Soldering material, soldering paste, foam solder, and solder joint

Abstract

The invention relates to a solder material, a solder paste, a foam solder and a solder joint. Provided is a solder material capable of suppressing the occurrence of electromigration, the solder material being provided with a spherical core (2A) comprising Cu or a Cu alloy, and a solder layer (3A) covering the core (2A), the solder material being a core sphere (1A), the Cu content is from 0.1% by mass to 3.0% by mass (inclusive), the Bi content is from 0.5% by mass to 5.0% by mass (inclusive), the Ag content is from 0% by mass to 4.5% by mass (inclusive), the Ni content is from 0% by mass to 0.1% by mass (inclusive), and the balance is Sn.

Description

This application is a divisional application of the invention patent application with application number 201880014631.5, international application date February 28, 2018, and invention name “Welding materials, solder paste, foam solder and solder joint”. Technical Field

[0001] The present invention relates to a solder material obtained by coating a metal core with a solder alloy, and a solder paste, a foam solder and a solder joint using the solder material. Background Art

[0002] In recent years, with the development of small information devices, the electronic components they carry are rapidly becoming smaller. To meet the demand for miniaturization, electronic components are increasingly using ball grid array (BGA) packages with electrodes on the back surface to meet the narrowing of connection terminals and the reduction of mounting area.

[0003] Among electronic components that utilize BGAs is a semiconductor package. In a semiconductor package, a semiconductor chip having electrodes is sealed with resin. Solder bumps are formed on the electrodes of the semiconductor chip. These solder bumps are formed by bonding solder balls to the electrodes of the semiconductor chip. A semiconductor package utilizing BGAs is placed on a printed circuit board (PCB) so that each solder bump contacts the conductive pads of the PCB. The solder bumps, melted by heating, bond to the pads and are then mounted on the PCB.

[0004] As connection terminals become narrower and mounting areas shrink, soldering leads to finer joints, increasing the current density in the joints. This increase in current density in the joints raises concerns about electromigration in the joints caused by the solder.

[0005] A technique for producing a solder material called a copper core solder ball has been proposed. The solder material is formed by coating a copper core having a 1.0 to 5.0 μm thick Ni layer on the surface of a copper ball with a diameter of 20 to 80 μm with a solder alloy layer composed of Sn-Ag-Cu (see, for example, Patent Document 1). Solder materials such as copper core solder balls, which cover a metal core with a solder layer, are known to suppress electromigration compared to solder materials called solder balls made of the same solder alloy composition but without a metal core.

[0006] Prior art literature Patent Literature Patent Document 1: Japanese Patent Application Laid-Open No. 2010-103501. Summary of the Invention

[0007] Problems to be solved by the invention However, as described above, the possibility of electromigration increases with miniaturization of joints, and therefore a solder material that can further suppress electromigration than the copper-core solder ball having a solder layer composed of Sn—Ag—Cu described in Patent Document 1 is desired.

[0008] The present invention has been made to solve such problems, and an object of the present invention is to provide a solder material capable of suppressing electromigration more than conventional solder materials, and a solder paste, foam solder, and solder joint using the solder material.

[0009] Means for solving problems The present inventors have discovered that by adding a certain amount of Bi to the solder layer of a solder material having a metal core and a solder layer covering the core, the temperature rise of the joint can be suppressed, thereby significantly suppressing the occurrence of electromigration compared to conventional solder balls or solder materials having a metal core.

[0010] Therefore, the present invention is as follows.

[0011] (1) A solder material comprising a metal core and a solder layer covering the core, wherein the solder layer contains Cu in a range of 0.1% by mass to 3.0% by mass, Bi in a range of 0.5% by mass to 5.0% by mass, Ag in a range of 0% by mass to 4.5% by mass, Ni in a range of 0% by mass to 0.1% by mass, and Sn as the remainder.

[0012] (2) A solder material comprising a metal core and a solder layer covering the core, wherein the solder layer has a Cu content of 0.1 mass % to 3.0 mass %, a Bi content of more than 1.0 mass % to 5.0 mass %, a Ni content of 0 mass % to 0.1 mass %, no Ag, and Sn as the remainder.

[0013] (3) The solder material according to (1) above, wherein the Ag content is greater than 1.5 mass % and not more than 4.5 mass %.

[0014] (4) The welding material according to any one of (1) to (3) above, wherein the core is composed of a single metal substance or an alloy of Cu, Ni, Ag, Au, Al, Mo, Mg, Zn, and Co.

[0015] (5) The welding material according to any one of (1) to (4) above, wherein the core is a spherical core ball.

[0016] (6) The welding material according to any one of (1) to (4) above, wherein the core is a columnar core column.

[0017] (7) The solder material according to any one of (1) to (6) above, wherein the core covered with a layer composed of one or more elements selected from Ni and Co is covered with a solder layer.

[0018] (8) A solder paste using the solder material described in any one of (1) to (7) above.

[0019] (9) A foam solder using the solder material described in any one of (1) to (7) above.

[0020] (10) A solder joint using the solder material described in any one of (1) to (7) above.

[0021] Effects of the Invention In the present invention, heat generated in the junction and heat transferred to the junction are dissipated through the metal core, thereby suppressing the temperature rise of the junction and keeping the metal elements in a state where they are difficult to migrate. Therefore, the electromigration suppression effect brought about by the inclusion of Bi can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] [ Figure 1 ] is a cross-sectional view showing the schematic structure of the core ball of this embodiment.

[0023] [ Figure 2 ] is a structural diagram showing an example of a solder bump formed by a core ball.

[0024] [ Figure 3 ] is a cross-sectional view showing a schematic structure of a Cu core column according to this embodiment. DETAILED DESCRIPTION

[0025] The solder material of this embodiment is composed of a metal core and a solder layer covering the core. When the core is a sphere, the solder material is called a core ball. The following embodiment describes the core ball.

[0026] Figure 1 1A is a cross-sectional view showing a schematic structure of a core ball according to the present embodiment. A core ball 1A according to the present embodiment is composed of a spherical core 2A and a solder layer 3A covering the core 2A.

[0027] Core 2A may be composed of a single element of Cu or an alloy containing Cu as its primary component. When core 2A is composed of an alloy, the Cu content is 50% by mass or greater. Core 2A may be composed of any metal element or alloy other than Cu, such as Ni, Ag, Au, Al, Mo, Mg, Zn, or Co, as long as its conductivity is superior to that of solder layer 3A, which is a Sn-based solder alloy.

[0028] From the viewpoint of controlling the gap (standoff) height, the sphericity (sphericity) of core 2A is preferably more than 0.95. Sphericity is more preferably more than 0.990. In the present invention, sphericity represents a deviation from a sphere. Sphericity can be obtained by various methods such as least square center method (LSC method), minimum zone center method (MZC method), maximum inscribed center method (MIC method), minimum circumscribed center method (MCC method), etc. In detail, sphericity refers to the arithmetic mean calculated when the diameter of each core 2A of 500 is divided by the major diameter, indicating that the closer the value is to 1.00 as the upper limit, the closer it is to a sphere. The length of the major diameter in the present invention and the length of the diameter refer to the length obtained by measuring the ULTRA QUICK VISION and ULTRA QV350-PRO measuring devices made by MITSUTOYO.

[0029] The diameter of the core 2A constituting the present invention is preferably 1 to 1000 μm. Within this range, the spherical core 2A can be stably manufactured and short circuits can be suppressed when the pitch between terminals is narrow.

[0030] The solder layer 3A is made of a Sn-Ag-Cu-Bi solder alloy or a Sn-Cu-Bi solder alloy. The core ball 1A is formed by solder plating the surface of the core 2A to form the solder layer 3A.

[0031] The Bi content is preferably 0.5% by mass or more and 5.0% by mass or less. If the Bi content is less than 0.5% by mass, sufficient electromigration suppression effect cannot be achieved. If the Bi content exceeds 5.0% by mass, the electromigration suppression effect will also decrease. The Bi content is preferably more than 1.0% by mass and less than 5.0% by mass, and more preferably 1.5% by mass and less than 3.0% by mass.

[0032] Regarding the content of Cu, it is 0.1% by mass or more and 3.0% by mass or less. If the content of Cu is less than 0.1% by mass, the melting temperature cannot be fully reduced, and heating at high temperature is required when joining the bonding material to the substrate, which may cause thermal damage to the substrate. In addition, the wettability is also insufficient, and the solder will not wet and spread during joining. In addition, if the content of Cu exceeds 3.0% by mass, the melting temperature rises and the wettability also decreases. Regarding the content of Cu, it is preferably 0.3% by mass or more and 1.5% by mass or less.

[0033] The Ag content is 0% to 4.5% by mass and is an optional element. Adding more than 0% to 4.5% by mass of Ag further enhances the electromigration suppression effect compared to alloys without Ag. If the Ag content exceeds 4.5% by mass, mechanical strength decreases. In the case of a Sn-Ag-Cu-Bi solder alloy, the Ag content is preferably 0.1% to 4.5% by mass, and more preferably 1.5% to 4.5% by mass.

[0034] The Ni content is 0% to 0.1% by mass and is an optional element. Adding more than 0% to 0.1% by mass improves wettability compared to alloys without Ni. If the Ni content exceeds 0.1% by mass, the melting temperature rises and wettability decreases. When Ni is added, the Ni content is preferably 0.02% to 0.08% by mass.

[0035] The diameter of the core ball 1A is preferably 3 to 2000 μm.

[0036] The core ball 1A may include a diffusion prevention layer 4 between the core 2A and the solder layer 3A. The diffusion prevention layer 4 is composed of one or more elements selected from Ni and Co, and prevents Cu constituting the core 2A from diffusing into the solder layer 3A.

[0037] Solder alloys containing Bi suppress the occurrence of electromigration. In core ball 1A in which solder layer 3A is formed on the surface of core 2A using a solder alloy containing Bi, the electromigration suppressing effect of Bi is maintained by core 2A.

[0038] Figure 2 FIG. 5 is a diagram showing an example of a solder bump formed by a core ball. The solder bump 5A connects the electrode 60A of the substrate 6A to the electrode 70A of the semiconductor package 7A using a solder alloy 30A. Figure 1 In the solder bump 5A of the core ball 1A shown, even if the weight of the semiconductor package 7A bonded to the substrate 6A by the solder alloy 30A is applied to the solder bump 5A, the core 2A, which does not melt at the melting point of the solder alloy 30A, can support the semiconductor package 7A. Therefore, the solder bump 5A is prevented from being crushed by the weight of the semiconductor package 7A itself.

[0039] The reason for this is that, because Bi has a greater electrical resistance than Sn, when current flows through a solder bump containing Bi, the solder bump temperature rises compared to a solder bump without Bi. This temperature rise becomes more pronounced as the current density increases due to miniaturization of the solder bump. Furthermore, heat generated in the semiconductor package, etc., is transferred to the solder bump, further increasing the solder bump temperature. It is believed that this increase in solder bump temperature facilitates the movement of metal atoms, leading to electromigration.

[0040] In contrast, in the core ball 1A of this embodiment, a solder layer 3A is coated on a Cu core 2A, which has a higher thermal conductivity than Sn. Solder bumps 5A formed from this core ball 1A incorporate the core 2A within the solder alloy 30A that joins the substrate 6A and the semiconductor package 7A. Consequently, heat generated in the solder bump 5A and heat transferred from the semiconductor package 7A are dissipated through the Cu core 2A, suppressing temperature increases in the solder bump 5A and keeping the metal elements in a state of low migration. Consequently, the electromigration suppression effect of the Bi inclusion is maintained.

[0041] Furthermore, Cu has higher conductivity than Sn. In solder bumps formed from solder balls, the current density at the solder bump surface increases. However, in solder bump 5A formed from core ball 1A, the current density at core 2A is higher than the current density at the solder bump 5A surface. Therefore, increases in current density in solder bump 5A are suppressed, and the occurrence of electromigration is suppressed.

[0042] Moreover, the solder bump 5A formed by the core ball 1A of this embodiment, in which the solder layer 3A is formed on the surface of the core 2A using a solder alloy containing Bi, can obtain the required specified strength, both against impact such as falling and against expansion and contraction caused by temperature changes called thermal cycles.

[0043] An example of application of the solder material according to the present invention is described. The solder material is used in a solder paste obtained by kneading solder powder, core balls 1A, and flux. When the core balls 1A are used in a solder paste, the "core balls" may also be referred to as "core powder."

[0044] "Core powder" is an aggregate of many core balls 1A, each possessing the aforementioned properties. For example, when used as a powder in solder paste, it differs from a single core ball in its usage form. Similarly, when used to form solder bumps, it is typically handled as an aggregate, so "core powder" used in this form differs from a single core ball. When used as "core powder," the core balls generally have a diameter of 1 to 300 μm.

[0045] The solder material of the present invention is used for a foam solder in which the core balls 1A are dispersed in solder. For example, a solder alloy having a composition of Sn-3Ag-0.5Cu (each value is in mass %) is used in solder paste and foam solder. It should be noted that the present invention is not limited to this solder alloy. Furthermore, the solder material of the present invention is used for solder joints in electronic components. Furthermore, the solder material of the present invention can also be applied in the form of columns, pillars, or pellets with a columnar Cu core.

[0046] Figure 3 This is a cross-sectional view showing a schematic structure of a Cu core pillar according to this embodiment. While the above example illustrates the use of a spherical core ball 1A as a soldering material, this is not limiting. For example, a cylindrical Cu core pillar 1B may also be used as a soldering material. It should be noted that the structure and materials of the Cu core pillar 1B are the same as those of the aforementioned Cu core ball 1A, and therefore only the differences will be described below.

[0047] The Cu core column 1B according to the present invention comprises a Cu column 2B, which is an example of a core having a predetermined size and ensuring a gap between the semiconductor package and the printed circuit board, and a solder layer 3B, which is an example of a covering layer covering the Cu column 2B. It should be noted that in this example, the Cu column 2B is cylindrical, but this is not limiting; for example, a quadrangular column (rectangular column) may also be employed.

[0048] The Cu column 2B preferably has a wire diameter D2 of 20 to 1000 μm and a length L2 of 20 to 10000 μm.

[0049] The thickness of the solder layer 3B is not particularly limited, but is sufficient to be, for example, 100 μm (on one side) or less, and can generally be 20 to 50 μm.

[0050] The Cu core column 1B preferably has a wire diameter D1 of 22 to 2000 μm and a length L1 of 22 to 20000 μm. Example

[0051] Core balls of Examples, core balls of Comparative Examples, and solder balls were prepared using the compositions shown in Table 1 below, and electromigration tests were performed to measure resistance to electromigration (EM) when a large current was applied. The composition ratios in Table 1 are in mass %.

[0052] In Examples 1 to 13 and Comparative Examples 1 to 7, core balls with a diameter of 300 μm were produced. In Comparative Examples 8 to 11, solder balls with a diameter of 300 μm were produced. In these core balls, a diffusion barrier layer with a thickness of 2 μm on one side of Ni was formed on a Cu core with a diameter of 250 μm, and a solder layer was then formed to a diameter of 300 μm. The solder layer was formed using a known electroplating method.

[0053] As well-known electroplating methods, there are the following methods: electrolytic plating methods such as barrel plating; a method in which a pump connected to a plating tank causes a high-speed vortex to be generated in the plating solution in the plating tank, and a plating film is formed on the spherical core by the vortex of the plating solution; a method in which a vibration plate is provided in the plating tank and vibrates at a specified frequency so that the plating solution is stirred by the high-speed vortex, and a plating film is formed on the spherical core by the vortex of the plating solution, etc.

[0054] In the electromigration test, core balls from each example, as well as core balls and solder balls from the comparative examples shown in Table 1, were used to produce packages by reflow soldering using a water-soluble flux on a 13 mm x 13 mm package substrate with a 0.24 mm diameter Cu electrode. Subsequently, solder paste was printed on a 30 mm x 120 mm, 1.5 mm thick glass epoxy board (FR-4). The resulting package was then mounted on the board and reflowed at a temperature range of 220°C or higher for 40 seconds, with a peak temperature of 245°C, to produce samples.

[0055] A 15 μm thick resist film was formed on a semiconductor package substrate for electromigration testing. An opening with a diameter of 240 μm was formed in the resist film, and core balls or solder balls of Examples or Comparative Examples were bonded in a reflow furnace.

[0056] In this manner, the semiconductor package substrate with the core balls or solder balls bonded thereto was mounted on a printed circuit board. A solder paste with a Sn-3.0Ag-0.5Cu alloy composition was printed on the printed circuit board to a thickness of 100 μm and a diameter of 240 μm. The semiconductor package substrate with the core balls or solder balls from the examples or comparative examples bonded thereto was then connected to the printed circuit board in a reflow oven. The reflow conditions included a peak temperature of 245°C in atmospheric air, preliminary heating at 140-160°C for 70 seconds, and main heating at a temperature above 220°C for 40 seconds.

[0057] In the EM test, the sample prepared above was connected to a compact variable switching power supply (PAK35-10A manufactured by Kikusui Electronics Industry Co., Ltd.) and heated in a silicone oil bath maintained at 150°C at a current density of 12 kA / cm 2Current was passed in a continuous manner. During the current application, the resistance of the sample was continuously measured. The test was terminated when the resistance value increased by 20% from the initial resistance value, and the test time was recorded. For samples with a test time exceeding 800 hours, the results of the EM test were considered to meet the electromigration evaluation (EM evaluation).

[0058] [Table 1] In the Cu core balls of Examples 1 to 10 having a solder layer composed of a Sn-Ag-Cu-Bi solder alloy with a Bi content of 0.5 mass % to 5.0 mass %, and the Cu core balls of Examples 11 to 13 having a solder layer composed of a Sn-Cu-Bi solder alloy with a Bi content of 0.5 mass % to 5.0 mass %, the test time for EM evaluation exceeded 800 hours.

[0059] In the Cu core ball of Example 2, which has a Bi content of 1.5 mass%, the EM evaluation test time exceeded 1300 hours. Although the EM evaluation test time tends to decrease with a Bi content of 5.0 mass% or more, the EM evaluation test time still exceeded 800 hours in the Cu core ball of Example 6, which has a Bi content of 5.0 mass%.

[0060] In contrast, the EM evaluation test time for the Cu core balls of Comparative Examples 1 and 2 having a solder layer composed of a Sn-Ag-Cu solder alloy containing no Bi, and the Cu core ball of Comparative Example 3 having a solder layer composed of a Sn-Cu solder alloy containing no Bi, was less than 800 hours.

[0061] Furthermore, even for Cu core balls having a solder layer composed of a Sn-Ag-Cu-Bi solder alloy, the test time for EM evaluation was less than 800 hours in Comparative Example 4, in which the Bi content was 0.2 mass%, and Comparative Example 5, in which the Bi content was 10.0 mass%. Thus, even for Cu core balls having a solder layer composed of a Sn-Ag-Cu-Bi solder alloy, if the Bi content was less than 0.5 mass% or greater than 5.0 mass%, the test time for EM evaluation was less than 800 hours, and the desired resistance to EM could not be achieved.

[0062] Furthermore, even for Cu core balls having a solder layer composed of a Sn-Cu-Bi solder alloy, the test time for EM evaluation was less than 800 hours in Comparative Example 6, in which the Bi content was 0.2 mass%, and Comparative Example 7, in which the Bi content was 10.0 mass%. Thus, even for Cu core balls having a solder layer composed of a Sn-Cu-Bi solder alloy, if the Bi content was less than 0.5 mass% or greater than 5.0 mass%, the test time for EM evaluation was less than 800 hours, and the desired resistance to EM could not be achieved.

[0063] In the solder ball of Comparative Example 8 composed of a Sn-Ag-Cu-Bi-Ni solder alloy having a Bi content of 3.0 mass% and a Ni content of 0.02 mass%, even though the solder alloy composition is the same as that of Example 5, the test time for EM evaluation is significantly less than 800 hours.

[0064] In the solder ball of Comparative Example 9 composed of a Sn-Ag-Cu-Bi solder alloy having a Bi content of 0.5 mass %, even though the solder alloy composition was the same as that of Example 1, the test time for EM evaluation was significantly less than 800 hours.

[0065] The solder balls of Comparative Example 10 and Comparative Example 11, which are made of the Sn—Ag—Cu solder alloy containing no Bi, also had EM evaluation test times significantly shorter than 800 hours.

[0066] As can be seen from the above, in a solder material in which the solder layer covering the metal core is composed of a Sn-Ag-Cu-Bi solder alloy or a Sn-Cu-Bi solder alloy, an electromigration suppressing effect can be achieved by controlling the Bi content to be between 0.5% and 5.0% by mass. Furthermore, it is understood that the preferred Bi content is between 1.5% and 3.0% by mass.

[0067] It should be noted that, it is known that by making the Cu content be more than 0.1 mass % and less than 3.0 mass %, the inhibitory effect of electromigration will not be suppressed. In addition, it is known that by making the Ag content be more than 0 mass % and less than 4.5 mass %, the inhibitory effect of electromigration can be obtained compared to the solder alloy that does not contain Ag. In Example 3 where the Ag content is 4.5 mass %, the test time of EM evaluation exceeds 1300 hours. Moreover, it is known that the Ni content is more than 0 mass % and less than 0.1 mass %, and the inhibitory effect of electromigration can also be obtained. In Example 4 where the Ni content is 0.1 mass %, the test time of EM evaluation exceeds 1400 hours.

[0068] Explanation of symbols 1A...core ball, 2A...core, 3A...solder layer, 30A...solder alloy, 4...diffusion prevention layer, 5A...solder bump, 6A...substrate, 60A...electrode, 7A...semiconductor package, 70A...electrode.

Claims

1. Welding material, characterized in that A metal core and a solder layer covering the core, wherein the solder layer is composed only of 0.1 mass % to 3.0 mass % Cu, 0.5 mass % to 3.0 mass % Bi, 3 mass % to 4.5 mass % Ag, 0 mass % to 0.1 mass % Ni, and the remainder Sn.

2. The welding material according to claim 1, characterized in that The core is composed of a single metal or an alloy of Cu, Ni, Ag, Au, Al, Mo, Mg, Zn, and Co.

3. The welding material according to claim 1 or 2, characterized in that The nucleus is a spherical nucleus.

4. The welding material according to claim 3, characterized in that The nucleus is a columnar nucleus.

5. The welding material according to any one of claims 1 to 4, characterized in that The core covered with the layer composed of one or more elements selected from Ni and Co is covered with the solder layer.

6. Solder paste, characterized in that The welding material according to any one of claims 1 to 5 is used.

7. Foam solder, characterized in that The welding material according to any one of claims 1 to 5 is used.

8. Solder joint, characterized in that The welding material according to any one of claims 1 to 5 is used.

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