A Design Method for the Thickness of Ni Barrier Layer in Solder Joints

Abstract

The present invention discloses a design method for the thickness of the Ni barrier layer in solder joints, including: welding the solder joint structure plated with the Ni barrier layer according to the actual assembly process conditions; performing morphological analysis on the microstructure of the solder joint structure after welding to determine the thickness consumed by the welding of the Ni barrier layer; numerically analyzing the Ni atomic flux in the Ni barrier layer at the cathode under temperature and current stresses to establish a service consumption thickness model of the Ni barrier layer at the cathode; based on the constructed service consumption thickness model of the Ni barrier layer, calculating the service consumption thickness of the Ni barrier layer in the solder joint under the actual working temperature, current stress and service time; obtaining the design thickness of the Ni barrier layer according to the remaining thickness of the Ni barrier layer, the welding consumption thickness and the design redundancy, as well as the service consumption thickness and the design redundancy. The present invention can provide a solution for the thickness design of the Ni barrier layer, effectively improve the packaging design efficiency of integrated circuits and reduce the development cost.

Description

Technical Field

[0001] The present invention belongs to the field of integrated circuit packaging design, and particularly relates to a method for designing the thickness of a Ni barrier layer in solder joints. Background Art

[0002] Solder joints are key structures for realizing electrical and mechanical interconnections in integrated circuit packaging. With the continuous increase in the integration density of integrated circuits, higher requirements are put forward for the size and density of their packaging, which is specifically manifested as the continuous reduction of the size of packaging solder joints. When tiny-sized solder joints are working, the high temperature and current stress they bear can easily cause alloying of the solder joints, and then generate an overly thick interfacial metal compound layer. For example, Cu atoms in the solder pad will react with Sn atoms in the solder to form layers such as Cu6Sn5 and Cu3Sn layers. When the proportion of Sn in the solder joint is relatively high or the size of the solder joint is small, even the phenomenon of complete alloying of the solder is likely to occur, greatly reducing the electrical and mechanical reliability of the solder joint interconnection. To inhibit the Cu-Sn reaction, the industry usually makes a Ni (nickel) metal barrier layer between the Cu solder pad and the solder to block the direct contact between the solder pad and the solder. Compared with Cu atoms, Ni is more difficult to react with Sn atoms, so the alloying process of the solder can be greatly slowed down. However, the thickness design of the Ni barrier layer is a problem that the industry has not yet solved, and the current public research on the Ni barrier layer for slowing down the alloying of the solder does not provide relevant solutions for the thickness design of the Ni barrier layer. Summary of the Invention

[0003] The purpose of the present invention is to provide a method for designing the thickness of a Ni barrier layer in solder joints, which can provide a solution for the thickness design of the Ni barrier layer, effectively improve the packaging design efficiency of integrated circuits, and reduce the development cost.

[0004] One aspect of the present invention provides a method for designing the thickness of a Ni barrier layer in solder joints, including:

[0005] Step S1, welding the solder joint structure plated with a Ni barrier layer according to the actual assembly process conditions, wherein the thickness of the Ni barrier layer can ensure that the Ni barrier layer is not completely consumed during the welding process;

[0006] Step S2, performing a morphological analysis on the microstructure of the solder joint structure after welding to determine the thickness of the Ni barrier layer consumed during welding and the composition of the generated Ni compound layer;

[0007] Step S3, numerically analyzing the Ni atomic flux in the Ni barrier layer at the cathode under temperature and current stress, and establishing a service consumption thickness model of the Ni barrier layer at the cathode:

[0008]

[0009] Wherein, is the thickness of the Ni barrier layer, t is the stress loading time, is the initial thickness of the Ni barrier layer, and

[0010]

[0011] wherein, , and are respectively the atomic concentration, thermal diffusivity and effective charge number of Ni atoms in the Ni barrier layer, is the diffusion coefficient at the interface between the Ni barrier layer and the Ni compound layer, is the resistivity of the Ni barrier layer, is the concentration gradient of Ni atoms at the interface between the Ni barrier layer and the Ni compound layer, is the unit charge amount, is the average current density, is the Boltzmann constant, is the Kelvin temperature;

[0012] Step S4, based on the constructed Ni barrier layer service consumption thickness model, calculate the service consumption thickness of the Ni barrier layer at the actual working temperature, current stress and service time of the solder joint;

[0013] Step S5, according to the remaining thickness, welding consumption thickness and design redundancy of the Ni barrier layer, and the service consumption thickness and design redundancy, obtain the design thickness of the Ni barrier layer.

[0014] Preferably, step S2 includes:

[0015] Use epoxy resin to encapsulate the solder joint structure after welding to obtain a cross-section slice of the welded part of the solder joint with a Ni barrier layer;

[0016] Use a scanning electron microscope to photograph the cross-section slice microstructure morphology of the welded part of the solder joint with a Ni barrier layer;

[0017] Analyze the photographed microscopic images to obtain the thickness of the Ni barrier layer consumed by welding and the composition of the Ni compound layer.

[0018] Preferably, the composition of the generated Ni compound layer is (Ni x Cu 1-x )6Sn5, where x represents the proportion of Ni atoms.

[0019] Preferably, the solder joint structure is a wafer-level copper pillar micro-solder joint structure, which includes a Ni barrier layer, a (Ni x Cu 1-x )6Sn5 layer, a Sn solder layer and a Cu pad from top to bottom.

[0020] Preferably, in step S3, the Ni atomic flux in the Ni barrier layer at the cathode is:

[0021] ,

[0022] where

[0023] ,

[0024] where represents the thermal diffusion flux of Ni atoms from the Ni barrier layer to the (Ni x Cu 1-x )6Sn5 layer, is the electromigration flux of Ni atoms from the Ni barrier layer to the (Ni x Cu 1-x )6Sn5 layer, is the thickness change value of the Ni barrier layer.

[0025] Preferably, in step S5, the designed thickness of the Ni barrier layer obtained is:

[0026] Designed thickness of Ni barrier layer = remaining thickness + (1 + design redundancy of welding consumption thickness) * welding consumption thickness + (1 + design redundancy of service consumption thickness) * service consumption thickness.

[0027] Preferably, the design redundancy of the welding consumption thickness is 20%, and the design redundancy of the service consumption thickness is 10%.

[0028] According to the method for designing the thickness of the Ni barrier layer in the solder joint in the above aspect of the present invention, a solution can be provided for the thickness design of the Ni barrier layer, effectively improving the packaging design efficiency of the integrated circuit and reducing the development cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the technical solutions of the present invention, the drawings used in the description of the embodiments of the present invention will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts:

[0030] Figure 1 is a flowchart of the method for designing the thickness of the Ni barrier layer in the solder joint in an embodiment of the present invention;

[0031] Figure 2 is a schematic diagram of the Ni atomic flux in the solder joint structure in an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0032] To make the objectives, technical solutions and advantages of the present invention more clear, the following will, in conjunction with the accompanying drawings, clearly and completely describe the technical solutions of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts fall within the scope of protection of the present invention.

[0033] An embodiment of the present invention provides a design method for the thickness of the Ni barrier layer in solder joints. As Figure 1 shown, the method of the embodiment of the present invention includes steps S1 to S5.

[0034] Step S1: Weld the solder joint specimen with a Ni barrier layer

[0035] This step is mainly used to obtain the high-temperature welding process parameters experienced by the solder joint. The reflow soldering required for encapsulating the solder joint structure with a Ni barrier layer is carried out under the same process conditions. The main process is as follows:

[0036] (1) According to the actual assembly process condition requirements of the integrated circuit product, clarify the high-temperature welding process parameters experienced by the encapsulation solder joint, including but not limited to the number of welds, the temperature curve used in each welding process, etc.;

[0037] (2) Weld the solder joint structure with a Ni barrier layer completely according to the actual assembly process conditions and procedures. Taking the wafer-level copper pillar micro-solder joint structure as an example, its copper pillar and solder interface are often plated with a Ni barrier layer. When assembling an integrated circuit using the wafer-level copper pillar micro-solder joint structure, it needs to go through two processes: high-temperature welding of the micro-solder joint and the substrate, and high temperature during the welding of the package-level solder joint and the PCB board.

[0038] It should be noted that at this time, it is required that the electroplated Ni barrier layer should have a certain thickness (generally not less than 2um) to ensure that the Ni barrier layer will not be completely consumed by reaction during the above welding process.

[0039] Step S2: Obtain the thickness of the Ni barrier layer consumed by welding

[0040] This step is mainly used to obtain the morphology of the microstructure of the solder joint structure after welding, and determine parameters such as the thickness of the Ni barrier layer consumed by welding through the analysis of the microstructure. The main process is as follows:

[0041] (1) Use epoxy resin to seal the welded specimen, and perform process treatments such as slicing, grinding, polishing, and etching on the sealed specimen to obtain a cross-section slice of the welded part of the solder joint with a Ni barrier layer;

[0042] (2) Use a scanning electron microscope to photograph the morphology of the cross-section slice structure of the welded part of the solder joint with a Ni barrier layer;

[0043] (3)Analyze the captured microscopic images using image processing software to obtain parameters such as the thickness of the Ni barrier layer, the thickness consumed by soldering, and the thickness of the Ni-Cu-Sn compound, and calculate the relationship formula of the Ni-Cu-Sn compound layer through atomic ratios, that is, confirm the coefficient x in (Ni x Cu 1-x )6Sn5.

[0044] Step S3: Construct a service consumption thickness model for the Ni barrier layer

[0045] This step is mainly used to establish a service consumption thickness model for the Ni barrier layer. Since the Ni barrier layer at the cathode is most easily consumed under the coupled stress of temperature and current, the method of this embodiment of the present invention establishes a service consumption thickness model based on the Ni barrier layer at the cathode.

[0046] (1)Numerical analysis of Ni atomic flux:

[0047] Figure 2 The schematic diagram of Ni atomic flux in the copper pillar micro-solder joint structure is shown. From top to bottom, it successively includes the Ni barrier layer, (Ni x Cu 1-x )6Sn5 layer, Sn solder layer, Cu6Sn5 layer, Cu3Sn layer, and Cu pad. When electrons (e - ) flow from Cupillar to Cu pad (the current direction is opposite), the Ni barrier layer and the side of the (Ni x Cu 1-x )6Sn5 layer are at the cathode position, and the side of the copper pad is at the anode position. Among them, represents the thermal diffusion flux of Ni atoms caused by the concentration gradient, and the direction is from the Ni barrier layer to the (Ni x Cu 1-x )6Sn5 layer. is the electromigration flux of Ni atoms from the Ni barrier layer to the (Ni x Cu 1-x )6Sn5 layer.

[0048] Therefore, for the cathode Ni barrier layer, the following can be obtained:

[0049] (Formula 1)

[0050] Among them:

[0051] (Formula 2)

[0052] In the formula, , and They are respectively the atomic concentration, thermal diffusivity, and effective charge number of Ni atoms in the Ni barrier layer; and They are respectively the thickness of the Ni barrier layer and the change in thickness, t is the stress loading time, is the diffusion coefficient at the interface between the Ni barrier layer and the (Ni x Cu 1-x )6Sn5 layer, is the resistivity of the Ni barrier layer, is the concentration gradient of Ni atoms at the interface between the Ni barrier layer and the (Ni x Cu 1-x )6Sn5 layer; is the unit charge amount, is the average current density, is the Boltzmann constant, is the Kelvin temperature.

[0053] (2) Ni barrier layer service consumption thickness model

[0054] Through the numerical analysis of the Ni atomic flux, the thickness change model of the Ni barrier layer can be established by transformation.

[0055] For the cathode Ni barrier layer, combining Equation 1 and Equation 2, we can obtain:

[0056] (Equation 3)

[0057] By integrating and solving Equation 3, we can obtain:

[0058] (Equation 4)

[0059] In the formula, is the initial thickness of the Ni barrier layer.

[0060] Step S4: Obtain the service consumption thickness of the Ni barrier layer

[0061] This step mainly obtains the service consumption thickness of the Ni barrier layer at the working temperature and current stress under the preset service life based on the parameters required by the established Ni barrier layer service consumption thickness model.

[0062] (1) Regarding the concentration of Ni atoms in the Ni barrier layer parameters, they can be calculated and obtained according to parameters such as the density and molar mass of the Ni barrier layer;

[0063] (2) Regarding the concentration gradient of Ni atoms at the interface between the Ni barrier layer and the (Ni x Cu 1-x )6Sn5 layer , it can be obtained by querying relevant literature or referring to relevant methods for testing and calculation based on information such as the composition of the (Ni x Cu 1-x )6Sn5 layer and the microstructural pictures taken;

[0064] (3) Regarding the thermal diffusion coefficient of Ni atoms at the interface between the Ni barrier layer and the (Ni x Cu 1-x )6Sn5 layer and the thermal diffusivity of Ni atoms in the Ni barrier layer , it can be obtained by querying relevant literature or referring to relevant methods for testing and calculation;

[0065] (4) Regarding the effective charge number of Ni atoms in the Ni barrier layer and the resistivity parameter of the Ni barrier layer , it can be obtained by querying relevant literature or referring to relevant methods for testing.

[0066] Finally, substituting the temperature , current stress conditions , service time requirements t borne by the actual application scenario of the solder joint, etc., and all the parameters required for model calculation, the service consumption thickness of the Ni barrier layer can be calculated.

[0067] Step S5: Obtain the appropriate design thickness of the Ni barrier layer

[0068] Based on the welding consumption thickness and service consumption thickness of the Ni barrier layer obtained in the above steps, and combined with the redundancy requirements designed by the user, the appropriate design thickness of the Ni barrier layer can be obtained, which is specifically as follows:

[0069] Design thickness of Ni barrier layer = remaining thickness + (1 + design redundancy of welding consumption thickness) * welding consumption thickness + (1 + design redundancy of service consumption thickness) * service consumption thickness (Formula 5)

[0070] For example, considering the possible consistency deviations in the encapsulation solder joint production process, welding process, and service environment, a 20% design redundancy is proposed for the welding consumption thickness of the Ni barrier layer, and a 10% design redundancy is proposed for the service consumption thickness of the Ni barrier layer. It is required that after meeting the service time requirements under the service temperature and current stress conditions, the remaining thickness of the Ni barrier layer is not less than 0.2 um. Then the design thickness of the Ni barrier layer is: 0.2 um + 120% welding consumption thickness + 110% service consumption thickness.

[0071] The following uses an example to illustrate the process of designing the thickness of the Ni barrier layer in the solder joint by using the method of the embodiment of the present invention.

[0072] For example, the design requirements for the solder joints of an integrated circuit product are as follows: a 20% design redundancy is proposed for the welding consumption thickness of the Ni barrier layer, and a 10% design redundancy is proposed for the service consumption thickness of the Ni barrier layer. The required temperature is 100 °C, and the current density is 2.0×10 4 A / cm 2 Under the condition that after 10,000 hours of service of the package solder joints, the remaining thickness of the Ni barrier layer is not less than 0.2 μm.

[0073] (1) Weld the solder joint specimens with Ni barrier layer according to step S1 to obtain the high-temperature welding process parameters experienced by the solder joints, and perform the reflow soldering required for packaging the solder joint structure with Ni barrier layer under the same process conditions;

[0074] (2) Obtain the welding consumption thickness of the Ni barrier layer according to step S2, obtain the morphology of the microstructure of the solder joint structure after welding, and through the analysis of the microstructure, determine that the welding consumption thickness of the Ni barrier layer is 1.5 μm, the thickness of the Ni-Cu-Sn compound layer is 0.5 μm, and the composition is (Ni 0.2 Cu 0.8 )6Sn5;

[0075] (3) Construct a service consumption thickness model of the Ni barrier layer according to step S3, establish a service consumption thickness model of the cathode Ni barrier layer, and obtain the relevant parameters according to the literature and related tests and theoretical calculations, as shown in Table 1 below;

[0076] Table 1 Parameters of the service consumption thickness model of the Ni barrier layer

[0077]

[0078] (4) Obtain the service consumption thickness of the Ni barrier layer according to step S4. Based on the established service consumption thickness model of the Ni barrier layer, substitute the required parameters to obtain that under the conditions of a working temperature of 100 °C and a current density of 2.0×10 4 A / cm 2 After 10,000 hours of service of the package solder joints, the consumption thickness of the Ni barrier layer is 0.15 μm respectively;

[0079] (5) Obtain the appropriate design thickness of the Ni barrier layer according to step S5. The welding consumption thickness of the Ni barrier layer is 1.5 μm, and a 20% design redundancy is proposed; the service consumption thickness of the Ni barrier layer is 0.15 μm, and a 10% design redundancy is proposed; the required temperature is 100 °C, and the current density is 2.0×10 4 A / cm 2Under the condition that the remaining thickness of the Ni barrier layer is still not less than 0.2 um after the encapsulated solder joint has served for 10,000 hours, the appropriate designed thickness of the Ni barrier layer is calculated as: 0.2 um + 1.5 um * 120% + 0.15 um * 110% = 2.165 um.

[0080] In summary, for the method of designing the thickness of the Ni barrier layer in the solder joint of the embodiment of the present invention, first, the solder joint structure plated with the Ni barrier layer is reflow soldered using the same process, and parameters such as the welding consumption thickness of the Ni barrier layer are obtained through microstructure analysis. Secondly, based on the numerical analysis of the migration flux of cathode Ni atoms under the coupled stress of temperature and current, a service consumption thickness model of the Ni barrier layer is established to obtain the service consumption thickness of the Ni barrier layer under the actual service temperature, current stress conditions, and service time requirements of the solder joint. Finally, the designed thickness of the Ni barrier layer is determined based on the welding consumption thickness and service consumption thickness of the Ni barrier layer. The method of the embodiment of the present invention is a quantitative design method for the solder joints of integrated circuit packages, which can effectively improve the efficiency of integrated circuit package design, reduce the relevant development costs, and has good engineering application value and great economic and social benefits.

[0081] Only some exemplary embodiments of the present invention have been described by way of illustration above. Undoubtedly, for those of ordinary skill in the art, the described embodiments can be modified in various different ways without departing from the spirit and scope of the present invention. Therefore, the above drawings and description are illustrative in nature and should not be construed as limiting the protection scope of the claims of the present invention.

Claims

1. A design method for the thickness of the Ni barrier layer in solder joints, characterized in that Including: Step S1: Weld the solder joint structure plated with a Ni barrier layer according to the actual assembly process conditions, where the thickness of the Ni barrier layer can ensure that the Ni barrier layer will not be completely consumed during the welding process; Step S2: Analyze the morphology of the microstructure of the solder joint structure after welding to determine the thickness of the Ni barrier layer consumed during welding and the composition of the formed Ni compound layer; Step S3: Numerically analyze the Ni atomic flux in the Ni barrier layer at the cathode under temperature and current stress, and establish a service consumption thickness model for the Ni barrier layer at the cathode: Among them, is the thickness of the Ni barrier layer, t is the stress loading time, is the initial thickness of the Ni barrier layer, and Among them, , and are respectively the atomic concentration, thermal diffusivity and effective charge number of Ni atoms in the Ni barrier layer, is the diffusion coefficient at the interface between the Ni barrier layer and the Ni compound layer, is the resistivity of the Ni barrier layer, is the concentration gradient of Ni atoms at the interface between the Ni barrier layer and the Ni compound layer, is the unit charge amount, is the average current density, is the Boltzmann constant, is the Kelvin temperature; Step S4: Based on the established service consumption thickness model of the Ni barrier layer, calculate the service consumption thickness of the Ni barrier layer of the solder joint at the actual working temperature, current stress, and service time; Step S5: Obtain the designed thickness of the Ni barrier layer according to the remaining thickness of the Ni barrier layer, the design redundancy of the welding consumption thickness, and the design redundancy of the service consumption thickness; 2. The method according to claim 1, characterized in that Step S2 includes: Seal the welded solder joint structure with epoxy resin to obtain a cross-sectional slice of the solder joint welded with the Ni barrier layer plated; Use a scanning electron microscope to photograph the cross-sectional slice microstructure morphology of the solder joint welded with the Ni barrier layer plated; Analyze the photographed microscopic images to obtain the thickness of the Ni barrier layer consumed during welding and the composition of the Ni compound layer; 3. The method according to claim 2, characterized in that, The composition of the formed Ni compound layer is (Ni x Cu 1-x )6Sn5, where x represents the proportion of Ni atoms.

4. The method according to any one of claims 1 to 3, characterized in that The solder joint structure is a wafer-level copper pillar micro-solder joint structure, which sequentially includes a Ni barrier layer, (Ni x Cu 1-x ) 6Sn5 layer, a Sn solder layer, and a Cu pad from top to bottom.

5. The method according to claim 4, wherein In Step S3, the Ni atomic flux in the Ni barrier layer at the cathode is: ， Where, ， Among them, represents the thermal diffusion flux of Ni atoms from the Ni barrier layer to the (Ni x Cu 1-x )6Sn5 layer, is the electromigration flux of Ni atoms from the Ni barrier layer to the (Ni x Cu 1-x )6Sn5 layer, is the thickness change value of the Ni barrier layer.

6. The method according to any one of claims 1 to 3, characterized in that, In Step S5, the designed thickness of the Ni barrier layer obtained is: Designed thickness of Ni barrier layer = Remaining thickness + (1 + Design redundancy of welding consumption thickness) * Welding consumption thickness + (1 + Design redundancy of service consumption thickness) * Service consumption thickness.

7. The method according to claim 6, wherein The design redundancy of the welding consumption thickness is 20%, and the design redundancy of the service consumption thickness is 10%.

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