Luu Nguyen

Fuming Acid Based Decapsulation Process for Copper-Aluminum Wirebond System Molded With Different EMCs

Decapsulation is one of the very powerful technique in failure analysis process. During this process, die and first level interconnects are exposed by dissolving molding compound around them using variety of methods. Typically decapsulation formulation uses red fuming nitric acid at elevated temperatures. This technique work for traditional Gold wire bonds, but does not work for its new alternative Copper. Gold, being inert metal does not react with acid. Copper on the other hand; tends to react with fuming nitric acid, and dissolves rapidly into acid. It is important to develop acid chemistry that can be successfully used to perform decapsulation of Cu-Al incorporated packages for different EMC’s.In this paper, decap process based on combination of red fuming nitric acid and concentrated sulfuric acid at elevated temperatures is presented. Reduction in wire diameter was monitored for all devices. For some devices decap process was evaluated based on comparison of WB shear strength of decaped part with unmolded part. SEM was used extensively to track down degradation of copper wires. These tests were performed on packages with different EMC’s, wire diameters, pad thickness and some active dies.Statistical principal components regression model has been developed correlating the decapsulation process parameters with the post decap wire diameter reduction. Principal component regression in conjunction with stepwise regression has been used to identify the influential variables, and to remove the multicollinearity between the predictor variables. Principal component analysis which combines two correlated variables into a single factor is a widely used image processing technique for pattern recognition and image compression. The post molded packages have then used to assess the effect of various decapsulation treatments.Copyright

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.

TCAD modeling of charge transport in HV-IC encapsulation materials

In high-voltage integrated circuits (HV-ICs) operating at high temperatures, the high electric field spreading out from the high-voltage bond-pad can induce charge transport phenomena in the encapsulation material. A new TCAD-based approach is proposed which is suitable for investigating the role played by the propagation of charge within the plastic mold over the surface of high-voltage circuits. Simulations are compared with experimental data carried out using a dedicated test chip. The proposed approach is able to predict the features of the leakage current curves measured on different sensors during a complete charging transient.

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.

Principal Components Regression Model for Prediction of Acceleration Factors for Copper-Aluminum Wirebonds Subjected to Harsh Environments

Migration to Cu wirebond from traditional gold wirebond is recent trend in packaging industry. Selection of different materials, such as EMC used in the molding process plays key role in defining lifetime for wirebond system. Effects of the individual variables such as pH value, ionic contamination, filler content on reliability of wirebond is available in literature. However, since all these parameters have combined effect on reliability, joint effect of all parameters on acceleration factor and their quantification is necessary for smooth transition to Cu wirebond system, keeping transition and testing time minimum. Effect of pH value and ionic contamination of EMC on life distribution was individually studied in the past. In this paper, predictive model for acceleration factor of copper based on PCR approach is presented. A set of parts, molded with different EMCs were subjected to high temperature environment (150°C-225°C). Resistance, IMC change and shear strength change were monitored during this study. Resistance spectroscopy was used for accurate resistance measurement. Dage 2400PC was used to calculate change in shear strength. Parts were cross-sectioned and polished along Cu-Al interface using SEM and EDX system after failure. Relation between resistance changes with change in shear strength was established. 20% change in resistance was considered as failure threshold. All parts were tested till failure. Time-to-failure data was used to calculate acceleration factor, with 150°C as a base temperature. Different AFs obtained for different EMCs were then regressed against environmental conditions, mechanical and chemical properties of molding compound. In this study, all packages had same architecture, wire diameter and pad thickness. Principal component analysis was used to identify influential variables and to remove multicollinearity in the data set. PCA technique allows dataset to be projected into lower dimension vector space, and remove variables which do not contribute towards variance of predictive variable. Prediction model will be able to predict AF when environmental conditions, properties of EMC are known. Model was validated by performing self-validation test, and by predicting AF for the dataset which was not part of study. This model will provide, educated estimation of time to failure for new set of EMCs, for desired operating condition.

Optimization of HV LDMOS devices accounting for packaging interaction

The sensitivity of HV RESURF LDMOS transistors to parasitic charging in molding compound is investigated in this work by incorporating the passivation and encapsulation layers in the TCAD setup and implementing the conductivity losses in the mold. The role played by field plates and multiple metal/poly-silicon floating rings on the overall RESURF are revisited by focusing on the breakdown voltage degradation under high-voltage, high-temperature stresses. The layout re-optimization of a Single- and a Triple-RESURF LDMOS device using only MET1 and poly-Si levels is presented to reach a stable breakdown voltage after HTRB stress.

Multiphysics-modeling of corrosion in copper-aluminum interconnects in high humidity environments

Copper aluminum interconnects are being used in automotive applications for deployment underhood, on-engine and on-transmission. Electronics is widely used for enabling safety function including lane departure warning systems, collision avoidance systems, antilock braking systems, and vehicle stability systems. Models for copper interconnect degradation are needed for life prediction modeling to ensure 10-year, 100,000 mile reliability for electronics in automotive applications. Small concentrations of chloride ions may diffuse towards the bond pad interface under temperature, humidity, and electrical bias. The chloride ions may act as a catalyst breaking down the passivation layer of aluminum pad and accelerate the micro-galvanic corrosion at the copper-aluminum leading to the failure of the wirebond. Models for prediction of the diffusion of the chloride ions and the corrosion of the copper-aluminum interface have been difficult to develop, because of the small scale of the interface and the lack of appropriate electro-chemical properties for the Cu-Al system and the Electronic Molding Compounds under conditions relevant to operation. In this effort, a multiphysics model for electrolytic corrosion in the presence of chloride has been presented. The contaminant diffusion along with the corrosion kinetics has been modeled. In addition, contaminated samples with known concentration of KCl contaminant have been subjected to the temperature humidity conditions of 130°C/100RH. The resistance of the Cu-Al interconnects in the PARR test have been monitored periodically using resistance spectroscopy. The diffusion coefficients of chloride ion has been measured in the electronic molding compound at various temperatures using two methods including diffusion cell and inductively coupled plasma (ICPMS). Moisture ingress into the EMC has been quantified through measurements of the weight gain in the EMC as a function of time. Tafel parameters including the open circuit potential and the slope of the polarization curve has been measured for both copper, aluminum under different concentrations of the ionic species and pH values in the EMC. The measurements have been incorporated into the COMSOL model to predict the corrosion current at the Cu-Al bond pad. The model predictions have been correlated with experimental data.

Optimum filler geometry for suppression of moisture diffusion in molding compounds

Inorganic fillers, such as fused silica or organic clay, help tailor/co-optimize the mechanical toughness, thermal conductivity, and moisture diffusivity of polymer mold compounds used to package microelectronic integrated circuits. Despite long history and wide-spread current use, the optimization of filler-infused composites is generally empirical and therefore time-consuming. A physics-based predictive modeling will improve application-specific design of composites that would offer optimum performance and reliability. As an illustrative example, in this paper, we develop a general theory of polymer composites that anticipates the suppression of moisture diffusion as a function of fill-fraction, size-dispersion, shape, and topology of filler nanoparticles. Our results show that the best performance is obtained by incorporation rod-shaped fillers, randomly closed packed at maximum density (~60%). Our numerical results are succinctly captured by the analytical model based on generalized Maxwell Garnett effective medium theory. The analytical model can be used for initial optimization of mold compounds before large-scale numerical modeling is invoked and characterization experiments are designed.

Multiphysics Life-Prediction Model Based on Measurements of Polarization Curves for Copper-Aluminum Intermetallics

Copper wire bonding is finding applications in automotive underhood electronics applications including lane departure warning systems, collision avoidance systems, and vehicle stability systems. The Cu-Al wire bond is susceptible to the corrosion and the reliability of Cu-Al wire bond is of great concern. Typical electronic molding compounds are hydrophilic and absorb moisture when exposed to humid environmental conditions and may contain ionic contaminants including chloride ions as a result of the chemical synthesis of the subcomponents of the resin, etching of metallization, the decomposition of the die-attach, epichlorohydrin in the resin as a flame retardant. The presence of moisture in the operating environment of semiconductor package makes the ion more mobile in the EMC. Models for prediction of the diffusion of the chloride ions and the corrosion of the copper-aluminum interface have been difficult to develop, because of the small scale of the interface and the lack of appropriate electro-chemical properties for the Cu-Al system and the Electronic Molding Compounds under conditions relevant to operation. In this effort, a multiphysics model for galvanic corrosion in the presence of chloride has been presented based on fundamental physics of failure measurements of the corrosion kinetics of Cu, Al, and IMCs. The specific IMCs measured include CuAl, CuAl2, and Cu9Al4. The contaminant diffusion along with the corrosion kinetics has been modeled. In addition, contaminated samples with known concentration of KCl contaminant have been subjected to the temperature humidity conditions of 130°C/100RH. Moisture ingress into the EMC has been quantified through measurements of the weight gain in the EMC as a function of time. Tafel parameters including the open circuit potential and the slope of the polarization curve has been measured for both copper, aluminum under different concentrations of the ionic species and pH values in the EMC. The measurements have been incorporated into the COMSOL model to predict the corrosion current at the Cu-Al bond pad and develop acceleration factors for copper-aluminum wirebond corrosion.