 is preferably 15% or less. Moreover, the porosity of solder layer 20 can be 0% or more, 5% or more, or 10% or more.

[0069] Therefore, it is possible to construct a semiconductor device with stable performance (solder joint 10).

[0070] Furthermore, in this embodiment, when the average diameter Da of the spacer 21 is in the range of 1 μm or more and 300 μm or less, the thickness of the solder layer 20 can be uniformly formed in the range of 1 μm or more and 300 μm or less, and the distance between the circuit board (first component 11) and the semiconductor element (second component 12) remains constant, enabling the manufacture of a semiconductor device (solder bond 10) with stable performance. The average diameter Da of the spacer 21 can also be described as the average height of the spacer 21 along the thickness direction of the solder layer 20.

[0071] The embodiments of the present invention have been described above, but the present invention is not limited thereto, and appropriate changes can be made without departing from the technical requirements of the present invention.

[0072] In this embodiment, the solder joint 10 is described as a semiconductor device formed by bonding a circuit board (first component 11) and a semiconductor element (second component 12) through a solder layer 20, but it is not limited to this and may be other solder joints.

[0073] Furthermore, while the solder powder is described as being composed of Au-Sn solder, it is not limited to this; for example, it could be other solder materials such as Sn-based solder or Pb-based solder. Additionally, it is preferable to select the material of the spacer 21 appropriately based on the material of the solder powder.

[0074] Example

[0075] The results of the verification experiments conducted to confirm the effectiveness of the present invention will be described below.

[0076] A solder paste containing the solder powder and spacers shown in Table 1 was prepared.

[0077] In addition, the average particle size of the solder powder recorded in Table 1 is the particle size (d50) at 50% of the volume cumulative frequency of the particles in the volume-based particle size distribution measured by a laser diffraction scattering particle size distribution measuring device.

[0078] The average diameter Da of cylindrical or tubular spacers was determined as follows: The spacers were placed on a carbon tape surface, and SEM images of the spacers were taken at 500x magnification using a Hitachi TM303 desktop microscope. From the obtained SEM images, five spacers whose overall shape could be identified as having a surface orthogonal to the length direction of the spacer were randomly selected. The distance between the two furthest points within a spacer was measured in the direction orthogonal to the length direction of the spacer, and this distance was taken as the diameter D of that spacer. The average of the diameters D measured in the five spacers was taken as the average diameter Da. Furthermore, the standard deviation σ was calculated based on the diameters D measured in the five spacers.

[0079] Furthermore, the lengths of the five spacers were measured, and the average of these lengths was taken as the average length L.

[0080] The solder paste is applied to an Al2O3 substrate whose surface has been metallized with Au, such that the thickness of the solder paste after reflow heating is equal to the diameter D of the spacer.

[0081] The formula for calculating the appropriate solder paste thickness is: ((volume of solder under the chip excluding spacers) / (volume ratio of solder in the solder paste excluding spacers) + (volume of spacers under the chip)) / (chip area)

[0082] Here, the solder volume is calculated based on the premise that the thickness of the solder layer is equivalent to the thickness of a spacer, and the spacer volume is calculated based on the spacer concentration in the solder paste.

[0083] Chips (1mm × 1mm) were stacked on the applied solder paste.

[0084] Then, using a reflow oven (Malcom SRS-1C), under a nitrogen atmosphere, heat treatment is performed according to the following heating mode to solder the circuit board to the chip.

[0085] Heat from room temperature to 200℃ at a heating rate of 1.5℃ / second, hold at 200℃ for 2 minutes, then heat from 200℃ to 300℃ at a heating rate of 1.5℃ / second, and hold at 300℃ for 5 seconds.

[0086] Regarding the solder joints obtained in the above manner, the porosity of the solder layer and the tilt of the chip were evaluated as follows. Furthermore, a comprehensive evaluation of the solder joints was conducted based on these evaluations.

[0087] (Porosity of the solder layer)

[0088] The porosity is calculated by observing the chip from directly above using a transmission X-ray device and taking the total area of ​​the projected pores relative to the chip's area. Three measurements are performed, and the average porosity is calculated.

[0089] (Chip tilt)

[0090] The solder joint is cut along a plane orthogonal to the centerline of the chip surface. The angle between the substrate surface and the chip surface (the tilt angle of the chip's bonding surface relative to the substrate's bonding surface) is measured and taken as the chip's tilt. The chip's tilt is determined by performing three measurements.

[0091] [Table 1]

[0092]

[0093] In Comparative Example 1, the solder paste did not contain spacers, and the chip had a relatively large tilt angle of 1.6°.

[0094] In Comparative Example 2, the spacer content was higher at 30% by mass, and the porosity of the solder layer was higher at 35%.

[0095] In Comparative Example 3, the diameter deviation σ / Da of the spacer was relatively large, at 10.0%, the porosity of the solder layer was relatively high, at 25%, and the tilt of the chip was relatively large, at 1.0°.

[0096] In Comparative Example 4, the diameter deviation σ / Da of the spacer was relatively large, at 10.0%, and the tilt of the chip was relatively large, at 1.4°.

[0097] In contrast, in Examples 1 to 9 of the present invention, the generation of pores in the solder layer can be suppressed, and the tilt of the chip can be suppressed.

[0098] Based on the results of the above confirmation experiments, it has been confirmed that, according to the present invention, a solder paste capable of producing a uniform thickness of solder layer when components are joined together and capable of suppressing the generation of pores in the solder layer, a solder joint formed by joining a first component and a second component via a solder layer of uniform thickness and few pores, and a method for manufacturing the solder joint can be provided.

[0099] Industrial availability

[0100] The solder paste of this embodiment is preferably applicable to various devices, such as LEDs or power modules, on which electronic components, such as semiconductor elements, are bonded to a circuit board.

[0101] Symbol Explanation

[0102] 10 Solder Joints

[0103] 11 First Component

[0104] 12 Second Component

[0105] 20 solder layers

[0106] 21 spacers

[0107] S01 Paste Coating Process

[0108] S02 Lamination Process

[0109] S03 Solder bonding process

Claims

1. A solder paste, characterized in that, It contains solder powder and spacers, wherein the content of the spacers is less than 1% by mass. The spacer has a higher melting temperature than the solder powder; even if heated for 5 minutes at a temperature 20°C above the melting point of the solder powder, the spacer will not dissolve in the solder. The spacer is cylindrical or tubular, and the diameter deviation σ / Da of the spacer, calculated from the average diameter Da and the standard deviation σ, is less than 6.0%.

2. The solder paste according to claim 1, characterized in that, The average diameter Da of the spacer is in the range of more than 1 μm and less than 300 μm.

3. A solder joint, characterized in that, It has a first component, a second component, and a solder layer. The first component and the second component are joined via the solder layer. Spacers are dispersed within the solder layer. The melting temperature of the spacers is higher than that of the solder layer. Even when heated for 5 minutes at a temperature 20°C above the melting point of the solder layer, the spacers will not dissolve. The spacer is cylindrical or tubular, and the diameter deviation σ / Da of the spacer, calculated from the average diameter Da and the standard deviation σ, is less than 6.0%. The content of the spacer in the solder layer is less than 1% by mass. The inclination angle of the mating surface of the second component relative to the mating surface of the first component is less than 1.0°, and the porosity of the solder layer is less than 20%.

4. The solder joint according to claim 3, characterized in that, The average height of the spacers is in the range of more than 1 μm and less than 300 μm.

5. The solder joint according to claim 3 or 4, characterized in that, The first component is a circuit board, and the second component is a semiconductor element.

6. A method for manufacturing a solder joint, wherein the solder joint is formed by bonding a first component and a second component via a solder layer, the method for manufacturing the solder joint being characterized by having: The solder paste application process involves applying the solder paste of claim 1 or 2 to either or both of the mating surfaces of the first component and the second component. The lamination process involves stacking the first component and the second component using the applied solder paste; and The solder bonding process involves heat-treating the first component and the second component, which are stacked with solder paste, to form a solder layer, and then bonding the first component and the second component together via the solder layer. The heating temperature in the solder bonding process is above the melting temperature of the solder powder and below the melting temperature of the spacer.

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