Method for enhancing flip-chip bonding strength of gold bumps

Abstract

The invention provides a method for enhancing flip-chip bonding strength of a gold bump, which comprises the following steps of: ball mounting: preparing the gold bump on the surface of a chip; leveling, wherein the chip is placed on a leveling substrate in an inverted mode, and ultrasonic pressure is applied to the chip through an ultrasonic pressure head; and flip-chip bonding: flip-chip bonding the chip on the bonding pad of the substrate with the bonding pad. Before the chip is formally welded, the ultrasonic pressure head is firstly used for ultrasonically pressurizing the chip on the leveling substrate, the salient points are deformed by utilizing ultrasonic energy and pressure, and the chip can be accurately welded by utilizing the principle that the energy is non-uniformly distributed on each salient point in the hot-pressing ultrasonic process. Each bump can obtain more uniform energy in the subsequent flip-chip bonding process, a better welding effect is achieved, the leveling substrate is made of an aluminum oxide ceramic material, the gold bumps cannot be welded with the aluminum oxide ceramic during leveling, the leveling substrate can be separated from the gold bumps conveniently after leveling, and the flatness of the gold bumps after leveling is guaranteed.

Description

Technical Field

[0001] The present application relates to the field of chip manufacturing technology, and in particular to a method for enhancing the welding strength of gold bump flip-chip soldering. Background Art

[0002] Chips and substrates are often required in electronic packaging, and electrical connections are established between chips. The traditional connection method is wire bonding, but the longer wires have large parasitic capacitance and inductance, and require a large space, which makes it difficult to meet the development of miniaturization and high performance of electronic products. Flip-chip bonding can directly solder the chip to the substrate through bumps. It has the advantages of low connection resistance, low parasitic capacitance and inductance, and high-density interconnection, and has received widespread attention and application. Depending on the size and purpose of the chip, the number of commonly used I / Os ranges from a few, dozens to hundreds or thousands. For example, storage requires miniaturization and thinness, and the number of I / Os is generally around 300; logic requires high integration, and the number of I / Os is as high as 3000-10000. In traditional flip-chip soldering, solder bumps are often used as connecting materials. Their performance has been widely studied and is suitable for flip-chip soldering of various I / O numbers. However, it also has some limitations. For example, the use of reflow soldering requires a temperature of up to 250°C to form solder interconnections, which makes it impossible to use this method for flip-chip soldering of many temperature-sensitive chips. In addition, although the use of adhesives can achieve interconnections at low temperatures, it is necessary to apply adhesives on the bumps, which is complex and costly. The use of gold bump hot pressing ultrasonic flip-chip soldering uses ultrasound to provide energy to the interface during the bonding process, which can reduce the requirements for force and temperature. It is particularly suitable for fragile and temperature-sensitive chip materials, such as the epitaxial layer of LEDs. At the same time, the use of gold as bumps has the advantages of good electrical conductivity, good thermal conductivity, and green environmental protection, which has attracted the attention of the market and the research of many scholars.

[0003] However, the process parameters and process window of gold bump flip-chip soldering will change with the number of I / Os. Sa-Yoon Kang et al. found that for chips with 30 I / Os, the optimal ultrasonic amplitude during flip-chip soldering is about 1.30 μm, and for chips with 1000 I / Os, the optimal ultrasonic amplitude during flip-chip soldering is reduced to 0.26 μm. When the amplitude is large, the new bonds formed in one cycle may be destroyed in the next cycle. On the other hand, too small an amplitude cannot generate enough bonding energy. In addition, the transmission of ultrasonic energy on the connection layer in each bump will be different, and there is a risk that each bump cannot be completely soldered. Kong Lingsong and his colleagues conducted research on multi-bump arrays through process experiments and modeling simulations. They found that the equivalent bonding pressure on the bumps at the edge of the array is slightly greater than that in the center, and that the equivalent vibration of the bumps at the edge will gradually deviate from the center. In other words, the load on each bump in multi-bump flip-chip soldering is uneven. In addition, the entire flip-chip soldering process is also affected by the flatness, position, and coplanarity of the substrate and bumps. Taking the coplanarity of the bumps as an example, the coplanarity indicates that there are certain differences in the height of the bumps in the bump array. The gold bumps prepared by the nail head bump method For example, due to the transmission error of the bonding machine and the random breakage of the tail wire, there are differences in the height between the bumps. In the hot-pressing ultrasonic flip-chip bonding, the higher bumps or the tips of the bumps first contact the substrate pads and feed back the contact pressure to the force sensor. As the chip and the substrate are continuously pressed together, the deformation of the higher gold bumps increases, and the contact pressure continues to increase. When the total pressure increases to the set value of the sensor, the chip and the substrate no longer produce relative movement. If the lower gold bumps have not yet contacted the substrate pads at this time, the gold and aluminum cannot be welded when ultrasound is applied, resulting in leaks. Similarly, areas with too high contact pressure will cause over-welding and damage the pads or even damage the chip, while areas with too low contact pressure will cause under-welding and low diffusion of gold and aluminum. And the difference increases with the number of bumps. Therefore, there are certain difficulties and challenges in flip-chip bonding of gold bumps with multiple I / Os.

[0004] For example, a Chinese patent with application number CN202210629031.8 discloses a metal bump leveling method and a flip chip method. This scheme thins the metal bumps by mechanical cutting, retains the bump ball-disk part, and keeps the consistency of the ball-disk part within 2μm. However, cutting the bump ball at micron-level precision requires high precision of the cutting equipment, has high procurement costs for the equipment, and requires regular replacement of cutting tools, which increases the manufacturing costs of the company. Summary of the invention

[0005] In order to solve the above problems, the present application proposes a method for enhancing the welding strength of gold bump flip-chip welding, by leveling the gold bumps through an alumina ceramic substrate before welding the gold bumps, thereby improving the consistency of the height of each gold bump, thereby improving the welding quality, and does not require the use of high-precision cutting equipment, thereby reducing the manufacturing cost of the chip.

[0006] This application is implemented through the following technical solutions:

[0007] The present application proposes a method for enhancing the welding strength of gold bump flip-chip soldering, comprising the following steps:

[0008] Ball planting, preparing gold bumps on the chip surface;

[0009] Leveling: Place the chip upside down on the leveling substrate and apply ultrasonic pressure on the chip through an ultrasonic indenter;

[0010] Flip chip bonding: flip the chip onto the pads of a substrate with pads.

[0011] Furthermore, the leveling substrate is an alumina ceramic substrate.

[0012] Furthermore, the gold bumps are prepared by a stud bumping method or an electroplating method.

[0013] Furthermore, the pressure applied by the ultrasonic pressure head to the chip is positively correlated with the number of gold bumps on the chip, and the pressure applied to a single gold bump is 0.4N-0.6N.

[0014] Furthermore, the ultrasonic power of the ultrasonic pressure head is positively correlated with the number of gold bumps on the chip, and the ultrasonic power on a single gold bump is 0.034W-0.042W.

[0015] Furthermore, the leveling substrate and the substrate with solder pads are respectively placed on different wafer platforms. After the chip is leveled, the ultrasonic pressure head drives the chip to rise. After the leveling substrate under the chip is switched to the substrate with solder pads through the wafer platform, the ultrasonic pressure head drives the chip to move downward, and the chip is flip-chip soldered on the pad of the substrate with solder pads.

[0016] Furthermore, before the leveling and flip-chip bonding, the parallelism between the chip and the leveling substrate and the substrate with pads needs to be adjusted.

[0017] Furthermore, the parallelism deviation between the chip and the flattened substrate and the substrate with pads is required to be ≤30 μm.

[0018] Furthermore, after the ball implantation, the height difference of the gold bumps on the chip is ≤12 μm.

[0019] Furthermore, the parallelism between the leveling and the flip-chip bonding is adjusted by a coupling on the flip-chip bonding machine.

[0020] The beneficial effects of the present application are as follows: before the chip is formally welded, an ultrasonic pressure head is used to ultrasonically pressurize the chip on a flattened substrate, and the ultrasonic energy and pressure are used to deform the bumps. The principle of uneven energy distribution on each bump during the hot pressing and ultrasonic process is used so that each bump can obtain more uniform energy in the subsequent flip-chip welding process to achieve a better welding effect. The flattened substrate is made of alumina ceramic. During leveling, the gold bumps cannot be welded to the alumina ceramic, which facilitates the separation of the flattened substrate and the gold bumps after leveling, thereby ensuring the flatness of the gold bumps after leveling. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a schematic diagram of the process of the present invention;

[0022] Figure 2 The surface flatness map of the single bump before and after the "pre-flip" treatment, (a) the morphology of the gold bump before the "pre-flip" treatment and the corresponding (c) height thermal map. (b) The morphology of the gold bump after the "pre-flip" treatment and the corresponding (d) height thermal map;

[0023] Figure 3 The surface height difference between each bump before and after the "pre-flip" process is shown in Figure 1. (a) The height of the gold bump array before the "pre-flip" process, (b) The height of the gold bump array after the "pre-flip" process;

[0024] Figure 4 Table of parameters (ultrasound power, pressure)-shear force optimized by orthogonal experimental method before using “pre-flipping” treatment;

[0025] Figure 5 Table of parameters (ultrasonic power, pressure)-shear force optimized by orthogonal experimental method after "pre-flipping" treatment; the descriptions of the reference numerals in the figure are as follows:

[0026] 1-chip, 2-flat substrate, 3-substrate with pads. DETAILED DESCRIPTION

[0027] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application, wherein the same or similar reference numerals throughout represent the same or similar elements or elements with the same or similar functions. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0028] It should be noted that all directional indications in the embodiments of the present application (such as up, down, left, right, front, back, etc.) are only used to explain the relative position relationship, movement status, etc. between the components under a certain specific posture (as shown in the accompanying drawings). If the specific posture changes, the directional indication will also change accordingly.

[0029] In addition, the descriptions of "first", "second", etc. in this application are only for descriptive purposes and cannot be understood as indicating or implying their relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of the features. In addition, the technical solutions between the various embodiments can be combined with each other, but they must be based on the ability of ordinary technicians in this field to implement them. When the combination of technical solutions is contradictory or cannot be implemented, it should be deemed that such combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0030] like Figures 1 to 3 As shown, an embodiment of the present invention provides a method for enhancing the welding strength of gold bump flip-chip soldering, characterized in that it includes the following steps: ball planting, preparing gold bumps on the surface of a chip; leveling, placing the chip flip-chip on a leveling substrate and applying ultrasonic pressure to the chip through an ultrasonic pressure head; flip-chip soldering, flip-chip soldering the chip to a solder pad on a substrate with a solder pad.

[0031] The present application can improve the height consistency of the gold bumps by adding a leveling step before flip-chip welding, so that each gold bump can obtain more uniform energy during flip-chip welding, thereby improving the welding quality. The leveling step does not require the introduction of other additional equipment, and only relies on a flip-chip welding machine, such as the CB-600 model, which has an ultrasonic hot pressing function. During leveling, the chip is pre-pressurized and ultrasonically flipped on a leveling substrate, and the bumps are deformed by using ultrasonic energy and pressure. The gold bumps are leveled by the leveling substrate, thereby improving the height consistency of the gold bumps.

[0032] Secondly, during ultrasonic welding, the transmission of ultrasonic energy on the connection layer will be different in each bump, and there is a risk that not every bump can be completely welded. The equivalent bonding pressure on the bumps at the edge of the array is slightly greater than that in the center area, and the equivalent vibration of the bumps at the edge will gradually deviate from the center position. In other words, the load on each bump in multi-bump flip-chip welding is uneven. Therefore, the purpose of this device is not to level all the gold bumps to the same height. The leveling step is equivalent to using a leveling substrate to replace the substrate with pads to perform a "simulated flip-chip welding" before the formal flip-chip welding. When using the principle that energy is unevenly distributed on each bump during the hot pressing ultrasonic process, the bumps with higher height or closer to the edge of the chip obtain higher energy and produce larger deformation, while the bumps with lower height or closer to the center of the chip obtain lower energy and produce smaller deformation. The height of the gold bumps after "simulated flip chip welding" will be different according to the different ultrasonic energy distribution. Compared with leveling all gold bumps to the same height, the gold bumps leveled by this device can obtain more uniform energy in the subsequent flip chip welding process, achieving better welding effect, such as Figure 2 As shown in the figure, the maximum height of the surface profile of a single bump before the "pre-flip" treatment is 9.4μm, and after the treatment is 1.5μm; Figure 3 As shown in FIG. 1 , the maximum height difference between the bumps before the “pre-flip” process is about 5 μm, and after the process is 2 μm; Figure 4 and Figure 5 As shown, the maximum shear force before the "pre-flip chip" treatment is 0.5N·bump-1, and after the treatment is 0.53N·bump-1; therefore, the "pre-flip chip" treatment enhances the soldering strength of the gold bump flip chip.

[0033] Preferably, the leveling substrate is an alumina ceramic substrate. The chip bump material is gold, and the leveling substrate is made of alumina ceramic. The gold and alumina ceramics will not be welded to each other during the hot pressing and ultrasonic process. The gold bumps will not adhere to the leveling substrate during the leveling process. When the gold bumps are separated from the leveling substrate, the alumina ceramic leveling substrate will not pull the gold bumps, so that the welding surface of the gold bumps remains smooth after leveling, and the height of the gold bumps will not change before and after the separation of the gold bumps from the leveling substrate, thereby ensuring the leveling effect.

[0034] Specifically, the gold bumps are prepared by stud bumping or electroplating processes, so that the gold bumps can be attached to the soldering points.

[0035] Preferably, the pressure applied by the ultrasonic pressure head to the chip is positively correlated with the number of gold bumps on the chip. The pressure applied to a single gold bump is 0.4N-0.6N. The ultrasonic waves on the ultrasonic pressure head concentrate the energy of the welder on the gold bumps. After the gold bumps are heated, they are pressed onto the flattened substrate by the ultrasonic pressure head, thereby improving the consistency of the height of the gold bumps.

[0036] Specifically, the ultrasonic power of the ultrasonic pressure head is positively correlated with the number of gold bumps on the chip. The ultrasonic power on a single gold bump is 0.034W-0.042W. Too high ultrasonic power can easily cause the gold bump to heat up, which can easily cause the gold bump to fall off during leveling. Too low ultrasonic power can easily cause the gold bump to have a low temperature, resulting in an unsatisfactory leveling effect.

[0037] Preferably, the flattened substrate and the substrate with pads are placed on different wafer stages, and after the chip is flattened, the ultrasonic pressure head drives the chip to rise, and after the flattened substrate under the chip is switched to the substrate with pads through the wafer stage, the ultrasonic pressure head drives the chip to move down, and the chip is flip-chip soldered to the pads of the substrate with pads. This application does not require the introduction of other equipment, and only relies on a flip-chip soldering machine, such as the CB-600 model, with ultrasonic hot pressing function. Before the formal flip-chip welding, a leveling substrate is used to replace the substrate with pads to perform a "simulated flip-chip welding". During leveling, the principle of uneven energy distribution on each bump during the hot pressing and ultrasonic process is used. The bumps with higher height or closer to the edge of the chip obtain higher energy and produce larger deformation, while the bumps with lower height or closer to the center of the chip obtain lower energy and produce smaller deformation. The height of the gold bumps after the "simulated flip-chip welding" will vary depending on the different ultrasonic energy distribution. Compared with leveling all the gold bumps to the same height, the gold bumps leveled by this device can obtain more uniform energy in the subsequent flip-chip welding process, achieving better welding effect.

[0038] In a preferred embodiment, before the leveling and flip-chip bonding, it is necessary to adjust the parallelism of the chip with the leveling substrate and the substrate with pads. After the gold bumps on the chip are leveled by the leveling substrate, the height of the gold bumps will vary depending on the ultrasonic energy distribution. Therefore, it is necessary to ensure the parallelism of the chip with the leveling substrate and the chip with pads to improve the welding strength of the chip after welding.

[0039] In a preferred embodiment, the parallelism deviation requirement between the chip and the flattened substrate and the substrate with pads is ≤30 μm, and the gold bumps on the chip can be effectively flattened and can be soldered on the substrate with pads.

[0040] Specifically, after the ball implantation, the height difference of the gold bumps on the chip is ≤12 μm, so as to avoid the leveling step being affected by the excessive height difference of the gold bumps.

[0041] In a preferred embodiment, the parallelism between the leveling and the flip chip is adjusted by a coupling on the flip chip machine. This application requires additional equipment, which can be achieved only by the existing machine, and the parallelism is adjusted by using the coupling on the existing machine.

[0042] Of course, the present application may have many other implementations. Based on the present implementation, other implementations obtained by ordinary technicians in this field without any creative work are all within the scope of protection of the present application.

Claims

1. A method for enhancing the soldering strength of gold bump flip-chip soldering, characterized in that: The following steps are involved: Ball planting, preparing gold bumps on the chip surface; Leveling: Place the chip upside down on the leveling substrate and apply ultrasonic pressure on the chip through an ultrasonic indenter; Flip chip bonding: flip the chip onto the pads of a substrate with pads.

2. The method for enhancing the soldering strength of gold bump flip-chip soldering according to claim 1, characterized in that: The leveling substrate is an alumina ceramic substrate.

3. The method for enhancing the soldering strength of gold bump flip-chip soldering according to claim 1, characterized in that: The gold bumps are prepared by a stud bump method or an electroplating method.

4. The method for enhancing the soldering strength of gold bump flip-chip soldering according to claim 1, characterized in that: The pressure applied by the ultrasonic pressure head to the chip is positively correlated with the number of gold bumps on the chip, and the pressure applied to a single gold bump is 0.4N-0.6N.

5. The method for enhancing the soldering strength of gold bump flip-chip soldering according to claim 1, characterized in that: The ultrasonic power of the ultrasonic pressure head is positively correlated with the number of gold bumps on the chip, and the ultrasonic power on a single gold bump is 0.034W-0.042W.

6. The method for enhancing the soldering strength of gold bump flip-chip soldering according to claim 1, characterized in that: The leveling substrate and the substrate with solder pads are placed on different wafer platforms respectively. After the chip is leveled, the ultrasonic pressure head drives the chip to rise. After the leveling substrate under the chip is switched to the substrate with solder pads through the wafer platform, the ultrasonic pressure head drives the chip downward, and the chip is flip-chip soldered on the pad of the substrate with solder pads.

7. The method for enhancing the soldering strength of gold bump flip-chip soldering according to claim 1, characterized in that: Before the leveling and flip-chip bonding, the parallelism between the chip and the leveling substrate and the substrate with pads needs to be adjusted.

8. The method for enhancing the soldering strength of gold bump flip-chip soldering according to claim 1, characterized in that: The parallelism deviation requirement between the chip and the flattened substrate and the substrate with pads is ≤30 μm.

9. The method for enhancing the soldering strength of gold bump flip-chip soldering according to claim 1, characterized in that: After the ball implantation, the height difference of the gold bumps on the chip is ≤12 μm.

10. The method for enhancing the soldering strength of gold bump flip-chip soldering according to claim 1, characterized in that: The parallelism between the leveling and the flip-chip bonding is adjusted by a coupling on the flip-chip bonding machine.

Images