CN107097012A - A kind of crystal grain is orientated consistent banjo fixing butt jointing electro-migration testing method - Google Patents

Abstract

The invention discloses a butt joint electromigration test method with consistent grain orientation, which belongs to the field of material preparation and connection. The two pads are remelted with solder paste to form a solder joint. The solder butt joint is adhered to the substrate with epoxy resin. At least one side surface of the solder joint is used as the cross section of the solder joint. Select Brazed butt joints with a single grain orientation under PLM; butt single crystal joints are wire-cut and finely polished to obtain EBSD data to determine the angle between the c-axis of the brazed butt joint grains and the current direction, with the help of epoxy resin Solder joints are adhered to the substrate for the relevant tests. On the premise that the angle between the c-axis of the single crystal solder joint grain and the current direction is consistent, a comparable solder joint reliability evaluation is obtained.

Description

A method for electromigration testing of butt joints with consistent grain orientation

technical field

The invention relates to an electromigration test method for a butt joint with consistent grain orientation, which belongs to the field of material preparation and connection, is suitable for preparing micro-brazed butt joints with consistent grain orientation, and is applied to electromigration reliability research. The method can effectively ensure the consistency of the size and grain orientation of the micro-butt joints, thereby ensuring the comparability of the electromigration reliability test data of the brazed butt joints.

Background technique

Solder joints are an integral part of microelectronic interconnection, playing the role of mechanical connection and electrical signal transmission. At present, the microelectronic packaging space is reduced, and the heat generation of the chip is intensified. On the one hand, the intermetallic compounds (Intermetallic Compounds, IMCs) generated by the reaction between the solder and the pad metallization layer during the formation of solder joints or the use of electronic products ) layer accounts for an increasing proportion of the entire solder joint, and its shape, size, crystal orientation, and thickness have an increasingly serious impact on the reliability of the solder joint. On the other hand, the current density borne by the solder joint continues to increase. Driven by kinetic factors, the IMCs formed by the liquid solder wetting on the solid pad during the remelting process will grow or dissolve, resulting in the failure of the solder joint. The reliability of the solder joint largely determines the entire electronic product. reliability and lifespan. Therefore, how to control the reaction behavior of IMCs at the interface is particularly important, which requires clarifying the interface reaction mechanism during solder joint formation and service.

Studies have shown that Sn-based lead-free interconnect solder joints prepared by remelting often exhibit single crystal or twin crystal structure, while the BCT crystal structure of β-Sn is anisotropic (a=0.5832, c=0.3182, c/a =0.546), the diffusion of Cu and other atoms in the solder joint will show strong anisotropy due to the different grain orientations of β-Sn, for example, at 25 °C, the diffusion rate of Cu along the c-axis of β-Sn lattice It is 2×10 ^-6 cm ² /s, which is 500 times of its diffusion rate along the a and b axes. This orientational diffusion behavior will have a serious impact on the electromigration behavior of solder joints, and has a c-axis parallel to the current direction. Sn-Ag or Sn-Ag-Cu solder single crystal solder joints are prone to premature failure, and the growth rate of IMCs at the interface is about 10 times that of single crystal solder joints or twin crystal solder joints with c-axis perpendicular to the current direction. At present, it is a thermodynamic problem to deeply understand and predict the growth mode of Sn dendrites. After the interconnection is completed, each solder joint has a unique crystal orientation, so it is inevitable that there will be some solder joints due to the growth of β-Sn grains. The orientation is unfavorable, and it will fail early during the use of electronic products, thereby reducing the service life of electronic products. It can be seen that the grain orientation of solder joints will seriously affect its service reliability. Therefore, it is necessary to find a suitable method to obtain single crystal solder joints with the same grain orientation, and to perform the characterization of the remelting state of the interface IMCs and the evolution of the interface IMCs during the aging process. The research on the behavior and the evolution behavior of interfacial IMCs during the electromigration process will greatly improve the understanding level of the interfacial reaction behavior of solder joints.

Contents of the invention

The purpose of the invention is to overcome the characteristic that the crystal grain orientation of the miniature brazed butt joint cannot be controlled, and produce a brazed butt joint with controllable solder joint size and consistent crystal grain orientation. At the same time, it is expected that by characterizing the remelting state of the butt single crystal solder joints with the same orientation, the aging process and the evolution behavior of the interface IMCs during the electromigration process, on the premise that the crystal orientation of the solder joints is consistent, the corresponding c-axis and current direction can be obtained. A series of reliability data of angle, and finally achieve the purpose of in-depth understanding of electromigration reliability of brazed butt joints.

In order to achieve the above object, the present invention adopts the following technical solutions.

A butt joint electromigration test method with consistent grain orientation, characterized in that it comprises the following steps:

(1) Remove oxides and organic pollutants on the surface of the pads, stick double-sided adhesive on the substrate, and place the two pads on the substrate to ensure that the welding surfaces of the two pads are parallel and have a certain distance , to ensure the consistency of weld size and width, and the welding surface is perpendicular to the substrate;

(2), apply the selected solder paste between the welding surfaces of the two pads, remelt, and then cool to obtain the corresponding solder butt joint; place the solder butt joint together with the substrate in acetone In the solution, the solder butt joint is removed from the substrate to obtain a brazed butt joint prepared by remelting with a certain grain orientation, and it is directly ground without inlaying to remove excess solder, and the brazed butt joint The surface that can be used as a section can be polished;

(3) Observing the surface cross-section of the polished butt joint by Polarized light microscopy (PLM), distinguishing the β-Sn grains with different crystal orientations, and selecting the solder that exhibits a single grain orientation under PLM. Welded butt joints;

Due to the anisotropic BCT crystal structure of β-Sn, when the light beam is incident on the surface of the polished β-Sn crystal, it will be decomposed into two beams of light and refracted in different directions to cause birefringence. Since the contrast of grains with different orientations is significantly different under Polarized light microscopy (PLM), the β-Sn grains with different crystal orientations can be distinguished by observing the cross-section of the polished brazed butt joint through PLM. Brazed butt joints exhibiting a single grain orientation under PLM;

(4), wire-cut the solder butt joint single crystal joint obtained by PLM observation, and obtain a plurality of micro-solder butt joints with the same grain orientation as the brazed butt joint prepared by remelting in step (2), to obtain Finely polish the micro-brazed butt joints, obtain Electron Backscattered Diffraction (EBSD) data, and determine the angle between the c-axis of the brazed butt joint grains and the current direction;

(5) Adhere the solder joint obtained by wire cutting in step (4) to the substrate with the aid of epoxy resin, and perform grinding and polishing of a specified section to finally obtain a brazed butt joint that can be used for electromigration testing, and Analyze the evolution behavior of IMCs in cross-section and top view direction of solder joints;

The use of epoxy resin is because it meets the temperature requirements of the electromigration reliability test of general solder solder joints, and at the same time can ensure the structural strength of solder solder joints;

(6) Conduct reliability tests related to electromigration tests, and obtain comparable solder joint electromigration reliability data on the basis of consistent grain orientation of brazed butt joints.

Processing of micro-brazed butt joints by wire cutting;

The substrate can withstand remelting temperature and electromigration temperature and is non-conductive, using a printed circuit board, etc.;

The pads are made of Cu, Cu/Ni/Au, Cu/Cu ₆ Sn ₅ ;

The solder solder paste is a Sn-based binary alloy, ternary alloy or quaternary alloy; preferably binary alloy SnCu series, SnAg series, SnZn series, SnBi series or SnIn series, ternary alloy SnAgCu series, SnAgBi series or SnAgIn series, quaternary SnAgBiIn series lead-free solder.

For remelting in step (2), the temperature range is from 200°C to 700°C; for cooling in step (2), choose furnace cooling, air cooling, air cooling, water cooling or oil cooling.

Relevant reliability tests in step (6) are observation of microstructure at remelted preparation interface, observation of crystal orientation and component analysis, aging experiment, electromigration experiment, and electromigration life test.

The invention has the advantages of being able to control the size of the brazed butt joint, ensuring the consistency of the crystal orientation of the butt joint; the process is simple and the cost is low; the butt joint obtained at the same time can meet various requirements of aging and electromigration tests, and the c-axis and current Comparable solder joint reliability evaluations are obtained under the premise that the included angles in the directions are consistent.

Description of drawings

Fig. 1: Micro-brazed butt joint photo obtained by wire cutting in embodiment 1

Figure 2: Micro-brazed butt joint orientation distribution diagram obtained by wire cutting in Example 1

Figure 3: Schematic diagram of the composition of the micro-brazed butt joint electromigration test structure in Example 1

Figure 4: Photo of micro-brazed butt joints used in electromigration-related tests in Example 1.

specific implementation

The present invention will be further described below in conjunction with the examples, but the present invention is not limited to the following examples.

Example 1

The following content specifically illustrates the embodiment of the present invention in conjunction with FIGS. 1 , 2 and 4 . Solder solder paste is generally stored in the refrigerator. It needs to be taken out of the refrigerator 4-8 hours in advance and placed at room temperature to restore the viscosity of the solder paste. It needs to be fully stirred before use.

Electromigration performance test of Cu/Sn3.5Ag/Cu butt single crystal joint with consistent grain orientation, cross-sectional size of 300μm×300μm, and weld width of 200μm.

1. Preparation of Cu pads: Make copper pads by wire cutting, the size of which is 500μm×10mm×20mm;

2. Put the Cu sheet with a purity greater than 99.99wt.% and a size of 500μm×10mm×20mm into the prepared 30% HNO ₃ aqueous solution for a few minutes to remove oxides and pollutants on the surface of the Cu pad, and then place it Put into acetone for further ultrasonic cleaning, after cleaning, dry for subsequent use;

3. Adhere double-sided tape on the 10mm×10mm×1.5mm printed circuit boards (Printed circuit boards, PCB), and place two Cu pads on it with a spacing of 200μm. At the same time, pay attention to ensure that the spacing between the pads is 200μm, and ensure the parallelism of the Cu pad;

4. Then use a thin paper cotton swab to apply the stirred Sn3.5Ag solder paste between the two Cu pads, and use a hot air rework bench (US PACE ST325) to conduct a remelting experiment, and set the remelting temperature The temperature is 245°C, the remelting time is set to 50s, air-cooled, and a Sn3.5Ag solder butt joint is obtained;

5. Put the sample together with the PCB in acetone solution, remove the linear solder joint from the PCB, grind it directly without inlaying, remove the excess solder, and obtain a Cu/Sn3.5Ag/Cu butt joint;

6. Grinding and polishing the specified cross-section, observing the grain orientation of the brazed butt joint prepared by remelting with the help of PLM, and selecting the butt joint that presents single crystal solder joints under PLM;

7. Carry out wire cutting on the selected single crystal solder joints, grind the obtained micro-brazed butt joints, remove excess solder, and perform fine polishing on the specified cross-section, and finally obtain a linear solder joint cross-sectional size of 300μm× 300μm, the solder joint thickness is 200μm, as shown in Figure 1;

8. Obtain the EBSD data of the finely polished section, as shown in Figure 2, determine whether the brazed butt joint is a single crystal solder joint with consistent grain orientation, and determine the angle between the c-axis of the grain and the current direction;

9. Adhere the sample to a PCB with a size of 10mm×10mm×1.5mm by means of epoxy resin, and perform grinding and polishing of a specified section, and finally obtain a linear solder joint with a cross-sectional size of 300 μm × 300 μm and a solder joint thickness of 200 μm, such as Figure 4 shows. The conductive area of the solder is 9×10 ⁴ μm ² , which is equivalent to the conductive area of the actual flip-chip solder ball;

10. Each single crystal solder joint butt joint is cut into multiple linear solder joints by wire cutting, and one of the solder joints is subjected to deep etching to obtain the initial interface IMCs state of the solder joint prepared by remelting;

11. Conduct electromigration experiments on a solder joint, conduct microstructure, crystal orientation observation and composition analysis on its cross section, and then conduct electromigration experiments on the same solder joint after 7 days, 14 days, 21 days, 28 days and 35 days Spot polished cross-section for microstructure observation and component analysis, and finally deep corrosion of solder joints after 35-day electromigration experiment;

12. A solder joint is subjected to an aging experiment at the temperature corresponding to the electromigration experiment, and its microstructure, crystal orientation observation and composition analysis are carried out, and the thickness of the interface IMCs layer is measured to obtain the internal and interface IMCs state of the initial cross section of the solder joint. After 7 days, 14 days, 21 days, 28 days and 35 days of aging treatment, the microstructure observation and component analysis of the polished cross-section of the same sample were carried out, and the solder joints were deeply etched after 35 days of aging;

13. Three solder joints are tested for electromigration failure time, and the average value is taken as the failure time. Since the solder has a large current-carrying area, the resistance is low. Therefore, when the failure time is reached, the resistance will suddenly increase. In this experiment, the conductive area of the solder is 9×10 ⁴ μm ² , which is equivalent to the failure area of the actual solder ball. Therefore, a similar phenomenon can also be seen in the resistance recording process. During the resistance recording process, the resistance of the solder increases sharply The local setting is the expiration time of the solder.

Claims (9)

1. a kind of crystal grain is orientated consistent banjo fixing butt jointing electro-migration testing method, it is characterised in that comprise the following steps：

(1) oxide and organic pollution of bond pad surface, are removed, double faced adhesive tape is adhered on substrate, and two pads are placed in On substrate, it is ensured that the solder side of two pads is parallel, and there is certain spacing, to ensure weld size and width consistency, welding Face vertical substrate；

(2), the solder soldering paste of selection is coated between the solder side of two pads, remelting is carried out, then cools down, obtains corresponding Solder banjo fixing butt jointing；Solder banjo fixing butt jointing is placed in acetone soln together with substrate, by solder banjo fixing butt jointing from base Removed on plate, obtain the soldering banjo fixing butt jointing prepared with certain grain-oriented remelting, without inlaying, directly grind, to go Except redundant brazing filler metal, and can be polished as the surface in section to soldering banjo fixing butt jointing；

(3) pricker polished, is observed by cross-polarization optics microscope (Polarized light microscopy, PLM) Banjo fixing butt jointing surface sections are welded, the β-Sn crystal grain of different crystal orientation is distinguished, is chosen under PLM and single grain-oriented pricker is presented Weld banjo fixing butt jointing；

(4) the solder docking monocrystalline joint obtained by, being observed as PLM carries out wire cutting, obtains what is prepared with step (2) remelting Soldering banjo fixing butt jointing has multiple miniature soldering banjo fixing butt jointings that same die is orientated, and obtained miniature soldering banjo fixing butt jointing is entered Row essence is thrown, and is obtained EBSD (Electron Backscattered Diffraction, EBSD) data, is determined pricker Weld the angle of banjo fixing butt jointing crystal grain c-axis and the sense of current；

(5), the solder connector that step (4) progress wire cutting is obtained is adhered on substrate by epoxy resin, and specified The grinding and polishing in section, finally give available for carry out electro-migration testing soldering banjo fixing butt jointing, and carry out solder joint cross section and Overlook direction IMCs develops behavioural analysis；

(6) the related reliability testing of electro-migration testing, is carried out, on the basis of soldering banjo fixing butt jointing crystal grain orientation is consistent, is obtained To the solder joint electromigration reliability data for possessing comparativity；

Solder soldering paste for institute's Sn bases bianry alloy, ternary alloy three-partalloy or quaternary alloy.

2. consistent banjo fixing butt jointing electro-migration testing method is orientated according to a kind of crystal grain described in claim 1, it is characterised in that Step (4) carries out the processing of miniature soldering banjo fixing butt jointing using wire cutting.

3. consistent banjo fixing butt jointing electro-migration testing method is orientated according to a kind of crystal grain described in claim 1, it is characterised in that Substrate is resistant to remelting temperature and electromigration temperature and non-conductive.

4. consistent banjo fixing butt jointing electro-migration testing method is orientated according to a kind of crystal grain described in claim 1, it is characterised in that Substrate uses printed circuit board (PCB).

5. consistent banjo fixing butt jointing electro-migration testing method is orientated according to a kind of crystal grain described in claim 1, it is characterised in that The pad is selected from Cu, Cu/Ni/Au, Cu/Cu₆Sn₅。

6. consistent banjo fixing butt jointing electro-migration testing method is orientated according to a kind of crystal grain described in claim 1, it is characterised in that Solder soldering paste is selected from bianry alloy SnCu series, SnAg series, SnZn series, SnBi series or SnIn series, ternary alloy three-partalloy SnAgCu series, SnAgBi series or SnAgIn series, quaternary SnAgBiIn series leadless solders.

7. consistent banjo fixing butt jointing electro-migration testing method is orientated according to a kind of crystal grain described in claim 1, it is characterised in that Remelting in step (2), temperature range selects 200 DEG C to 700 DEG C.

8. consistent banjo fixing butt jointing electro-migration testing method is orientated according to a kind of crystal grain described in claim 1, it is characterised in that Cooling in step (2), selects the type of cooling of furnace cooling, air cooling, air-cooled, water cooling or oil cooling.

9. consistent banjo fixing butt jointing electro-migration testing method is orientated according to a kind of crystal grain described in claim 1, it is characterised in that Related reliability testing in step (6) be remelting prepare interface microstructure observation, crystal orientation observation and constituent analysis, Timeliness experiment, electromigration experiment, electromigration lifetime test.