Yihua Luo

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.

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.

Resistance Spectroscopy Based Assessment of Degradation in Cu-Al Wire Bond Interconnects

Escalation of the expense of gold has resulted in industry interest in use of copper as alternative wire bonds interconnect material. Copper wire has the advantage of lower price and comparable electrical resistance to gold wire. In this paper, 32-pin copper-aluminum wire bond chip scale packages are aged at three types of environment conditions separately. Environmental conditions included: 200°C for 10 days, 85°C and 85% RH for 8 weeks and −40°C to 125°C for 500 thermal cycles. The resistances of the wire bond are obtained every 24 hours for 200°C environment, every 7 days for 85C/85RH environment and every 5 days (50 thermal cycles) for the thermal cycling environment. A leading indicator has been developed in order to monitor the progression effect of the different thermal aging condition on the package and prognosticate remaining useful life based on the resistance spectroscopy. The Cu-Al wire bond resistance has been measured using a modified Wheatstone bridge. It has been shown previously that precise resistance spectroscopy is able to offer the failure of a leading indicator prior to the traditional definition of failure. The prognostic health management is qualified to be an efficient and accuracy tool for assessment of the remaining life of the wire bond. The ability to predict the remaining useful life of Cu-Al wire bond provides several advantages, including increasing safety by providing warning ahead of time before the failure.Copyright

Package-level multiphysics simulation of Cu-Al WB corrosion under high temperature/humidity environmental conditions

Copper wire has found extensive applications in microelectronics packaging industry due to its low material cost, advanced electrical, mechanical and thermal properties compared to gold wire. Studies have shown that copper wire performs better than gold wire under high temperature operational conditions as there is little or no kirkendall void formation at Cu-Al wire bond interfacial area. However, when functioning under high humidity conditions, Cu-Al wire bond is more susceptible to bond interfacial corrosion compared to Au-Al wire bond. Experimental results on high humidity reliability of Cu-Al wire bond show that galvanic corrosion at ball bond interface is one of the main causes of failure. Despite reliability performance of wire bond being a constant research subject for decades, there is no time-to-failure model available to describe progression of corrosion. In this paper, a COMSOL multiphysics package level of Cu-Al wire bond corrosion model is developed. The package level model focus on capturing the progression of corrosion as a result of mold compound degradation, chlorine transport and micro-galvanic corrosion. The model is characterized by Nernst-Planck equation and interfacial electrolytic corrosion. Three-electrode electrochemical polarizations are performed to quantify the corrosion rate of wire bond. Diffusion cell experiment and molding compound degradation experiments are performed to quantify the diffusion rate of chlorine and the release rate of chlorine in a particular type of molding compound. Those experimental results are then incorporated into COMSOL multiphysics software to simulate the corrosion process and calculate lifetime of Cu-Al wire bond. The simulation results are then verified by the experimental results.

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.

De-bonding simulation of Cu-Al wire bond intermetallic compound layers

Copper wire bonding is being increasingly used as an alternative to gold wire bonding in electronics packaging industry. Copper wire has advantages over gold wire including lower cost, higher electrical and thermal conductivity and also higher mechanical strength, making it a good alternative for the high power interconnection and fine pitch bonding applications. However, introduction of copper wire bonding has also created new sets of challenges including the high susceptibility of copper and Cu-Al intermetallic compound to oxidation. Wire bond reliability especially intermetallic cracking is a predominant failure mode resulting from thermal aging or temperature humidity exposure. In this paper, an IMC grain-level finite element model has been developed to simulate the interfacial de-bonding behavior in order to study the influence of the IMC microstructure characteristics on the mechanical reliability of Cu-Al wire bond. Voronoi tessellations have been used to construct both regular and irregular IMC grain shapes geometry. Intrinsic cohesive zone model has been adapted to model interactions between neighboring grain boundaries including the effect of uniform interfacial strength and Weibull distributed grain interfacial strength. Finally, Cu-Al IMC growth and phase transformation are modeled. Simulation results indicate Cu-Al IMC microstructure characteristics not only influence bond strength but also influences the crack initiation and propagation. Regular-shaped IMC grain provides Cu-Al wire bond with more bond strength while non-uniform grains reduce bond strength. Results also indicate that the increase of IMC thickness makes wire bond less reliable while the crack propagation mode changes with the phase transformation.

Chlorine-Ion Related Corrosion in Cu-Al Wirebond Microelectronic Packages

The increasing price of gold has resulted in industry interest in use of copper as alternative wire bonds interconnect material. Copper wire has the advantages of the lower cost, lower thermal resistivity, lower electrical resistivity, higher mechanical strength and higher deformation stability over the gold wire. In spite of the upside above, the Cu-Al wire bond is susceptible to the electrolytic 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. EMC contain ionic contaminants including chloride ions as a result of the chemical synthesis of the subcomponents of the resin, etching of metallization and the decomposition of the die-attach glue. The presence of moisture in the operating environment of semiconductor package makes the ion more mobile in the EMC. The migration of chloride ions to the Cu-Al interface may induce electrolytic corrosion inside the package causing degradation of the bond interface resulting in eventual failure. The rate at which the corrosion happens in the microelectronic packages is dependent upon the rate at which the ions transport through the EMC in addition to the reaction rate at the interface. 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 (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 at various temperatures in the molding compound have been measured using inductively coupled plasma. Measured diffusion coefficients have been incorporated into the COMSOL multiphysics model. Moisture ingress into the EMC has been quantified through measurements of the weight gain in the EMC. Predictions from the COMSOL multiphysics model have been correlated with experimental data.Copyright

Multiphysics Model for Chlorine-Ion Related Corrosion in Cu-Al Wirebond Microelectronic Packages

The increasing price of gold has resulted in industry interest in use of copper as alternative wire bonds interconnect material. Copper wire has the advantages of the lower cost, lower thermal resistivity, lower electrical resistivity, higher mechanical strength and higher deformation stability over the gold wire. In spite of the upside above, the Cu-Al wire bond is susceptible to the electrolytic 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. EMC contain ionic contaminants including chloride ions as a result of the chemical synthesis of the subcomponents of the resin, etching of metallization and the decomposition of the die-attach glue. The presence of moisture in the operating environment of semiconductor package makes the ion more mobile in the EMC. The migration of chloride ions to the Cu-Al interface may induce electrolytic corrosion inside the package causing degradation of the bond interface resulting in eventual failure. The rate at which the corrosion happens in the microelectronic packages is dependent upon the rate at which the ions transport through the EMC in addition to the reaction rate at the interface. In this effort, a multiphysics model for galvanic 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.Copyright