Wire-bonded Devices Research Articles

Comparative Study of Chloride and Fluoride Induced Aluminum Pad Corrosion in Wire-Bonded Device Packaging Assembly

The introduction of copper as wire bonding material brings about a new challenge of aluminum bond pad bimetallic corrosion at the copper/aluminum galvanic interface. Aluminum is well known to undergo pitting corrosion under halide-contaminated environments, even in slightly acidic conditions. This paper aims to study the corrosion morphology and progression of aluminum influenced by different halide contaminations in the presence and absence of galvanic contact with copper. We used a new corrosion characterization platform of the micropattern corrosion screening to simulate the copper wire bonding on the aluminum bond pad. The corrosion screening data and subsequent SEM–EDX analyses showed a striking difference in morphology and progression between chloride-induced and fluoride-induced aluminum corrosion. The corrosion products formed play a vital role in the resulting morphology and in sustaining further aluminum corrosion.

Accelerated reliability testing of Cu-Al bimetallic contact by a micropattern corrosion testing platform for wire bond device application

Accelerated reliability testing of integrated circuit (IC) packages, such as wire-bonded devices, is a useful tool for predicting the lifetime corrosion behavior of real-world devices. Standard tests, such as highly accelerated stress test, involves subjecting an encapsulated device to high levels of humidity and high temperature (commonly 85–121 ⁰C and 85–100% relative humidity). A major drawback of current reliability tests is that mechanistic information of what occurs between t = 0 and device failure is not captured. A novel method of in-situ investigation of the device corrosion process was developed to capture the real time mechanistic information not obtained in standard reliability testing [1]. The simple, yet effective methodology involves:•Immersing a micropattern or device directly into contaminant-spiked aqueous solution, and observing its morphological changes under optical microscope paired with a camera.•Short (2–48 h) time required for testing (compared to 24–300 h of standard tests).•No need for humidity chambers.

Mechanistic study of copper wire-bonding failures on packaging devices in acidic chloride environments

Microelectronic reliability requirements are tightening to a standard of near zero ppb defects due to the evolution of self-driving cars and wearable electronics. The successful transition from gold (Au) to copper (Cu) in wire-bonding introduces corrosion related reliability concerns to wire-bonded devices in integrated circuit (IC) packaging. This report studies the effect of bimetallic contact, Cu wire vs. aluminum (Al) bond pad, on the corrosion failure of Al bond pads when exposed to low ppm levels of chloride (Cl−) contamination. Real time corrosion screening was carried out on simulating Cu/Al micro patterns for time-dependent observation of corrosion progression in 5–20 ppm Cl− solutions at pH 5. Al in Cu/Al bimetallic couple corroded at accelerated rate compared to Al without bimetallic contact. Cathodic hydrogen evolution was found to be the key factor driving this aggressive Al bond pad corrosion under the influence of the peripheral Cu/Al bimetallic contact in acidic chloride solution. Chemically modifying the surface of Cu wires to prevent this H2 evolution reaction resulted in the inhibition of Al bond pad corrosion.

Galvanic Aluminum Bond Pad Corrosion in Copper Wire Bonded Device Assembly – Mechanism and Prevention

The explosion of microelectronics use in automobiles has made the microelectronic corrosion control more critical over the past decade. Wire bonded devices form an integral part of the microelectronic systems used in automobiles. With the reliability requirements of automobile microelectronics pushing towards ppb levels of failure, halide induced corrosion issues have to be controlled to achieve such high reliability goals. The corrosion of Aluminum bond pads in slightly acidic chloride solutions (ppm level) has been one of the main modes of wire-bonding failure in wire bonded devices. Earlier researches pointed out the possibility of intermetallic compound formation as the main reason for this corrosion. However, our investigation of the electrochemical nature of the corrosion led to the discovery that bimetallic contact of the aluminum bond pads with copper wires is the main reason for this type of corrosion. Copper and Aluminum being apart in the galvanic series can act as a galvanic corrosion cell under suitable conditions. Furthermore, the presence of halides causes de-passivation of Aluminum oxide layer making the Aluminum bond pad more susceptible to corrosion. In this report we introduce corrosion screening as a mimicking platform for studying wire-bond device corrosion. For the first time we reported that Hydrogen evolution is the cathodic reaction and is responsible for the explosive nature of this bond pad corrosion. By selective surface treatment of the Cu wires (cathodic part), we were able to prevent the corrosion by blocking the cathodic reaction and thereby stopping the electron flow required for the corrosion cycle to continue. The prevention treatment was then applied to commercial wire-bonded device and excellent corrosion prevention was observed. The corrosion inhibition coating exhibited high thermal stability up to 260 O C. We also report a novel testing method for corrosion testing of molded wire-bonded device with internal chloride ion contamination. Electrical continuity and delamination analysis showed that the chosen method of inhibition treatment was able to prevent corrosion even in cases of high chloride contamination and severe delamination. The results of the Pressure Cooker Test showed that the inhibitor coating is suitable for prevention of corrosion in even 100% Relative Humidity environments. Work is now in progress to apply these treatments to Palladium coated Copper wire bonded devices to check corrosion prevention ability of the reported corrosion prevention treatment.

Comparative Study of Chloride and Fluoride Induced Al Pad Corrosion in Wire Bonded Packaging Assembly

Microelectronics are used in virtually all electronic equipment’s nowadays ranging from medical instruments, automobiles, computers, cell phones and home appliances. Wirebonding process which connects the chip to the lead frame in IC packaging plays a very important role in the commercial production of IC’s. Chip usually consists of Al bond pads with Cu/Au wires. Thin film Al bond pads are easily vulnerable to corrosion in presence of contaminants and moisture. Bond pad corrosion is one of the common modes of corrosion failure in wire-bonded devices IC packaging. Improper rinsing, packaging and passivation defects allows moisture and contaminants into bondpads resulting in severe corrosion. Most common source of this corrosion comes from the presence of halide ions contamination as (Cl-, F-, Br- etc.) However, the corrosion morphology and the severity varies from different ions contamination as the chemistry governing the corrosion differs from one ion to another. Our research is focused on key findings on corrosion of Aluminum bond pad due to chloride (Cl-) and fluoride (F-) contamination. Our central motivation is to identify this F- corrosion mechanism, so that it will help to develop a strategy to prevent it and study the comparison between Cl- and F- Corrosion using Micro-pattern corrosion screening technique, Wire-bonded device (WBD) and other characterization technique such as GC-MS, SEM and tafel. GC-MS reveals that H2 gas evolution is observed during Cl- corrosion of the WBD, whereas little or no H2 evolution is observed during F- corrosion. SEM and EDX reveals no presence of Cl- in the corroded areas of its corrosion, whereas strong F- signal is observed in F- corrosion of the bondpads revealing, that the corrosion products formed on the surface has very poor solubility showing that Cl- and F- has a complete different corrosion morphology. Striking contrasts in corrosion morphology observed during the Cl- and F- corrosion were explained in the present work and will lead to better understanding of factors influencing the bond pad corrosion leading to the wire bond failure in IC packaging devices. [Fig. 1] Figure 1

Electrothermal coupling analysis and experimental verification for wirebond devices

The thermal performance of a powered wirebond device with package level and board level test specimens was investigated by both analytical and experiment methods. The effects of thickness and thermal conductivity of the molding compound and heat spreader attached to the top surface of the molding compound on the performance of the Au wire and silicon die were modeled and evaluated by three-dimensional electrothermal coupling analysis. An advanced quad flat no-lead (QFN) sample was selected to experimentally measure the maximum allowable current in Au wire for packages either with or without molding compound. Two failure modes, namely the fusing of the wire and the decomposition temperature of the molding compound, were established in analysis. A board level test specimen with a thermal test die was also employed to measure the real time package thermal performance. The major achievement of this work is in the complete combination of modeling, experiment, and optimization for thermal performance evaluation purpose of a powered wirebond device. Results of this physical analysis can provide a reliable and useful guide to estimate the maximum allowable currents in Au wires for a wirebond device under practical application conditions.

Effective decapsulation of copper wire-bonded microelectronic devices for reliability assessment

The wire bonding industry has made a major shift in wire materials from gold to copper, primarily due to cost concerns. Copper wire-bonds are now present in many commercial off-the-shelf (COTS) devices but minimally used in automotive, industrial, or military-grade applications due to lack of detailed understanding about reliability concerns. A thorough study of wire bond reliability includes performing bond shear and pull strength measurements before and after stress testing. This in turn requires a special decapsulation procedure for copper wire-bonded devices because, unlike gold, copper is chemically potent. Many techniques for copper wire-bonded device decapsulation exist and can be categorized into laser-, plasma-, and acid-based processes. This paper reviews some of these techniques and discusses the decapsulation mechanism, which involves decomposition of the epoxy resin. By understanding the decapsulation mechanism and available techniques, a unique decapsulation method was developed. The effectiveness of this method is presented along with scanning electron microscopy (SEM) images of the results, which indicate minimal etching of copper wire bonds. The critical parameters of this technique are also identified, a suitable range of input for each parameter is analyzed theoretically, and a design of experiment (DOE) is conducted to find optimal values for each parameter. Several SEM images are provided to show both the good and bad results from the DOE. An image method for measuring effectiveness of decapsulation is also presented.

Reliability of wire-bonded electronic devices in combined high temperature and vibrational environments

High temperature electronic systems need to operate in extreme conditions that exceed the current levels specified by the military and aerospace standards. In high temperature and wide operating temperature applications, electronics can suffer from failure modes caused by a combination of environmental and operational loadings such as temperature, vibration, humidity, pressure, external stresses, etc. This work reports the findings and observations of the investigation of the combined effects of thermal and vibrational environments on wire-bonded interconnections used in high temperature electronic components. The work is focused on combined testing at temperatures up to 250°C and vibration loading with the sine frequency swept cycles ranging from 5Hz to 2000Hz and acceleration up to 20g rms. Analysis is focusing on the wire bond connections which has shown an increasing occurrence of wire deformation at a test temperature of 250°C whilst being subjected to vibration, particularly for wires with large loop shapes. Wire bonded samples have been evaluated through visual inspection, electrical characterization and ball shear tests to identify failure mechanisms.

On the implementation of computed laminography using synchrotron radiation

Hard x rays from a synchrotron source are used in this implementation of computed laminography for three-dimensional (3D) imaging of flat, laterally extended objects. Due to outstanding properties of synchrotron light, high spatial resolution down to the micrometer scale can be attained, even for specimens having lateral dimensions of several decimeters. Operating either with a monochromatic or with a white synchrotron beam, the method can be optimized to attain high sensitivity or considerable inspection throughput in synchrotron user and small-batch industrial experiments. The article describes the details of experimental setups, alignment procedures, and the underlying reconstruction principles. Imaging of interconnections in flip-chip and wire-bonded devices illustrates the peculiarities of the method compared to its alternatives and demonstrates the wide application potential for the 3D inspection and quality assessment in microsystem technology.

The viability of anisotropic conductive film as a flip chip interconnect technology for MEMS devices

The use of anisotropic conductive film (ACF) and stud bumping to form interconnects between die and substrates is one variation of current flip chip technologies with potential applications to MEMS devices. The key concerns associated with ACF are its long-term mechanical stability and consistent electrical performance. During this study a process methodology was developed for using ACF interconnections on MEMS devices by investigating key electrical parameters, and a practical example was investigated by packaging a pressure sensor using ACF flip chip techniques. Results of process development and contact resistance measurements using glass substrates and the anisotropic conductive film Sony FP1526 are discussed and analyzed. All required processing steps, such as stud bumping, application of ACF and thermocompression bonding of the die and substrate, were carried out using a digital wire bonder and a bench-top flip chip system. FP1526 yielded an average contact resistance of 10.23 mΩ, a stray capacitance measurement of 10 femtofarad and less than 0.1% change in contact resistance after being encapsulated in parylene C and being immersed in DI water for 72 h. Finally, the performance of an ACF-packaged MEMS piezoresistive pressure sensor was compared to that of a conventional wire-bonded device and showed a significant improvement in temperature stability (<0.34% deviation in offset voltage) with essentially no change in sensitivity.

Impact of Die Attach Material and Substrate Design on RF GaAs Power Amplifier Devices Thermal Performance

The latest commercial applications for microelectronics use GaAs material for RF power amplifier (PA) devices. This leads to the necessity of identifying low cost packaging solutions with high standards for reliability, electrical, and thermal performance. A detailed thermal analysis for the wirebonded GaAs devices is performed using numerical simulations. The main interest of the study focuses on the impact of die attach thermal conductivity (1.0–50.0 W/mK), substrate’s top metal layer thickness (25–50 μm), and via wall thickness (25–50 μm) on GaAs IC device overall thermal performance. The study uses a two-layer organic substrate. The peak temperatures reached by the PA stages range from 99.6°C to 120.3°C, below the prohibitive/critical value of 150°C (based on 85°C ambient temperature). The increase of die attach thermal conductivity from 1.0 to 7.0 W/mK led to a decrease in peak temperatures of up to 18°C, with larger decay between 1 and 2.4 W/mK. The largest temperature differences were obtained by varying the thermal via thickness, as opposed to only increasing the top metal layer thickness. The peak temperatures and corresponding junction-to-ambient thermal resistances are thoroughly documented. With the same die attach thickness, for a thermal conductivity much larger than 7 W/mK, the impact on the PA’s peak temperature is insignificant. The die attach solder material (with a large thermal conductivity) leads to only a small (2.5°C) decrease in the PA junction temperature.