Masood Murtuza

Computer Aided Stress Modeling for Optimizing Plastic Package Reliability

A computer-aided stress analysis program has been applied to reliability prediction of VLSI plastic packages. The process of plastic encapsulation and the testing by temperature cycling produce stresses in the silicon chip as well as in the molding material. These stresses must be minimized through specific choices of material and package design. Finite element stress modeling is used to quantify and display the effect of material choices, form factors, and innovative processing techniques. Excellent correlations have been achieved between the model predictions and the actually observed failures and quantitative stress measurements using strain gauge test structures. The mechanical and thermal problems associated with VLSI assembly and packaging can then be minimized.

Reliability of lead-free SAC electronics under simultaneous exposure to high temperature and vibration

Electronics installed in automotive systems are subjected simultaneously to mechanical vibrations and thermal loads in underhood applications. Typical failure modes include solder joint failure, pad cratering, chip-cracking, copper trace fracture, and underfill fillet failures. The solder interconnects accrue damage much faster when vibrated at elevated temperatures. Industry migration to lead free solders has resulted in a proliferation of a wide variety of solder alloy compositions. Presently, the literature on mechanical behavior of lead free alloys under simultaneous harsh environment of high-temperature vibration is sparse. In this paper, a test vehicle with a variety of lead-free SAC305 daisy chain components including BGA, QFP, SOP, TSOP has been tested to failure by subjecting it to two elevated temperatures and harmonic vibrations at its first natural frequency. The test matrix includes variation in the amplitude of vibration from 10G to 14G as well as variation in temperature. Full field strain on the PCB has been extracted using high speed cameras operating at 100,000 fps in conjunction with digital image correlation. The vibration simulation using global-local finite element models is correlated with the system characteristics such as modal shapes and natural frequencies. The vibration simulation provides a fatigue life prediction that has been validated with the experimentally obtained cycles to failure. In addition, the packages have been cross-sectioned to study the failure modes. A comparison of failure modes for different surface mount packages at elevated test temperatures and vibration has been presented in this study.

Prognostication of copper-aluminum wirebond reliability under high temperature storage and temperature-humidity

Gold wire bonding has been widely used as first-level interconnect in semiconductor packaging. The increase in the gold price has motivated the industry search for alternative to the gold wire used in wire bonding and the transition to copper wire bonding technology. Potential advantages of transition to Cu-Al wire bond system includes low cost of copper wire, lower thermal resistivity, lower electrical resistivity, higher deformation strength, damage during ultrasonic squeeze, and stability compared to gold wire. However, the transition to the copper wire brings along some trade-offs including poor corrosion resistance, narrow process window, higher hardness, and potential for cratering. Formation of excessive Cu-Al intermetallics may increase electrical resistance and reduce the mechanical bonding strength. Current state-of-art for studying the Cu-Al system focuses on accumulation of statistically significant number of failures under accelerated testing. In this paper, a new approach has been developed to identify the occurrence of impending apparently-random defect fall-outs and pre-mature failures observed in the Cu-Al wirebond system. The use of intermetallic thickness, composition and corrosion as a leading indicator of failure for assessment of remaining useful life for Cu-al wirebond interconnects has been studied under exposure to high temperature and temperature-humidity. Damage in wire bonds has been studied using x-ray Micro-CT. Microstructure evolution was studied under isothermal aging conditions of 150°C, 175°C, and 200°C till failure. Activation energy was calculated using growth rate of intermetallic at different temperatures. Effect of temperature and humidity on Cu-Al wirebond system was studied using Parr Bomb technique at different elevated temperature and humidity conditions (110°C/100%RH, 120°C/100%RH, 130°C/100%RH) and failure mechanism was developed. The present methodology uses evolution of the IMC thickness, composition in conjunction with the Levenberg-Marquardt algorithm to identify accrued damage in wire bond subjected to thermal aging. The proposed method can be used for quick assessment of Cu-Al parts to ensure manufactured part consistency through sampling.

Microstructural Indicators for Prognostication of Copper–Aluminum Wire Bond Reliability Under High-Temperature Storage and Temperature Humidity

Gold wire bonding has been widely used as the first-level interconnect in semiconductor packaging. The increase in the gold price has motivated the industry search for an alternative to the gold wire used in wire bonding and the transition to a copper wire bonding technology. Potential advantages of transition to a Cu-Al wire bond system include low cost of copper wire, lower thermal resistivity, lower electrical resistivity, higher deformation strength, damage during ultrasonic squeeze, and stability compared with gold wire. However, the transition to the copper wire brings along some tradeoffs, including poor corrosion resistance, narrow process window, higher hardness, and potential for cratering. Formation of excessive Cu-Al intermetallics may increase the electrical resistance and reduce the mechanical bonding strength. Current state of the art for studying the Cu-Al system focuses on the accumulation of statistically significant number of failures under accelerated testing. In this paper, a new approach has been developed to identify the occurrence of impending apparently random defect fall-outs and premature failures observed in the Cu-Al wire bond system. The use of intermetallic thickness, composition, and corrosion as a leading indicator of failure for the assessment of the remaining useful life for Cu-Al wire bond interconnects has been studied under exposure to high temperature. Damage in the wire bonds has been studied using an X-ray micro-Computed Tomography (CT). Microstructure evolution was studied under the isothermal aging conditions of 150 °C, 175 °C, and 200 °C until failure. Activation energy was calculated using the growth rate of intermetallic at different temperatures. An effect of temperature and humidity on a Cu-Al wire bond system was studied using the Parr bomb technique at different elevated temperature and humidity conditions (110 °C/100%RH, 120 °C/100%RH, and 130 °C/100%RH), and a failure mechanism was developed. The present methodology uses the evolution of the intermetallic compound thickness and composition in conjunction with the Levenberg-Marquardt algorithm to identify accrued damage in wire bond subjected to thermal aging. The proposed method can be used for a quick assessment of Cu-Al parts to ensure manufactured part consistency through sampling.

Degradation mechanisms in electronic mold compounds subjected to high temperature in neighborhood of 200°C

Plastic encapsulated microelectronics (PEMs) has found wide spread applications in automotive environments for varied roles. Transition to hybrid electric vehicles and fully electric vehicles has increased the trend towards greater integration of electronics in automotive under hood environments. Electronics in such applications may be mounted directly on engine and on transmission. Electronics under hood may be subjected to temperatures in neighborhood of 200°C. Commercially available PEMs are able to operate in the neighborhood of 175°C. However, sustained operation at temperatures of 200°C or higher is beyond the state of art. Materials and processing techniques needed for sustained high temperature operation for 10 years and 100,000 miles of vehicle operation are yet unknown. There is need for studies for understanding the failure mechanisms of PEMs at sustained high temperature. In this paper, new approach is discussed to study physical and chemical stability of molding compound when it is subjected to very high temperature for prolonged duration. Four mold compound candidates were selected for test purpose. They were subjected to thermal aging at 200°C and 250°C, for 5000 hours. For degradation study, bulk mold compound specimens as well as 20 pin SOIC devices, encapsulated with MC candidates were used. Test vehicle was bonded with gold wires, and Pd coated Al pad. For bulk mold compound samples, weight loss test, DMA, FTIR, XPS tests were performed at fixed time intervals. To study integrity of SOIC devices, resistance spectroscopy, x-ray inspection and current leakage tests were selected. Another set was subjected to 120 hours of aging at 130°C/100%RH condition to check leakage current. Performance of MC candidates at high temperature was evaluated using all these tests. Sensitivity of each test towards detecting degradation of EMCs is also discussed and most effective tests are suggested.

Root Cause of Black Pad Failure of Solder Joints with Electroless Nickel/Immersion Gold Plating

Publications on the so called black pad defect, seen in electroless nickel/immersion gold (ENIG) surface finishes, often cite high phosphorous (P) content in the nickel (Ni) plating as a key factor in the defect. Therefore, one proposed solution is to decrease the P content in the electroless Ni plating. In a contrary approach, some researchers propose a high P content to avoid the black pad defect. In the present work, the solder reaction with ENIG plating and the resulting interfacial structures were studied. Focused ion beam (FIB) was used to polish the cross sections to reveal details of the microstructure of the ENIG plated pad with and without soldering. High speed pull testing of solder joints was performed to expose the pad surface. Results of SEM/EDX analysis of the cross sections and fractured pad surfaces support the suggestion in the literature that black pad is the result of galvanic hyper-corrosion of the plated electroless Ni by the gold (Au) plating bath. High P content in the fractured surface on the pad side is not the signature of black pad. New criteria are proposed for diagnosing black pad in ENIG

Performance comparison of thermally enhanced plastic (TEP) packages

Plastic packages have long been the mainstay of VLSI packaging due to a combination of important characteristics such as cost, reliability, and mass producibility. However standard plastic packages and higher pin-count standard plastic quad flat pack (PQFP) packages do not readily lend themselves to applications over 1-2 watts. Over the last few years several techniques have been used to enhance the thermal performance of plastic packages. These include the addition of a thin metal sheet referred to as a heat spreader or a thicker piece of metal called a heat slug internal to the package. These thermal enhancers behave differently depending on the external environment and the printed circuit boards ability to carry heat away from the package. They also have important differences in other aspects such as cost and manufacturability. This paper discusses several different types of thermally enhanced plastic (TEP) packages, their thermal performance under different ambient conditions, relative costs and manufacturability, and the effects on thermal performance depending on the test environment and how the measurements are made. Such an analysis is useful in selecting the right type of thermally enhanced packages for a given application.

Prognostic indicators for Cu-Al wirebond degradation under operation at elevated temperature and combined temperature humidity

Gold wire bonding has been widely used as first-level interconnect in semiconductor packaging. The increase in the gold price has motivated the industry search for alternative to the gold wire used in wire bonding and the transition to copper wire bonding technology. Potential advantages of transition to Cu-Al wire bond system includes low cost of copper wire, lower thermal resistivity, lower electrical resistivity, higher deformation strength, damage during ultrasonic squeeze, and stability compared to gold wire. However, the transition to the copper wire brings along some trade-offs including poor corrosion resistance, narrow process window, higher hardness, and potential for cratering. Formation of excessive Cu-Al intermetallics may increase electrical resistance and reduce the mechanical bonding strength. Current state-of-art for studying the Cu-Al system focuses on accumulation of statistically significant number of failures under accelerated testing. In this paper, a new approach has been developed to identify the occurrence of impending apparently-random defect fall-outs and pre-mature failures observed in the Cu-Al wirebond system. The use of intermetallic thickness, composition and corrosion as a leading indicator of failure for assessment of remaining useful life for Cu-al wirebond interconnects has been studied under exposure to high temperature and temperature-humidity. Damage in wire bonds has been studied using x-ray Micro-CT. Microstructure evolution was studied under isothermal aging conditions of 150°C, 175°C, and 200°C till failure. Activation energy was calculated using growth rate of intermetallic at different temperatures. Effect of temperature and humidity on Cu-Al wirebond system was studied using Parr Bomb technique at different elevated temperature and humidity conditions (110°C/100%RH, 120°C/100%RH, 130°C/100%RH) and failure mechanism was developed. The present methodology uses evolution of the IMC thickness, composition in conjunction with the Levenberg-Marquardt algorithm to identify accrued damage in wire bond subjected to thermal aging. The proposed method can be used for quick assessment of Cu-Al parts to ensure manufactured part consistency through sampling.

Delamination prediction in lead frame packages using adhesion measurements and interfacial fracture modeling

A combined adhesion characterization and fracture mechanics based modeling methodology has been developed to predict delamination in lead frame packages. This methodology allows experimental data from a simple adhesion test such as button shear test to be used along with the finite element model to ultimately predict whether a particular interface will fail in reliability testing. The methodology was validated using existing package reliability data across several package types, materials and package interfaces. In each case, the delamination prediction was compared to the actual occurrence of delamination post reliability testing. The validation showed that delamination could be predicted with 80–90% accuracy, depending on the interface and the temperature condition considered.

Damage mechanisms in copper-aluminum wirebond under high temperature operation

Wire bonding is predominant mode of interconnect in electronics packaging. Traditionally material used for wire bonding is gold. But industry is slowly replacing gold wire bond by copper-aluminum wire bond because of the lower cost and better mechanical properties than gold, such as high strength, high thermal conductivity etc. Numerous studies have been done to analyze failure mechanism of Cu-Al wire bonds. Cu-Al interface is a predominant location for failure of the wirebond interconnects. In this paper, the use of intermetallic thickness as leading indicator-of-failure for prognostication of remaining useful life for Cu-Al wire bond interconnects has been studied. For analysis, 32 pin chip scale packages were used. Packages were aged isothermally at 200°C and 250°C for 10 days. Packages were withdrawn periodically after 24 hours and its IMC thickness was measured using SEM. The parts have been prognosticated for accrued damage and remaining useful life in current or anticipated future deployment environment. The presented methodology uses evolution of the IMC thickness in conjunction with the Levenberg-Marquardt Algorithm to identify accrued damage in wire bond subjected to thermal aging. The proposed method can be used for equivalency of damage accrued in Cu-Al parts subjected to multiple thermal aging environments.