Learn About the Gold Nail Head Bump (SBB) Technology of Flip Chip

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Learn About the Gold Nail Head Bump (SBB) Technology of Flip Chip

This article details the gold nail head bump technology in flip chip technology.

I. the development of semiconductor packaging technology
Microelectronic packaging technology has developed along with the development of device forms, and its development history is also a history of continuous improvement of device performance and continuous miniaturization of systems. From the classification of the installation method of the device on the substrate, microelectronic packaging can be divided into the following development stages:
The first stage was the through-hole mounting (THD) era before the 80s of the 20th century, represented by TO type packages and dual in-line packages. The function of the IC is relatively simple, the number of leads is small, the package can be manually inserted into the through hole of the PCB, the lead pitch is fixed, the increase in the number of leads will mean an increase in the package size, and the maximum mounting density of the package is 10pin/cm2.
The second stage was the surface mount (SMT, surface mount/surface mount) era in the 80s of the 20th century, which was represented by small outline package (SOP) and flat package (QFP), which greatly increased the number of pins and assembly density, and was a revolution in packaging technology at that time. The design concept of these packages differs from the DIP (Dual In-Line Package) in that the size of the package body is fixed, and the lead pitch around the perimeter varies as needed, which also improves productivity, with a maximum lead count of 300 and a mounting density of 10-50pin/cm2, which is also the golden age of metal leaded plastic packaging.
The third stage is the solder ball array package (BGA) / chip size package (CSP) era in the 90s of the 20th century, the lead pitch of BGA is mainly 1.5mm and 1.27mm, the expansion of the lead pitch greatly promotes the progress of installation technology and the improvement of production efficiency, the installation density of BGA package is about 40-60pin/cm2, and then Japan used the concept of BGA at the chip level and developed a CSP package with a smaller lead pitch, The lead pitch can be as small as less than 1.0mm, and the CSP package further reduces the size and weight of the product, improving the competitiveness of the product, and the BGA era has transitioned to the BGA/CSP era.

There are four main technologies to achieve chip-scale packaging: Wire Bonding (WB), Tape Automated Bonding (TAB), Flip Chip (FC), and Through Silicon Via (TSV). WB technology refers to the bonding of metal leads and pads under the action of ultrasonic, and is divided into thermoultrasonic spherical bonding and ultrasonic wedge bonding according to the bonding method. WB accounts for 90% of the chip scale packaging market due to its excellent reliability, but because the connection formed by wire bonding has a certain height that affects the size of the package, generates the delay of the electrical signal, and increases the resistance value, the search for a new primary packaging technology suitable for small size packaging has become a research hotspot.
TAB technology is a technology that bonds chips to carrier tapes in a single time with lead strips under a hot stamping die, and the manufacturing of metal bumps, carrier tapes and hot stamping dies in this technology brings great challenges to mass production.
TSV technology is an emerging new technology, the connection in this technology mainly depends on the connection between the Cu bump and the pre-plated Au layer in the through-silicon via, which is suitable for 3D laminated packaging, and because the electronic products have high requirements for package size, the size of the through-silicon vias is very small, so the uniformity of the Au coating in the through-silicon vias and the reliability of the connection have brought great challenges to the development and application of this technology.

II. FlipChip technology
FlipChip (FC) technology is a method of inverting the active side of the chip to align the substrate for micro-connection, the inversion of the active device reduces the package size of electronic products, and due to the controllable size of the solder joint, this method is suitable for the packaging of high-integration and high-power electronic products with fine pin pitch. The schematic diagram of the flip chip is as follows:

In order to realize the flip chip process, it is necessary to realize the production of bumps on the surface of the chip, and there are six common bump formation methods: Stud Bump Bond, evaporation solder bump, electroplating solder bump, printed solder bump, ball bump, and solder transfer bump. In the mobile phone camera packaging we use in daily life, the technology used to connect the imaging chip to the substrate is the gold nail head bump (SBB) connection in the flip chip:


III. What is SBB(SBB,Stud Bump Bond)?
The fabrication of flip chip nail head bump is to use an airless balloon (SBB,Stud Bump Bond) formed by metal wire to interconnect the chip I/O port with the package pin or the wiring solder area on the substrate;
Under the combined action of ultrasonic energy, bonding pressure and other factors, the oxides and dirt on the surface of the bonding interface are removed, and the plastic deformation of the bonding interface occurs at the same time, so that the dislocation occurs in the metal of the bonding interface and the atomic diffusion is stimulated, forming a firm metal flip nail head bump.
For gold wire hot pressing ultrasonic bonding, the diameter of gold wire is generally between 0.5mil ~ 2.5mil (1mil = 25μm), the material of flip chip pad is generally aluminum pad (there are also gold plated pads), and the surface is plated with aluminum (gold) with a thickness of about 2μm.
The following diagram shows the equipment and accessories required to make the gold-plated bumps, including the bonding machine, capillary (Capillary capillary), and gold wire:

Among them, the Kulicke and Soffa (KS) type bonding machine is more widely used in the bonding machine industry, and the internal structure of the equipment is shown below:

The welding head part of the equipment is the key part of the production of the gold nail head bump, as shown in the figure below, the welding head part includes a lead tensioner, a glass lead tube, an electrode (also known as a lighter), a capillary (also known as a capillary), and a gold wire clamp.

In the bonding preparation stage, the wire clamp is opened, and the heating block has been heated to a certain temperature; The capillary is moved down some distance so that the mouth of the capillary is close to the lighter. At this time, the electronic ignition system releases about 2000V of high-voltage electricity in a very short time, so that a loop is formed between the gold wire tail wire at the tip of the capillary and the electrode of the lighter, so that a small section of gold wire exposed to the capillary mouth is formed under the action of current FAB (Free air ball), and then the capillary continues to move downward, so that the FAB is in contact with the chip pad, and the FAB forms a fixed pie shape under the action of bonding pressure, and then the pressure of the capillary is reduced. The ultrasonic energy begins to act to form a firm connection between the FAB and the pad, after the bonding is completed, the capillary moves up a distance so that the capillary nozzle can leave a small piece of gold wire, so as to form a FAB for the next bonding spark plug work, the capillary stops rising after moving up a distance, the wire clamp tightens the gold wire, the capillary continues to move up with the wire clamp and the gold wire, and the gold wire is broken in the process of moving up, leaving a nail head bump.
The first layer of gold nail head bump is bonded on the aluminum pad, and on the basis of completing the first layer of gold nail head bump, the bonding of the second layer of gold nail head bump is made to realize the bonding of the entire laminated gold nail head bump, and the whole bonding process is similar to the bonding process of the first layer of gold nail head bump. The bonding process of gold nail head bumps is mainly affected by the bonding pressure, bonding power and bonding time. The formation process of the gold nail head bump is shown in the following figure:

The gold nail head bump bonding process is mainly divided into three stages: the first stage is the collision stage, that is, the bonding pressure concentration stage, which is characterized by the maximum bonding pressure and the bonding power is not applied in this stage. The second stage is the bonding preparation stage, where the capillary will prepare for the bonding action between the bump of the gold nail head and the pad; At this stage, the bonding pressure decreases. The third stage is the bonding stage, which is the stage where the gold bump and the pad form a bond, and the stage where the bonding power and bonding pressure work together; At this stage, the capillary begins to move violently under the action of ultrasound, the bonding surface is destroyed, and a strong bond is formed quickly in a very short time.

IV.Factors influencing the bump of the gold nail head
1,The choice of capillary
In the flip laminated gold nail head bump bonding process, the consistency of each gold nail head bump bond is a key factor to ensure the success of the bond. The size of the capillary determines the bonding characteristics of the laminated gold nail head bump and the geometric characteristics of the gold nail head bump. Therefore, in order to obtain a gold nail head bump with good bond consistency, it is necessary to choose a suitable capillary. The size of the capillary hole (H), the diameter of the chamfer (CD) and the chamfer angle (CA) are usually the most important reference factors for the selection of the capillary.
The below figure is relevant parameter of Kulicke and Soffa(a capillary)：


2,The effect of the first layer of gold nail head bump effect
Flip laminated gold nail head bump bonding is to complete the first layer of gold nail head bump bonding, and then the second layer of gold nail head bump bonding, that is, the laminated gold nail head bump is composed of the first layer of gold nail head bump and the second layer of gold nail head bump. The following figure is a microstructure diagram of the first layer of gold nail head bump and the laminated gold nail head bump respectively.
The first layer of gold nail head bump bonding is a component of laminated gold nail head bump bonding, and the quality of the first layer of gold nail head bump bonding and its size parameters have an impact on the second layer of gold nail head bump bonding.


The key size parameters of the first layer of gold nail head bump are shown in the figure below, where d is the diameter of the gold wire, which is determined by the gold wire used for bonding, the height h is determined by the shape of the bonding capillary, and the height of the gold nail head bump H and the maximum radial diameter D of the gold nail head bump are jointly determined by the bonding process parameters.

Improving the quality of the gold nail head bump is mainly through the optimization of the following factors：
(a)(Bump Placement)
(b)(Bump Shear)
(c)( Bump Diameter)
(d)(Bump Thickness)
(e)(Bump Height)
(f)(Crater Test)
(g)(IMC)
The measurement of the thrust of the golden ball is tested according to the following picture:

For some common problems in practical applications, they can be improved from the following aspects:


V. Simulation analysis of gold nail head bumps
Through the simulation and analysis of the whole process of gold wire bonding, the simulation data is shown in the following figure:

In the collision stage of gold nail head bump, the stress distribution of gold nail head bump is uneven, and the stress level is relatively high, and the area with large stress level is located inside the gold nail head bump and the contact surface between the gold bump and the pad, and these areas are the areas where the bonding pressure is concentrated.
The following figure is a positive view of the pad stress distribution, the stress of the pad is concentrated in the circular area with the bonding center as the center of the circle, in which the larger stress is distributed in the peripheral area of the circle, and there is a clear boundary with the smaller stress area, where the deformation of the pad will be more intense, and the violent deformation will cause more dislocations and make the bond easier to form bonds. The image on the right shows the bond trace of the gold nail head bump. The off-white area is the bond-forming area, and it can be seen that the bonding is mainly formed in the peripheral area of the concentric circle centered on the geometric center of the pad, which corresponds to the larger stress distribution area during the flip gold nail head bump bonding process.

The bonding of flip laminated gold nail head bumps is to complete the second layer of gold nail head bumps on the basis of completing the first layer of gold nail head bumps. During the whole bonding process, the capillary has an effect on both the stress and strain of the first layer of gold nail head bumps and the second layer of gold nail head bumps.

As shown in the following figure, in the flip laminated gold nail head bump bond collision stage, the higher stress level of the flip laminated gold nail head bump is mainly distributed in the inside of the two gold nail head bumps close to the contact surface of the upper and lower gold nail head bumps, in which the larger stress is concentrated in the second layer of gold nail head bumps, and the maximum stress appears at the bonding contact surface of the first layer of gold nail head bumps and the second layer of gold nail head bumps.

VI,Sixth, the gold nail head bump technology summary
Compared with the traditional wire bonding technology, the bump electrodes in the bonding solder zone of flip chip bonding technology are not only distributed along the edge around the chip, but can be distributed by rewiring, so flip chip bonding technology has the following advantages:
(1)The interconnect wires are very short, and the stray capacitance, interconnect resistor, and interconnect inductance generated by the interconnect are much smaller than those of the WB. So as to be more conducive to the application of high-frequency and high-speed electronic products.
(2)Chip-mounted interconnects occupy a small substrate area and have a high chip-mounted density.
Bibliography：
(1)Kong Lingsong: Research on Quality Control of Gold Bump Thermal Ultrasonic Flip Chip Bonding (2) Wang Jiao, Forming and Interface Reaction Mechanism of Nail Head Bump _Sn-based Brazing Metal Joints
(3)Tang Wenliang, Simulation and reliability research of flip laminated gold nail head bump key

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