阅读说明：本技术 一种耐电子迁移焊料、焊点、封装结构及制备方法、应用 (Electron migration resistant solder, welding spot, packaging structure, preparation method and application ) 是由 华菲 于 2021-09-23 设计创作，主要内容包括：本申请实施例提供一种耐电子迁移焊料、焊点、封装结构及制备方法、应用,涉及无铅焊料领域。耐电子迁移焊料包括锡基无铅焊料和掺杂物,掺杂物包含至少选自掺杂元素Al、Cr、Ge、Si和它们的氧化物、它们的氮化物组成的组中的至少一种,掺杂物的掺杂量至少为0.02wt％,且小于2wt％。耐电子迁移焊点采用上述的耐电子迁移焊料制成,其至少有10％的低角度晶界。本申请实施例在锡基无铅焊料里掺杂一些特定的合金元素以增强其抗电子迁移能力,并通过改进焊点微观结构实现理想的晶界配置,适用于各种焊料封装,包括倒装式粘结焊点互连。(The embodiment of the application provides an electron migration resistant solder, a welding spot, a packaging structure, a preparation method and an application, and relates to the field of lead-free solder. The electromigration-resistant solder comprises a tin-based lead-free solder and a dopant, the dopant comprises at least one selected from the group consisting of doping elements Al, Cr, Ge, Si, and oxides and nitrides thereof, and the doping amount of the dopant is at least 0.02 wt% and less than 2 wt%. The electromigration resistant solder joints are made of the electromigration resistant solder, and have at least 10% of low angle grain boundaries. According to the tin-based lead-free solder, the specific alloy elements are doped in the tin-based lead-free solder so as to enhance the anti-electron migration capability of the tin-based lead-free solder, and ideal grain boundary configuration is realized by improving the microstructure of the solder joint, so that the tin-based lead-free solder is suitable for various solder packages, including flip-chip bonding solder joint interconnection.)

1. An electron migration resistant solder, characterized by comprising a tin-based lead-free solder and a dopant containing at least one selected from the group consisting of doping elements Al, Cr, Ge, Si and oxides thereof, nitrides thereof, the doping amount of the dopant being at least 0.02 wt% and less than 2 wt%.

2. The electron mobility resistant solder according to claim 1, wherein the alloy of the tin-based lead-free solder is selected from one of Sn-Ag, Sn-Cu, Sn-Sb, Sn-In, Bi-Sn, Sn-Ag-Cu, and Sn-Ag-In;

and/or the alloy of the tin-based lead-free solder comprises 0.5-5 wt% Ag, and/or 0.2-2 wt% Cu, and/or 0.5-5 wt% Sb, and/or 0.5-5 wt% In, and at least 30% Sn;

optionally, the alloy of tin-based lead-free solder comprises Bi-Sn or Sn-In an amount of at least 30% by weight Sn.

3. An electromigration resistant solder joint formed using an electromigration resistant solder as claimed in any of claims 1 to 2 having at least 10% low angle grain boundaries.

4. The electromigration resistant weld of claim 3 wherein at least 20% of the bulk grain boundary area of the electromigration resistant weld is decorated with a dopant element and/or an oxide thereof and/or a nitride thereof.

5. The electron mobility resistant solder joint of claim 3, wherein at least one doping element and/or its oxide and/or its nitride is isolated within the grain boundaries;

optionally, the doping element is converted to an oxide or nitride and isolated within the grain boundaries.

6. The electron mobility resistant solder joint of claim 3, wherein the low angle grain boundaries exist in a horizontal direction and are directionally solidified by a liquid or gaseous coolant entering the solder joint area.

7. The electron mobility resistant solder joint of claim 3, wherein the low angle grain boundaries exist in a vertical direction and are formed by a vertical direction temperature gradient and vertical solidification;

alternatively, the low angle grain boundaries exist in a vertical direction and are formed by vertical strain of the solder joints and vertical rearrangement of atoms at warm temperatures.

8. The electromigration resistant solder joint of claim 3 wherein the electromigration resistant solder joint has a lattice size of less than 500 nanometers.

9. The electromigration resistant solder joint of claim 8 wherein the lattice size of the electromigration resistant solder joint is obtained by one of:

by introducing a plastic deformation and a recrystallization,

through the phase change of the heat cycle,

through the doping of the nano-particles,

by introducing artificial defects or dispersoid particles.

10. A method of making an electromigration resistant solder joint as set forth in claim 3 wherein the electromigration resistant solder is positioned on the contact pads so as to be electrically connected to another mating device, and the electromigration resistant solder is melted by increasing the temperature and then cooled to solidify and controlled to produce at least 10% low angle grain boundaries.

11. Use of an electromigration resistant solder as claimed in any of claims 1 to 2 in flip chip packaging and 3D, 2.5D advanced packaging.

12. A package structure comprising an electron mobility resistant solder joint according to claim 3.

Technical Field

The application relates to the field of lead-free solders, in particular to an electron migration resistant solder, a welding spot, a packaging structure, a preparation method and an application.

Background

Modern electronic equipment requires packaging and assembly of devices including computer Central Processing Units (CPUs), with the current trend toward ever smaller packages, and shrinking size has increased the current density of electronic chips and devices to higher levels, from 10⁵A/cm²To 10⁶A/cm². When the current density of the device is increased, the resistive heating generated by the current is also increased, so that the device and the interconnection/packaging structure are influenced, the phenomenon of electronic migration of the welding spot is more obvious, and finally the welding spot is failed due to the electronic migration and the thermal migration.

The electromigration phenomenon of a solder joint represents the movement of atoms when current flows through a conductor (such as an interconnection line of the solder joint), when the current causes the atoms to migrate to the anode side of the solder joint, lattice voids can appear on the cathode side of the solder joint, the lattice voids are accumulated continuously to form voids, and the void defects increase continuously to finally cause the solder joint to fail. Lead-free solders in particular are more susceptible to electromigration because they generally have a higher melting point than conventional eutectic Pb-Sn solders.

Various improvements have been sought in solders, including lead-free solders, to enhance the electromigration resistance of solder joints, but no means has yet been found to be very effective in mitigating the electromigration problem.

Disclosure of Invention

An object of the embodiments of the present application is to provide an electron migration resistant solder, a solder joint, a package structure, a method for manufacturing the same, and an application thereof, in which a certain specific alloy element is doped in a tin-based lead-free solder to enhance the electron migration resistant capability thereof, and an ideal grain boundary configuration is realized by improving a microstructure of the solder joint, so that the solder joint is suitable for various solder packages, including flip-chip bonding solder joint interconnection.

In a first aspect, embodiments of the present application provide an electron migration resistant solder, including a tin-based lead-free solder and a dopant, the dopant including at least one selected from the group consisting of doping elements Al, Cr, Ge, Si, and oxides and nitrides thereof, and a doping amount of the dopant being at least 0.02 wt% and less than 2 wt%.

In the technical scheme, the tin-based lead-free solder is doped with one or more dopants consisting of doping elements, wherein the dopants are specifically pure doping elements such as Al, Cr, Ge and Si or corresponding oxides, nitrides or combinations thereof, and after the electron migration resistant solder is melted, the dopants are preferentially separated to the surface or the grain boundary surface so as to slow down the kinetic force of atomic interface diffusion, thereby slowing down the kinetic force of electron migration and enhancing the electron migration resistance. The doping amount of the dopant in the present application is at least 0.02 wt% and less than 2 wt%, which significantly enhances the electron transfer resistance.

In one possible implementation, the alloy of the tin-based lead-free solder is selected from one of Sn-Ag, Sn-Cu, Sn-Sb, Sn-In, Bi-Sn, Sn-Ag-Cu, and Sn-Ag-In;

and/or the alloy of tin-based lead-free solder comprises 0.5-5 wt% Ag, and/or 0.2-2 wt% Cu, and/or 0.5-5 wt% Sb, and/or 0.5-5 wt% In, and at least 30 wt% Sn;

optionally, the alloy of tin-based lead-free solder comprises Bi-Sn or Sn-In an amount of at least 30% by weight Sn.

In the technical scheme, the alloy composition of the tin-based lead-free solder covers almost all conventional solders, and the solder formed by the tin-based lead-free solder and the dopant has a proper melting point and can also enhance the electron migration resistance.

In a second aspect, embodiments of the present application provide an electromigration-resistant solder joint made using the electromigration-resistant solder provided in the first aspect, which has at least 10% low-angle grain boundaries.

In the technical scheme, the inventor finds that in the research process, besides the addition of the dopant, the grain boundary configuration also plays an important role in the electron migration kinetic, and the mismatch rate along the crystal direction is higher due to the higher diffusion speed of the grain boundary, and vice versa. Since the grain boundary configuration plays an important role in the electron mobility, the high-angle grain boundaries (the higher the degree of lattice mismatch between adjacent grains) make the grain boundaries diffuse faster, so it is necessary to provide as many low-angle grain boundaries as possible. Creating low angle grain boundaries can therefore reduce grain boundary diffusion of the solder alloy and mitigate electromigration-induced failure. The electron migration resistant welding spot has at least 10% of low-angle grain boundaries, the lattice mismatch of the grain boundaries is less than 30 degrees, and compared with the similar welding spots without the low-angle grain boundaries, the electron mobility is reduced by at least 50%, but the atom rearrangement of the low-angle grain boundaries is not formed.

In one possible implementation, at least 20% of the entire grain boundary region of the electron-migration resistant solder joint is decorated with doping elements and/or oxides thereof and/or nitrides thereof.

In the above technical solution, the inventors found in the research that: in many solder alloy interconnects and packages, it is necessary to slow the kinetic diffusion kinetics of atoms as the grain size becomes smaller due to orders of magnitude faster diffusion of atoms along the material surface or through the crystal interface, electron transport at room or elevated temperatures tends to accelerate, which makes the electron transport induced atom transport faster during early device failure. One way to achieve such surface/interface diffusion kinetic changes is to coat or decorate the surface of the weld or the interfaces within the weld (e.g., grain boundaries) with atomic species to prevent the atoms from moving along the grain boundaries. Complete coverage of the grain boundaries by atoms is beneficial to maximize electromigration resistance, however from a mechanical performance point of view, such full grain boundary doping may degrade the mechanical flexibility and superplasticity of the solder joint, which is intended to relieve unwanted stresses in the solder joint. Taken together, the grain boundary doping of the doping element and/or its oxide and/or its nitride should cover at least 20% of the entire grain boundary area.

In one possible implementation, at least one doping element and/or its oxide and/or its nitride is isolated within the grain boundaries;

optionally, the doping element is converted to an oxide or nitride and isolated within the grain boundaries.

In the above solution, since the solubility of the dopant in the tin-based solder is very low, the dopant is dissolved in the molten tin-based solder, and when the solder is cooled and solidified, these elements are separated to the interface (e.g., grain boundary or other defect boundary in the solder) or the surface (e.g., outer surface of the solidified solder joint). When dopants such as Al, Cr, Ge, Si, or combinations thereof segregate to grain boundaries, interfacial diffusion kinetics are slowed, thereby increasing electromigration resistance. Whereas the doping elements are more easily oxidized (or nitrified if the solder melting and solidification process is carried out in a controlled nitrogen or ammonia-type atmosphere) when exposed to the external atmosphere, the melting temperature of such oxidized or nitrided layers is much higher and therefore the diffusion rate of atoms is much slower, and such a reduction in interface/surface diffusion rate greatly retards the electromigration kinetics. It is because the selected dopant elements have a high tendency to oxidize (or nitride), a small portion of the dopant alloy may be internally oxidized (or nitrided), thereby producing an oxide or nitride at the grain boundaries, which also improves electromigration resistance.

In one possible implementation, the low angle grain boundaries exist in a horizontal direction and are formed by the directional solidification of a liquid or gaseous coolant entering the weld zone.

In the technical scheme, the solder is melted and transversely cooled through the fluid channel to induce directional solidification from the outer edge, a low-angle grain boundary is created, grain boundary diffusion of solder alloy components (such as Sn, Sb, Cu, Ag, In and Bi) can be reduced, and failure caused by electron migration is relieved.

In one possible implementation, the low angle grain boundaries exist in a vertical direction and are formed by a vertical direction temperature gradient and vertical solidification;

alternatively, low angle grain boundaries exist in the vertical direction and are formed by the vertical strain of the solder joint and the vertical rearrangement of atoms at warm temperatures.

In the technical scheme, the directional solidification is caused by the temperature gradient In the vertical direction In the cooling and solidification process of the molten welding point, a low-angle crystal boundary can be generated, the crystal boundary diffusion of the components (such as Sn, Sb, Cu, Ag, In and Bi) of the solder alloy is reduced, and the failure caused by electron migration is slowed down. Or low-angle grain boundaries are formed through forced recrystallization and texture formation, and the electron migration of the welding spots is reduced by introducing mechanical strain in the vertical direction at a proper temperature.

In one possible implementation, the electron mobility resistant solder joint has a lattice size of less than 500 nanometers.

In the above technical solution, the inventor found in the research process that the anisotropy of the diffusivity, and the accelerated electron migration in a certain crystal direction in the textured solder joint can cause faster electron migration failure, and if the solder joint has an ultra-fine or nano-lattice structure, the failure can be alleviated. Grain boundary diffusion contributes to electromigration and thus smaller grains are not always desirable, however, when localized electromagnetic failures (e.g., a multiplicity of voids) occur, the nano-grains provide more alternative current paths, and thus some solder joints with smaller grains or nano-grains may also improve electromigration resistance. When the material in the solder joint has a preferred grain orientation, certain crystallographic orientations (e.g., tin along the c-axis may cause copper and nickel to diffuse in the interstitial at a rate that is about 2 orders of magnitude faster than along the a-or b-axis) exhibit faster atomic diffusion (faster electromigration failure) than other grain orientations, and a lattice size of less than 500nm may mitigate this risk when the preferred structure is potentially dangerous and accelerated atomic diffusion along certain crystallographic directions may lead to early electromigration failure.

In one possible implementation, the lattice size of the electron-migration resistant solder joint is obtained by one of the following methods:

by introducing a plastic deformation and a recrystallization,

through the phase change of the heat cycle,

through the doping of the nano-particles,

by introducing artificial defects or dispersoid particles.

In the above technical solution, in order to make the electron migration resistant solder joint have a nanocrystalline microstructure, a unique processing method can be adopted, for example, by introducing plastic deformation and recrystallization, by thermal cycle phase transition, by nanoparticle incorporation, or by introducing artificial defects or dispersed particles.

In a third aspect, embodiments of the present application provide a method for preparing an electromigration-resistant solder joint according to the second aspect, wherein an electromigration-resistant solder is positioned on a contact pad to be electrically connected to another mating device, and the temperature is increased to melt the electromigration-resistant solder, and then the solder is cooled to solidify and control at least 10% of low-angle grain boundaries.

In the technical scheme, the electron migration resistant welding spot can be manufactured according to the preparation method, and packaging is realized.

In a fourth aspect, the embodiments of the present application provide an application of the solder with electron mobility resistance provided in the first aspect, which is used in flip chip packaging and advanced packaging of 3D and 2.5D.

In the above solution, electromigration-resistant solder is used for various interconnections and packages of flip chips or other related devices.

In a fifth aspect, embodiments of the present application provide a package structure, which includes the electromigration-resistant solder joint provided in the second aspect.

In the above technical solution, the electromigration-resistant solder joints not only realize the packaging, but also constitute an important part of the packaging structure.

Drawings

In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings that are required to be used in the embodiments of the present application will be briefly described below, it should be understood that the following drawings only illustrate some embodiments of the present application and therefore should not be considered as limiting the scope, and that those skilled in the art can also obtain other related drawings based on the drawings without inventive efforts.

FIG. 1 is a schematic diagram of solder (dopant Al, Cr, Ge or Si) in a molten state versus a solidified state;

FIG. 2 is a schematic diagram of different segregation configurations of the weld spot (dopant is Al, Cr, Ge or Si) grain boundary;

FIG. 3 is a schematic diagram of different segregation configurations of the grain boundaries of the solder joint (the dopant is oxide or nitride);

FIG. 4 is a first illustration of a method for creating low-angle grain boundaries;

FIG. 5 is a schematic diagram of a second way of creating low-angle grain boundaries;

FIG. 6 is a schematic diagram of a third way of creating low-angle grain boundaries.

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions of the embodiments of the present application will be clearly and completely described below. The examples, in which specific conditions are not specified, were conducted under conventional conditions or conditions recommended by the manufacturer. The reagents or instruments used are not indicated by the manufacturer, and are all conventional products available commercially.

The following describes the electromigration-resistant solder, solder joint, package structure, manufacturing method and application of the embodiments of the present application.

The alloy components of the conventional tin-based lead-free solder are improved to obtain the electron migration resistant solder, which comprises the tin-based lead-free solder and a dopant, wherein the dopant comprises at least one element selected from doping elements Al, Cr, Ge, Si and oxides Al thereof₂O₃、Cr₂O₃、GeO₂、SiO₂Their nitrides AlN, CrN, Ge₃N₄、Si₃N₄At least one of the group consisting of, for example, the dopant is one, two or more of pure doping elements Al, Cr, Ge and Si; or the dopant is an oxide Al of a doping element₂O₃、Cr₂O₃、GeO₂、SiO₂One, two or more; or nitride AlN, CrN, Ge with doping element as dopant₃N₄、Si₃N₄One, two or more; or the dopant is one, two or three of doping elements and oxide or nitride of the same doping element; or the dopant is one or two of the doping elements and oxide or nitride of different doping elements; or the dopant is a pure doping element, an oxide of a doping element, or a nitride of a doping element (the doping elements may be the same or different). In the present application, the doping amount of the dopant is at least 0.02 wt%, and less than 2 wt%, optionally less than 0.5 wt%, further optionally less than 0.2 wt%, and even optionally less than 0.1 wt%. The doping amount in the present application refers to the amount of the dopant relative to the tin-based lead-free solder (base solder), i.e., the weight of the dopant is the weight percentage of the tin-based lead-free solder.

In the present application, the alloy of the tin-based lead-free solder can be a binary or ternary composition of Sn and Sb, Cu, Ag, In, Bi, for example, the composition of the alloy is specifically selected from one of Sn-Ag, Sn-Cu, Sn-Sb, Sn-In, Bi-Sn, Sn-Ag-Cu and Sn-Ag-In. And/or the tin-based lead-free solder may be a multi-component tin-based solder alloy consisting of 0.5-5 wt% Ag, and/or 0.2-2 wt% Cu, and/or 0.5-5 wt% Sb, and/or 0.5-5 wt% In, and at least 30 wt% Sn; at least 30% by weight of Sn In the tin-based lead-free solder may also be provided by Bi-Sn or Sn-In. In addition to the above elements, the alloys of the tin-based lead-free solders may also be binary, ternary, quaternary or multicomponent compositions containing one or more other elements. It should be noted that "and/or" in the present application, such as "scheme a and/or scheme B" means that the three modes of scheme a alone, scheme B alone, scheme a plus scheme B may be used.

In addition, the microstructure of the conventional tin-based lead-free solder joint is improved, so that the electron migration resistant solder joint is obtained, and the electron migration resistant solder joint is made of the electron migration resistant solder, and has at least 10% of low-angle crystal boundary. The shape of the solder joint is not particularly limited in the present application, and may be generally a filament, a strip, a sheet, a sphere, or a column. The fabrication process may be performed by positioning the electromigration resistant solder on the contact pad (bonding location) after the electromigration resistant solder is prepared (e.g., by adding flux to the solder powder or depositing patterned islands of a solder layer) and electrically connecting it to another mating device, such as a flip chip device to a base circuit substrate, and then increasing the temperature to melt the electromigration resistant solder and cooling it to solidify and control it to have low angle grain boundaries of at least 10%, and optionally at least 30%. A low angle grain boundary is a special grain boundary that is created when a slight misorientation occurs between two crystals. The 10% low angle grain boundary in the present application means that the region where the low angle grain boundary is generated occupies 10% of the entire grain boundary region.

The size of the crystal lattice of the electron migration resistant welding spot is a nanometer size, and the size of the crystal lattice is generally less than 500 nanometers, optionally less than 100 nanometers, further optionally less than 50 nanometers, and even optionally less than 20 nanometers. Compared with the welding spot with large lattice size, the anti-fatigue life of the welding spot is improved by at least 2 times, and the anti-electron migration capability is improved by at least 2 times. The lattice size of the electron migration resistant solder joint is obtained by one of the following methods: a. by introducing plastic deformation and recrystallization, b, by thermal cycling phase transitions, c by nanoparticle incorporation, d, by introducing artificial defects or dispersoid particles.

In one embodiment, the solder of the present application is prepared by positioning the solder on a substrate (e.g., a circuit substrate), electrically connecting the substrate to a device to be packaged (e.g., a flip chip), increasing the temperature to melt the electromigration-resistant solder, and cooling the solder to solidify the solder to form a ball-shaped solder joint. It should be noted that in all the schematic diagrams of the present application, the upper board represents a device to be packaged, the lower board represents a substrate, and the middle circle represents a solder joint.

Fig. 1 is a schematic diagram showing a comparison between a molten state and a solidified state of a solder (a dopant is Al, Cr, Ge, or Si), in which fig. 1, an upper diagram is a schematic diagram in the molten state, and a lower diagram is a schematic diagram in the solidified state. As can be seen from fig. 1, after the weld spot is cooled from the molten state, the dopant element atoms of the dopant solidify as they segregate to the weld spot surface and the grain boundary surface (interface), and have different possible segregation configurations.

FIG. 2 shows different segregation configurations of the grain boundaries of the solder joint (the dopant is Al, Cr, Ge or Si), wherein (a) shows a continuous arrangement, (b) shows a semi-continuous shape, and (c) shows discrete islands in FIG. 2. (a) Complete coverage of the grain boundaries as shown is beneficial to maximize electromigration resistance. However, from a mechanical performance point of view, such a doping of the whole crystal boundary may deteriorate the mechanical flexibility and superplasticity of the solder joint, which sometimes serves to relieve unwanted stresses in the solder joint. (b) The semi-continuous grain boundary segregation doping shown still slows down the grain boundary diffusion kinetics of the anti-electromigration. The dopant segregation may have a dot structure as shown in (c).

FIG. 3 shows a solder joint (the dopant is Al oxide)₂O₃，Cr₂O₃，GeO₂Or SiO₂Or nitride AlN, CrN, Ge₃N₄Or Si₃N₄) The segregation configuration of a very thin oxide or nitride layer (Al, Cr, Ge, or Si) isolated at grain boundaries is represented by (a) a continuous arrangement, (b) a semi-continuous shape, and (c) discrete islands in fig. 3.

The present application selects Al, Cr, Ge, Si or mixtures thereof as doping elements because these elements also have a low solubility in the tin matrix and do not interact with tin to form intermetallic compounds according to their binary phase diagram with tin-based solders (Sn-Al binary phase diagram, Sn-Cr binary phase diagram, Sn-Ge binary phase diagram, Sn-Si binary phase diagram). These doping elements are thus dissolved in the molten tin-based solder, but when the solder joint is cooled to solidify, these elements can segregate to interfaces (e.g., grain boundaries or other defect boundaries in the solder) or surfaces (e.g., the outer surfaces of the solidified solder joint), which behavior is shown in fig. 1 and 2.

In addition, by selecting the grain boundary segregation configuration of the doping element or the oxide and nitride according to fig. 2 and 3, at least 20%, optionally at least 40%, and further optionally at least 70% of the entire grain boundary region of the electron mobility resistant welding spot of the present application is decorated with the doping element and/or the oxide and/or the nitride thereof, so as to achieve the ideal grain boundary configuration.

When the doping elements Al, Cr, Ge, Si or a combination thereof segregate to grain boundaries, the interfacial diffusion kinetics are slowed down, thereby increasing electromigration resistance, these selected doping elements also have a high tendency to oxidize (or nitridize), so that a small portion of the dopant may internally oxidize (or nitridize), thereby creating oxides or nitrides at the grain boundaries, which also increase electromigration resistance, as shown in fig. 3, at least one of the doping elements and/or oxides and/or nitrides thereof being segregated within the grain boundaries. As shown in fig. 1 to 3, the dopant element is converted into fine oxide or nitride and isolated in the grain boundary, and the melting temperature of the oxide or nitride layer or the mixture thereof is much higher, so that the diffusion speed of atoms is much slower, and the electron mobility is greatly delayed.

Since the grain boundary configuration also plays an important role in the electron mobility, the solidification process can be used to obtain low-angle grain boundaries, in the following specific manner:

FIG. 4 is a schematic view of one way to create low angle grain boundaries, FIG. 4, in which a molten solder joint is cooled laterally by fluid channels to induce directional solidification from the outer edges to reduce grain boundary diffusion of the solder alloy and mitigate electromigration induced failures. In some embodiments of the present application, therefore, low angle grain boundaries are present mostly in the horizontal direction, and are formed by directional solidification of a liquid or gaseous coolant entering the weld zone, specifically by lateral cooling of the molten weld spot through a fluid channel, inducing directional solidification from the outer edge.

Fig. 5 is a schematic diagram of a second method for creating low-angle grain boundaries, in which in fig. 5, a vertical temperature gradient is set when the molten solder joint is cooled and solidified, so that low-angle grain boundaries are generated, grain boundary diffusion of solder alloy components is reduced, and failure caused by electron migration is slowed down. Thus in some other embodiments of the present application, the low angle grain boundaries are mostly present in the vertical direction, and are formed by a vertical direction temperature gradient and vertical solidification, and the vertical direction temperature gradient causes the low angle grain boundaries to be generated when the molten weld spot is cooled and solidified.

Fig. 6 is a schematic diagram of a third method for creating low-angle grain boundaries, and in fig. 6, forced recrystallization and texture formation can create low-angle grain boundaries and reduce electromigration of a solder joint by introducing mechanical strain in a vertical direction at a proper temperature. Thus in some other embodiments of the present application, the low angle grain boundaries exist primarily in the vertical direction, formed by the vertical strain of the solder joint and the vertical rearrangement of atoms at warm temperatures, specifically by forced recrystallization and texturing to create low angle grain boundaries, and by introducing mechanical strain in the vertical direction at the appropriate temperature, the electromigration of the solder joint is reduced. Forced recrystallization and texturing is accomplished by applying one or more cycles of push-pull deformation with a vertical stress (e.g., by using a vacuum chuck in the upper tool array) to force the atoms to realign in the vertical stress direction, preferably at a strain greater than 0.7T_mAt a temperature of-230 c (e.g., Sn-Ag or Sn-Cu solder, which is carried out at 150 c), the mechanical properties of the solder joint alloy are softer and easily strained without causing failure during elastoplastic deformation.

Based on the superior performance of the electromigration-resistant solder, the embodiment of the application also provides an application of the electromigration-resistant solder, which is generally used for flip chip packaging, but does not exclude the packaging application of other elements, such as 3D and 2.5D advanced packaging. Therefore, the electromigration resistant solder is used for various interconnections and packaging of flip chips or other related devices, and after the packaging is completed, the electromigration resistant solder can become an electromigration resistant welding spot and become a part of a packaging structure.

The embodiment of the application provides a packaging structure, which comprises an electron migration resistant welding spot formed by the electron migration resistant welding flux, wherein the packaging structure specifically comprises a flip chip and the electron migration resistant welding spot, and the packaging structure is suitable for products such as mobile phones, computers, personal wearable equipment, entertainment audio-visual equipment/systems, information storage and processing equipment, avionics, automotive electronics, medical and health electronics and the like.

The features and properties of the present application are described in further detail below with reference to examples.

Example 1

The present embodiment provides a package structure, which is formed by packaging a flip chip with an electron migration resistant solder, wherein the electron migration resistant solder is a mixture of Sn-Ag type solder alloy and dopants Al and Cr, and wherein the dopants account for 0.1 wt% of the Sn-Ag type solder alloy. The packaging method comprises the following steps:

the method comprises the steps of positioning the electron migration resistant solder on a circuit substrate to enable the electron migration resistant solder to be electrically connected with a flip chip, raising the temperature to enable the electron migration resistant solder to be molten, cooling and solidifying, wherein the cooling mode is that the molten solder joint is cooled transversely through a fluid channel, directional solidification is induced from the outer edge, and a spherical solder joint with 20% of low-angle crystal boundary is formed.

Example 2

This embodiment provides a package structure, which is formed by packaging a flip chip with an electromigration-resistant solder, wherein the electromigration-resistant solder is Sn-Ag type solder alloy and GeO dopant₂Wherein the dopant comprises 0.1 wt% of the Sn-Ag type solder alloy. The packaging method comprises the following steps:

the solder resistant to electron migration is positioned on the circuit substrate to be electrically connected with the flip chip, the temperature is raised to melt the solder resistant to electron migration, and then the solder resistant to electron migration is cooled and solidified, and a temperature gradient in the vertical direction is set when the molten solder is cooled and solidified, so that a spherical solder joint with 20% of low-angle crystal boundary is formed.

Example 3

The embodiment is providedA package structure is provided, which is formed by packaging a flip chip with an electron migration resistant solder, wherein the electron migration resistant solder is Sn-Ag type solder alloy and adulterants Si and Al₂O₃、Cr₂O₃Wherein the dopant comprises 0.1 wt% of the Sn-Ag type solder alloy. The packaging method comprises the following steps:

the method comprises the steps of positioning an electron migration resistant solder on a circuit substrate to be electrically connected with a flip chip, raising the temperature to melt the electron migration resistant solder, cooling and solidifying, forcibly recrystallizing and forming a texture structure in the cooling process, and forming a spherical welding point with 20% of low-angle crystal boundary by introducing mechanical strain in the vertical direction at a proper temperature.

Comparative example 1

This comparative example provides a package structure that is substantially the same as example 1 except that: and adding no dopant to obtain the final package structure.

Comparative example 2

This comparative example provides a package structure that is substantially the same as example 1 except that: and forming a spherical welding spot with 5% of low-angle crystal boundary by adopting a conventional cooling mode, and finally obtaining the packaging structure.

The following was conducted to evaluate the electron mobility resistance of solder bumps in the package structures of the examples and comparative examples.

The electron migration resistance of the welding spot is evaluated through an electron migration test, and the evaluation method comprises the following steps: and applying a certain current to the packaging structure for a certain time, and observing the resistance change and the formation condition and size of the welding spot section hole.

Because the poorer the electron migration resistance of the welding spot, the more easily the hole appears at the welding spot, the larger the hole size along with the time, and the larger the resistance is. Therefore, the resistance can be observed firstly in the experimental process, if the resistance is found to be obviously increased, the welding spot is sliced, and the formation condition and the size of the hole in the section of the welding spot are observed.

The evaluation results of the electron mobility resistance of the solder joints in the package structures of examples 1 to 3 and comparative examples 1 to 2 were as follows: during the experiment, the resistance of examples 1-3 did not change significantly; the resistance of comparative examples 1-2 became significantly large and voids were found in the solder joint sections.

In summary, the electron migration resistant solder, the solder joint, the packaging structure, the preparation method and the application of the solder joint in the embodiment of the present application are characterized in that some specific alloy elements are doped in the tin-based lead-free solder to enhance the electron migration resistant capability of the tin-based lead-free solder, and ideal crystal boundary configuration is realized by improving the microstructure of the solder joint, so that the tin-based lead-free solder is suitable for various solder packaging, including flip-chip bonding solder joint interconnection.

The above description is only an example of the present application and is not intended to limit the scope of the present application, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, improvement and the like made within the spirit and principle of the present application shall be included in the protection scope of the present application.