The effectiveness and dependability of an integrated circuit (IC) chip in its various applications are heavily reliant on the quality of its wire bond interconnection. A robust and reliable wire bond interconnection is essential for ensuring the seamless performance and long-term functionality of the IC chip. Conversely, a poor and inconsistent wire bond interconnection can have a detrimental impact on the reliability and dependability of the device, potentially leading to operational failures and malfunctions.
Understanding the determinants of wire bond quality is crucial in ensuring the overall reliability of IC chips. The quality of a wire bond is intricately tied to the strength and integrity of the interfaces between the bond wire and the bond pad, as well as the bond wire and the lead finger. These interconnections play a critical role in maintaining the structural and electrical integrity within the IC chip. Any weaknesses or inconsistencies in these interfaces can compromise the performance and reliability of the device, highlighting the significance of maintaining high-quality wire bond interconnections.
To evaluate and ensure the quality of wire bond interconnections, a comprehensive set of testing methodologies and procedures have been established. These methodologies serve as critical tools for assessing and validating the reliability of wire bond interconnections, ultimately contributing to the overall performance and dependability of IC chips.
One of the established testing methodologies for evaluating wire bond interconnections is the bond pull test. This test involves subjecting the bond wire to a controlled force to measure its adhesion strength and evaluate its ability to withstand mechanical stress. By conducting bond pull tests, manufacturers and engineers can gain valuable insights into the reliability and durability of wire bond interconnections, identifying any potential weaknesses or defects that may compromise the functionality of the IC chip.
Similarly, the bond shear test is another essential method employed to assess the quality of wire bond interconnections. During this test, a shear force is applied to the bond wire to test its resistance to lateral stress. The results of bond shear tests aid in evaluating the stability and robustness of wire bond interconnections, ensuring that they can withstand the mechanical stresses encountered during operation.
In addition to mechanical testing, visual inspection remains a crucial component of evaluating wire bond quality. Visual inspections allow for the detection of visible defects or irregularities in the wire bond interconnections, such as bond wire looping, lifted bonds, or improper bonding geometries. Identifying and addressing these anomalies during visual inspections is vital for maintaining the high-quality standards of wire bond interconnections and safeguarding the reliability of IC chips.
Furthermore, specialized testing procedures play a pivotal role in assessing the quality of wire bond interconnections, addressing specific aspects related to their reliability and performance under varying conditions. These specialized tests may encompass advanced techniques such as bond ultrasonic testing, thermal cycling tests, or wire bond cratering analysis, providing in-depth analysis of wire bond interconnections and their ability to withstand environmental and operational challenges.
Therefore, the significance of wire bond interconnections in determining the reliability and performance of IC chips cannot be understated. Employing established testing methodologies such as bond pull tests, bond shear tests, and visual inspections, as well as specialized testing procedures, underscores the commitment to ensuring the high-quality and consistent nature of wire bond interconnections. By rigorously assessing and validating the reliability of wire bond interconnections, manufacturers and engineers can instill confidence in the overall dependability and performance of IC chips across a diverse range of applications, ultimately contributing to the advancement of electronic technology.
The Bond Pull Test
The bond pull test is widely used to assess and regulate the quality of wire bonds. It is also utilized for controlling the wire bonding process and optimizing it during assembly manufacturing. This test involves placing a hook under the wire and applying a normal upward force until the wire fails, as depicted in figure 1.
Figure 1. Bond pull test
If a ball lift failure or weld lift failure occurs during the bond pull test, even at very high pull values, it is considered unacceptable and indicates poor bond quality. In such cases, thorough analysis of the cause of the non-stick is required, and appropriate remedial action must be taken. Figure 2 illustrates examples of ball lift and weld lift failures.
Figure 2. Ball lift and wedge (weld) lift failure
Failure Modes and their Interpretation in Pull Tests
The gold ball bond pull test can manifest different types of failures, including ball lift (Type A), ball neck break (Type B), mid span break (Type C), heel break (Type D), and weld lift (Type E), as illustrated in figure 3. In some cases, failures may also occur at other interfaces, such as the lift-off of bond pad metallization, caterings under the bond pad, and peel-off of lead finger metallization. Let's delve into a detailed review and discussion of these five failure modes.
Ball Lift Failure
The occurrence of ball lift failure signifies a very weak interface between the ball and the bond pad. This type of failure indicates either a lack of bond formation or a weakened bond. Various factors can contribute to ball lift, including poor wire bonder setup and contamination of the bond pad surface. Inadequate setup, characterized by improper parameter settings, unstable work piece holders, and worn-out tools, can lead to subpar initial welding and insufficient intermetallic formation between the bond pad and the ball.
Ball Neck Failure
The failure of the wire just above the ball neck, as depicted in figure 4, is the predominant failure mode observed during bond pull tests. Neck breaking involves the severing of the wire from its ball bond due to a fracture in the neck, which is the portion where the wire meets the ball bond. This type of failure is often attributed to poor wire bonder setup, such as improper bonding parameter settings, bonehead movement settings, and worn-out or contaminated tools. Incorrect bonding parameters can result in excessive deformation of the bond, leading to a thin, weak, or cracked neck that is prone to fracture. Crack propagation can also be induced by wire defects caused by worn-out and contaminated tools.
Figure 4. Shows neck break, midspan break and heel break
Wire Break at Mid Span
The failure of the bond wire at the midspan, as shown in figure 4, indicates that the wire failed due to plastic deformation and ductile fracture, as opposed to brittle fracture at the heel or the ball neck. Typically, this type of failure yields the highest bond pull value, closely approaching the ultimate tensile strength (UTS) of the wire.
Heel Break
Heel breaking involves the severing of the wire from its wedge or crescent bond due to a fracture in the heel, which is the portion where the wire tapers off into the bond. During bond pull tests, heel break failure, as shown in figure 4, usually indicates damage to the heel area. Improper bonehead movements and low loop settings can subject the wires to excessive stresses, pulling them backward and away from the bonds, resulting in significant heel cracks that may propagate to fracture. The presence of a micro crack at the heel is a major cause of heel break and may be formed during bonding. High bonding force and excessive ultrasonic energy due to bonding parameters, as well as damaged, clogged, or overused capillary, are potential causes of micro cracks.
Weld Lift
Weld lifting refers to the detachment of a wedge bond from the bond pad or bonding post, or the crescent bond from the lead frame bonding finger, as depicted in figure 5. During bond pull tests, weld lift is an indication of improper process optimization. Similar to ball lift failure, weld lift, even at very high bond pull values, is indicative of poor quality and should not be accepted. Causes of weld lifting may include low bond parameters, contamination on the lead frame, hard metallization, or improper ultrasonic coupling between the capillary and the lead finger, possibly due to lead frame fragility or improper clamping during bonding.
Figure 5. Weld lift
Ball-Bond Shear Test
The destructive wire bond pull test is limited in its ability to provide comprehensive information regarding the strength and overall quality of the initial bond. Although a simple wire pull test does not fully capture the characteristics of a ball bond, high-quality ball bonds can withstand up to ten times the force required to destruct a wire bond, while low-quality bonds still require more force to be pulled off than in a wire pull test. Thus, pull tests are ineffective in determining the true strength of ball bonds. As a result, the development of a complementary wire bond test methodology, known as the ball-bond shear test, has been prompted.
Ball shear testing is utilized to evaluate the integrity of the gold ball-to-bonding pad interface in the gold ball bonding process. This destructive test provides data on intermetallic formation and bond coverage, measured in gram force over the area of the ball formation.
Description of the Test
The test involves the utilization of a shear tool, which applies sufficient force to the ball bond during testing. Proper positioning of the tool is crucial during shear testing, with the shear ram being strategically placed just above the bond pad and in close proximity to the center of the ball. The shear tool then moves parallel to the bonded surface, shearing the ball bond. Figure 6 provides a visual depiction of the placement of the shear tool.
Improper positioning of the tool is a common issue in shear testing. The positioning of the shearing ram during the ball shear test must adhere to the specified criteria in the ball shear test specification. For a normally deformed ball, the tool should approach from 2.5 to above the substrate, and for large, high balls, it must not be positioned any higher, as doing so could cause the tool to drag on the substrate. Figure7 illustrates the step-by-step movement of the shear ram and the failure modes of the ball bond.
Figure 6. Ball shear test
The observed failure modes during a ball shear test include ball lift/partial metallization lift off, ball shear (weld interface separation), bond pad lift (substrate metallization removal), and cratering. Each of these failure modes will be discussed in further detail.
1.Ball lift/Partial Metallization Lift Off
Ball lift is defined as a separation at the bonding pad interface with little or no intermetallic formation present or remaining. Figure7 (a) illustrates part of the Al metallization being sheared with the ball, leaving partial metallization on the bond pad during the shear test.
2. Ball Shear (Weld Interface Separation) During the ball shear test, if pronounced intermetallic formation is present, the ram may shear the entire ball, leaving gold or an intermetallic on the bond pad, as depicted in figure7 (b).
3. Bond Pad Lift (Substrate Metallization Removal) Another failure mode during the shear test is bond pad lift, where separation between the bond pad and the underlying substrate can be observed. The aluminum metallization is peeled off, leaving it exposed. This may occur due to improper adhesion of the aluminum metallization. In this failure mode, the interface between the ball bond and the residual pad attached to the ball remains intact, as illustrated in figure7(c).
4. Cratering Cratering is a general term used to describe a form of bonding failure resulting from mechanical damage to the bond pad or underlying material. This type of failure occurs on the underside of a wire bond during bond pull or shear test, typically as a result of cracks generated under the bond pad during bonding. Figure7 (d) shows the cratering effect during the ball shear test.
Figure 7. Shear test failure modes (a) Ball lift (b) Ball shear (c) Bond pad lift (d) Cratering
Visual Inspection of Bond Quality
The inspection of wire bonding has traditionally been carried out visually, as human ability to identify faults has been superior to the capabilities of available machines. This method is utilized to ensure the proper formation of both ball and wedge bonds. Visual indications of issues such as cracked heels, wedge tearing, misplaced wires, inconsistent wire placement, "golf-clubbing" at the gold ball, excessive neck down, and others signify that the wire bonding process is not under control. Visual inspection also verifies the proper placement of bonds in relation to the bond pads and bond fingers of the lead frame/substrate. Additionally, visual inspection screens potential bond defects that could lead to open or short circuits, based on specific criteria related to wire clearance and the proximity of each bond to others.
As previously mentioned, bond pull and bond shear tests are utilized to assess the quality of bonded interfaces, both on the chip and the lead finger. However, there are numerous other failure modes that can only be identified through visual inspection, such as deformed bonds, wire bonding and probe marks. As a result, visual inspection is critically important in today's wire bonding process. Inspection of bond quality encompasses both "Pre-Bonding Inspection" and "Post-Bonding Inspection." Pre-Bonding Inspection is essential for ensuring process consistency and reliability, while Post-Bonding Inspection is necessary to ensure package reliability remains intact.
Pre-Bonding Inspection
In the final stages of integrated circuit manufacturing, it is crucial to conduct inspection of the probe marks on bonding pads resulting from electrical testing of the integrated circuit wafers. This pre-bonding inspection aims to identify potential problem areas, such as probe marks that extend beyond the pad boundary, excessively large marks, or scratch marks introduced during electrical testing. The evaluation of various parameters, including pad size, center location, mark size and coverage percentage, as well as the largest mark's size and location, facilitates the inspection of bond pads to quantify these defects.
Additional issues in the bonding area that can impact bond quality are the discoloration of bond pads due to corrosion and the presence of photo resist residue. Visual inspection is essential for identifying and addressing these problems. Photo resist residue is not visible under normal lighting conditions, but using polarized UV lighting under a microscope enables the identification of passivation and photo resist residue problems.
Post Bonding Inspection
Following the bonding process, a range of problems may arise that can only be identified through visual inspection. Post-bonding inspection encompasses the identification of bond quality issues, such as smashed bonds, skinny bonds, misplaced and deformed bonds, as well as wire conditions including loop height. Visual inspection can be carried out manually with the assistance of an operator or with automatic visual inspection systems.
Special Tests for Quality
The industry employs a range of methods to assess the quality of bonds. Visual inspection, bond pull, bond shear test, bond etching, electrical testing, bake test, thermal cycle/thermal shock testing, electromagnetic resonance, and surface analysis are among the most commonly utilized techniques. While some of these tests are suitable for monitoring production processes, others are specifically designed for laboratory use in the evaluation and advancement of wire bond technology.
Etching of Bonds
The evaluation of layers beneath bond pads following bonding is achieved through bond etching. This method involves removing the ball (wire) and pad metallization to inspect the underlying materials for defects. Wet etching serves not only to detect subsurface damage resulting from equipment parameters, but also as a means to optimize bonding parameters. Typically, chemical solutions such as NaOH or KOH are employed to eliminate the interconnection material and expose the underlying structure. For instance, in assessing cratering beneath the bond pad, the bond pad metallization is removed using a potassium hydroxide (KOH) solution.
Electrical Evaluation
The quality of the bond is assessed through electrical testing. A bond that does not adhere properly leads to an electrical open, while two adjacent bonds in contact result in an electrical short. Specialized techniques like "Four Point Probe Testing" are used to measure the electrical resistance of the Au ball-Al bond pad interface. The resistance increases with the growth of intermetallic compound, and an open circuit failure indicates extensive voiding at the interface.
Thermal Testing
The bake test quantifies the formation of intermetallic compounds between gold and aluminum (Au/Al). Excessive diffusion of aluminum atoms into gold can cause Kirkendall voiding at the Au/Al interface, leading to bond lifting. By subjecting bonded devices to high temperatures (175°, 240°, or 300°) for varying durations, one can assess the long-term reliability of the bond.
Surface Analysis
Surface analysis techniques, such as energy-dispersive X-ray analysis (EDX), wave-length dispersive X-ray analysis (WDX), auger, and scanning electron microscopy (SEM), are employed as failure analysis tools to detect contamination, determine the extent of intermetallic compound formation, and identify bond irregularities.
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