Shantanu Deshpande

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.

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.

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.

Study of electromigration in Cu and Ag wirebonds operating at high current in extreme environments

Small form factor of the latest electronics has significantly increased the current densities in the interconnects. Under high temperature operating conditions, these interconnects fail prematurely due to electromigration phenomenon as well as Joule heating. Electromigration (EM) is a mass transport of a material due to the momentum transfer between conducting electrons and atoms. High current densities in the wires can start mass transfer from one place to another, depending on the direction of the current. Joule heating of the wirebonds can significantly increase EMC-wirebond contact temperature and cause localized degradation of the EMC; which results into rapid degradation of the bond wire. Mechanisms of electromigration behavior of Palladium coated Copper (PCC), Copper (Cu), Silver (Ag) wirebonds are not widely reported in the literature. Relation between microstructural changes due to electromigration with the change in electric response of these interconnects is not yet reported. In this paper, combined effect of high current and high temperature on PCC, Ag, Cu, and Au wirebonds was evaluated. The tests were performed at ambient temperature with current density of 8.5∗104A/cm2. High resistance of the package increased the temperature of the package till 152°C. Change in electric response was correlated with change in morphology of bond-pad interface. Failure mechanisms for Cu-Al, PCC-Al, Ag-Al, and Au-Al system were developed.

Effect of EMCs on the high current reliability of Cu wirebonds operating in harsh environments

Copper (Cu) wire bonding, which is a newer alternative to Gold (Au) wire bonding, gets affected greatly by the variety of operating conditions. Selection of different materials, such as epoxy molding compound (EMC) used in the molding process plays key role in defining lifetime for wirebond system. Higher ionic contamination adversely affects the reliability of Cu wirebonds. Interaction of the EMCs with different properties with the Cu wirebond under harsh environment in presence of bias has not been fully understood. Quantification of the acceleration of the wirebond degradation under bias conditions is yet to be established. Previous research mainly investigates failure mechanisms upon failure, however does not report progression of damage which is leading to the failure. This information and understanding of the progression mechanism can yield into development of prognostics based life prediction models. In this paper, Cu wire bonded parts were subjected to high temperature aging conditions. One set of packages was subjected to unbiased test, and another set was subjected to biased condition. Change in electric response of both sets was monitored and was correlated with degradation of Cu-Al interface using ball shear test. Effect of EMC properties as well as voltage bias on the wirebond was then established and discussed in details.

Model for Interaction of EMC Formulation with Operating Current and Reliability of Cu-Al Wirebonds Operating in Harsh Environments

The migration of high-reliability applications requiring sustained operation in harsh environments needs a better understanding of the acceleration factors under the stresses of operation. Prolonged exposure of the copper wire to elevated temperatures results in growth of excessive intermetallics and degradation of the interface. Behavior of Copper wirebond under high current-temperature conditions is not yet fully understood. Exposure to high current may induce Joule heating and electromigration, and thus significantly increase the degradation rate in comparison with low current operating conditions. Further, the accelerated test results of unbiased conditions cannot be used for life prediction of such high powered parts. EMCs used for encapsulation of the chip and the interconnects may vary widely in their formulation including pH, porosity, diffusion rates, levels and composition of the contaminants. Selection of different materials, such as EMC used in the molding process plays key role in defining lifetime for wirebond system. There is need for predictive models which can account for the exposure to environmental conditions, operating conditions and the EMC formulation in order to be realistically representative of the expected reliability. In this paper, a set of parts, molded with different EMCs were subjected to high temperature-current environment (temperature range of 150°C-200°C, 0.2A-1A). An artificial neural network (ANN) driven predictive model for estimation of the beta-sensitivities of the input variables has been developed for computation of the acceleration factor for the Cu-Al WB under high voltage and high temperature.

Development of Model for Identification of Process Parameters for Wet Decapsulation of Copper–Aluminum Wirebond in PEMs

Decapsulation is a failure analysis technique often used to expose the die and first-level interconnects such as wirebonds by dissolving the surrounding epoxy molding compound (EMC). The wet decapsulation technique, which uses fuming acids, works very well for traditional gold (Au) wirebonds. On the other hand, its latest alternative, copper (Cu) wirebond, reacts with the nitric acid vigorously and undergoes severe corrosion. It is important to develop an acid chemistry that can be used to perform decapsulation of Cu–Al incorporated plastic encapsulated microelectronics (PEMs) without damaging the Cu wires. This paper presents the wet decapsulation technique based on different ratios of the red fuming nitric acid and concentrated sulfuric acid. Quality of post-decap part was examined using reduction in the wire diameter and changes in ball shear strength. Reduction in wire diameter was monitored with the scanning electron microscopy (SEM), and shear strength was measured using a DAGE2400 shear tester. These tests were performed on the PEMs molded with different EMCs, wire diameters, and pad thickness to cover process as well as geometric variability. Artificial neural network with Bayesian regularization-based model has been developed correlating the decapsulation process parameters with the post-decap wire diameter reduction and shear strength of the wirebond. Stepwise regression was used to identify the significant variables that affect the decap process. The influential variables were then used to develop a predictive model for prediction of the percent reduction in wire diameter and change in shear strength separately. Models were then validated with the test data set.