Semiconductor devices are commonly encapsulated with Epoxy-based Moulding Compounds (EMC) to form an electronic package. EMC typically occupies a large volume within a package, and thus, governs its thermomechanical behaviour. When exposed to high temperatures (150°C and above), electronic packages predominantly show oxidation of the outer layer of EMC. Oxidized EMC exhibits notably different material properties, resulting in a modified deformation pattern of a thermally aged package under varying thermal loads. As the oxidation layer grows in thickness, its mechanical properties also evolve, indicating distinct phases of the oxidized material at different stages of thermal ageing. Reflecting these changes (i.e., the current state of degradation) into a Finite Element (FE) model-based analysis can provide better insights into failure prediction and component reliability. It requires updating the geometry and material behaviour as a function of ageing. This paper presents a systematic procedure to build a continuously updated physics-based Digital Twin of a thermally aged flip-chip package that can represent intermediate oxidation stages. First, experimental measurements are carried out to quantify the growth of the oxidation thickness at 150°C and a diffusion-dominant mathematical model is proposed. Then, an accurate geometry of the test package is prepared with a parametric outer layer from all exposed sides of EMC to represent the oxidized layer at different stages of thermal ageing. Next, the experimental characterization of a few partially oxidized EMC specimens is done, and analytical methods are utilized to extract the thermomechanical properties of the oxidized EMC at different stages of ageing. Experimental warpage data of aged test packages are utilized to verify the defined material-model parameters that represent curing shrinkage, thermal expansion, glass transition, and corresponding elasticity moduli of the oxidized EMC at select stages of ageing. Then, a workflow to establish continuity in the material model is presented. Finally, the developed Digital Twin is utilized for an FE analysis to study the change in the trend of out-of-plane package deformations as a function of several stages of EMC oxidation.@en

Epoxy Mold Compounds (EMC) are used to protect integrated circuits (IC) from environmental influences, with one of these influences being moisture ingress, causing corrosion. To obtain the needed thermal and mechanical properties EMCs require a high loading of (silica) fillers, introducing a large amount of interface. While silane coupling agents promote good binding, they have shown to introduce an interphase volume that exhibits a faster moisture transport between epoxy and SiO₂ in glass fiber filled epoxy. In this work, we investigate if such an interphase volume is also introduced by the filler particles in EMC and if it influences the moisture diffusion coefficient of the composite. We compare moisture uptake measurements performed by dynamic vapor sorption (DVS) with predictions from effective medium theory, as well as with numerical simulations based on micro-CT scans of our samples for a model epoxy system containing different filler levels and commercial EMC samples with two different filler levels. From the measured DVS data, we observe an effective diffusion coefficient, that is higher than predicted for an absence of any interphase for both the EMC and the model system. This suggests that an interphase layer should be present.

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The risk of corrosion poses a challenge to meet the stringent reliability requirements of microelectronic devices that are used in harsh environments. Microelectronic devices are often encapsulated in polymer packaging materials, which protect them from corrosion. These polymers are, however, not completely hermetic and thus allow small amounts of ions and moisture to reach the device, which might cause corrosion of the microelectronic circuitry. To improve and predict the reliability of the device, it is important to quantify the ion diffusivity in these materials. Previously reported values for the ion diffusivity vary by multiple orders of magnitude for a single class of material. Here, we investigate the causes for this discrepancy using three experimental methods: (i) saltwater immersion, (ii) diffusion cell measurements, and (iii) transient electric current measurements. Several materials, such as silicone, epoxy, and polyamide, were tested to cover the broad spectrum of polymers used by the microelectronics industry. We found that the discrepancies are likely due to the strong dependence of the ion diffusivity on both the moisture content within the polymers, as well as on the salt concentration and pH of the solutes. Furthermore, we found that the very low ion diffusivity causes long measuring times, and thus a large risk for errors from contamination, leakage, or minor defects in the samples.

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The reliability of LEDs decreases in moist environments. One potential gateway of moisture ingress, reducing the product lifetime is the lens. In white LEDs, phosphor particles are embedded into the optical silicone of the lens to convert the blue light emitted by the diode down in frequency and achieve a light output that appears white. In this study, the influence of these phosphor particles on the moisture sorption, permeation and diffusion in optical silicones is investigated by comparing two silicone resins that are commonly used in LEDs, both with and without the addition of phosphor particles. The results of two methods are compared: the wet-cup method and a gravimetric approach of dynamic vapour sorption (DVS). Diffusion coefficients between 20 and 75 °C are reported as well as sorption isotherms, activation energy and sorption enthalpy. It is concluded that the addition of phosphor particles only has a very small impact on the moisture transport properties of the silicones.@en

Tarnishing of the reflective silver layer in LED packages is an important failure mechanism, leading to both a decrease in luminous flux by up to 75% and a change in color spectrum.

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We present a time-dependent numerical model for corrosion in microelectronics, focusing on aluminum bondpads, which can be very beneficial for the design as well as the interpretation of reliability data of microelectronics. The model includes charge transport through the polymer microelectronics encapsulant as well as the formation of layers of space charge at all polymer interfaces, which strongly influences the electrochemical charge transfer rate at the polymer¿metal interfaces via the generalized Frumkin¿Butler¿Volmer equation. The system we consider consists of two parallel gold bondwires that are each electrically connected to two aluminum bondpads, which are assumed to contain weak spots in the protective native oxide layer, i.e. pits. This system is encapsulated in an epoxy molding compound, which is the usual low-conductive, and slightly hydrophilic, microelectronic encapsulant. We assume that a cathodic reaction takes place at the gold wires, and an anodic reaction at the weak spots of the aluminum bondpads. Furthermore,weassume the presence of a large excess of inert supporting salt compared to the reactive hydroxyl ions. Numerical calculations were made in a two-dimensional geometry as function of the applied voltage difference between the two wires, the concentration of absorbed moisture in the encapsulation, and the ambient temperature. We show that the model results predict trends similar to the empirical industrial standards for failure of microelectronic products due to corrosion.@en

Despite extensive research over the past decades, corrosion of aluminum bond pads is still a major reliability risk for plastic encapsulated microelectronics. Nowadays even an increase in susceptibility for corrosion is observed for new waferfab technologies and encapsulation materials during reliability tests. The recent trend for the new generation encapsulation materials is to decrease the glass transition temperature. As a result reliability tests could be performed at temperatures well above this temperature, such that traditionally used translations between test and operational lifetime, i.e. the acceleration factor, are no longer valid. Consequently, a new acceleration model for bond pad corrosion is required, which is capable of predicting the lifetime of products from test performed at temperatures above glass transition temperature. In this work we will show the merits of electrochemical cell modeling on the understanding of corrosion failures in microelectronics. The model is based on the Poisson-Nernst-Planck equation for the transport of ions coupled to a generalized Frumkin corrected Butler-Volmer equation for the kinetics of the electrochemical reactions. We present results as an example to clarify our approach.@en

In this paper, the interaction between chip and package is investigated with the focus on low ppm-level failures. More specifically, the failure mode of inter-metal shorts is investigated, caused by either electrical discharges (ESD) or internal/external mechanical forces. It is demonstrated that forces induced by the filler particles in the molding compound can cause these shorts. Finite element simulations are performed in order to estimate the stress levels in the backend stack of the integrated circuit (IC). Nano-indentation experiments are performed to measure the hardness of different passivation materials. The simulation and indentation results are combined with estimations and measurements of the particle size distribution, flow modeling and statistical methods. As such, the ppm-level of the failures could be attributed to the low chance that a filler particle would land on the critical location. Measures to prevent these failures are to be found in the area of improved passivation materials and/or recipes in combination with other molding compounds. For succesful development of IC backend structures and processes, it is essential to take into account the influence of the package in the earlier phase of IC backend development.@en

Plastic encapsulations will absorb moisture in humid environments due to their hydrophilic nature, this in combination with the inherent ionic contamination of the plastic will result in an electrolyte. This electrolyte might pose several reliability issues for the package and the encapsulated microelectronic circuit, such as electrochemical migration and bond pad corrosion. The corresponding failure mechanisms are associated with ionic currents through the package and electrochemical processes at e.g. the leads or bond wires. In this paper we present a generic mathematical framework for modeling ionic currents and electrochemical processes. We will apply this framework to electrochemical migration of metal between the leads of a plastic encapsulation, and show results for the transient formation of migration fluxes through the plastic encapsulation.@en

The supply current of plastic encapsulated microelectronic devices in the presence of a high potential source can increase abnormally due to parasitic gate leakage. According to reliability qualification standards, stress during a parasitic gate leakage test is applied by a corona discharge at a thin tungsten needle placed a few centimeters above the devices under test. The gate leakage sensitivity factor obtained from this test lacks any physical basis and is therefore not believed to be useful. Here we show that this sensitivity factor can be replaced by a physical model for charge transport through the encapsulation material. The model is used to explain why devices encapsulated by a molding compound with a low volume resistivity of 6x10(11) Ohm.cm, at high temperature, 150º C, are more prone to fail the test on an increased current, compared to devices encapsulated by a compound having a high resistivity of 4x10(13) Ohm.cm at the same temperature. Furthermore, we discuss an alternative test setup where the potential difference between two parallel electrodes sandwiching the devices is used as the source of stress. It is suggested in literature that this setup yields identical results as the current setup. However, using both setups on the same product did not result in an equal outcome, which indicates that both tests do not trigger the same failure mechanism to the same extent.@en

When high electrical fields are applied, and especially above the glass transition temperature, ion transport through the epoxy molding compounds that encapsulate the integrated circuit (die) strongly increases, leading to the accumulation of charge at the passivation-epoxy interface, and possibly to leakage currents because of the formation of parasitic transistors. Recovery from this failure mechanism is possible after annealing the package in the absence of the applied voltage, at sufficiently high temperature. The high electrical fields originate either from within the package due to high-voltage bondpads or are due to an external high-voltage source. We have developed a numerical transport model that is able to describe ion transport through IC packages including the formation of 'ionic diffuse layers', or 'polarisation layers', at the passivation-epoxy interface. In these layers the ion concentration strongly increases leading to a modification of the electrostatic boundary conditions (Gauss' law), as well as to a voltage step over the passivation-epoxy interface. The exact distribution of the ions in this diffuse layer is a very important quantity as it strongly influences the voltage profiles in the bulk of the material via the boundary conditions, and therefore the ion transport rates. We will briefly outline how the relation between charge and voltage (integral capacitance) can be obtained experimentally from the dielectric response when a DC voltage is applied across a thin polymer sample. The data that are obtained can be used to derive realistic microscopic models for the ion distribution in the diffuse layer formed within an epoxy resin next to electrodes and passivation layers.@en

To efficiently select qualification and reliability monitoring programs, structural similarity rules for Integrated Circuit designs, wafer fabrication processes and/or package designs are currently used by the industry. By following the package structural similarity rules, the numbers of reliability qualification tests may be greatly reduced. However, when looking at the present rules it is clear that they are not reliably defined. For instance, geometrical parameters such as die-to-pad ratio are not quantitatively included and it seems that linear relationships are assumed. Besides that, these rules are mainly deducted from experience and industrial trial and error results, not from reliability physics. Driven by the present development trends of microelectronics (miniaturization, integration, cost reduction, etc) it is urgently needed to develop `advanced based structural similarity rules¿ based on reliability physics (physics of failures), to meet the industrial development trends. In this study, we have used DOE/RMS techniques to deduct advanced structural similarity rules through simulation-based optimisation techniques. Parametric 3D non-linear FE models are used to explore the responses of the complete ball grid array (BGA) package family for both the thermo-mechanical and moisture-diffusion responses as function of six parameters among which the die-to-pad ratio and the body size. In this way, advanced structural similarity rules are deduced which can be used to shorten design cycles. Even more, by using the accurate 3D non-linear reliability prediction models easy tools can be created for package designers. By using such a tool, the number of reliability qualification tests can be reduced. More importantly, possible failure mechanisms can be (better) understood and predicted.@en