A. Mavinkurve

Lifetime prediction of Cu-Al wire bonded contacts for different mould compounds

Large scale conversion of gold to copper wiring in microelectronics can only become successful when all the failure mechanisms that can be encountered during reliability testing, or during product application life are understood. One of these mechanisms is corrosion of the contact between the copper (Cu) ball and the aluminum (Al) bond-pad, consisting of various intermetallic compounds (IMCs), which are more sensitive to corrosion compared to gold (Au) Al IMC. This study elaborates on three corrosion mechanisms present in the Cu-Al system: interfacial Cu-Al IMC corrosion, bulk Cu-Al IMC corrosion and Al bond pad corrosion. For the first mechanism, which is dominant, an empirical corrosion model is introduced. To gather data for this model, a recently developed method for analyzing the IMC contact area to study the dynamics of the dominant mechanism is used. Data was collected from various devices, which were exposed to accelerated aging conditions. The focus of this paper is on unbiased conditions using a wide temperature and humidity range. In total four epoxy molding compound types have been investigated and are compared to each other, using the empirical model proposed in this paper. Finally, it is shown that the model allows the prediction of the life time of Cu-Al ball contacts for different application conditions and also allows the selection of an appropriate mould compound type.

Degradation of Cu-Al wire bonded contacts under high current and high temperature conditions using in-situ resistance monitoring

To evaluate the dynamics of Cu-Al bond contact degradation, the evolution of the intermetallic electrical interface resistance was monitored in-situ in a test device exposed to high temperatures (140 to 200°C) while conducting high current densities (7.0 × 103 to 4.5 × 104 A/cm2). The degradation is quite different from what is known for Au-Al contacts. The influence of different stimuli like current direction, current density, temperature, and temperature gradients is studied and discussed. On top of that the dynamics in relation to material properties is assessed in detail from which the concept of an incubation time until IMC degradation starts is proposed.

Electrical characterization of plastic encapsulations using an alternative gate leakage test method

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 6 times 1011 Ohm-cm, at high temperature, 150degC, are more prone to fail the test on an increased current, compared to devices encapsulated by a compound having a high resistivity of 4 times 1013 Ohmtimescm 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.

Copper wire interconnect reliability evaluation using in-situ High Temperature Storage Life (HTSL) tests

Abstract This paper describes the use of in-situ High Temperature Storage Life (HTSL) tests based on a four point resistance method to evaluate Cu wire interconnect reliability. Although the same set up was used in the past to monitor Au–Al ball bond degradation, a different approach was needed for this system. Using conventional statistical methods of failure probability distributions and a fixed failure criterion were found to be unsuitable in this case. Besides this, tests usually take very long until a sufficient percentage of the population have failed according to that criterion. A simple physical model was used to electrically quantify ball bond degradation due to the prevailing failure mechanism in a substantially smaller amount of test time. The method enabled the determination of activation energies for a number of moulding compounds and is extremely useful for a fast screening of such materials regarding their suitability for Cu wire.

A multi-scale approach to the thermo-mechanical behaviour of silica-filled epoxies for electronic packaging

Delamination of mating interfaces can cause serious reliability problems in different application areas. The causes of delamination are multiple. In the case of leadframe-based chip packages, a critical interface is that between the leadframe and the moulding compound. Delamination can magnify stress levels at the interface and can lead to fatigue of interconnects.

On electrochemical cell modeling as basis for predicting corrosion failures in plastic encapsulated microelectronics

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.

Multi-Physics Based Structural Similarity Rules for the BGA Package Family

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-topad 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 ‘multi-physics 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 BGA 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 nonlinear reliability prediction models an Excel-based tool is created for package designers. By using this tool, the number of reliability qualification tests can be reduced. More importantly, possible failure mechanisms can be (better) understood and predicted.

Crevice Corrosion of Ball Bond Intermetallics of Cu and Ag Wire

In this work we present results, both experimental and theoretical, related to corrosion of intermetallic layers that are formed at the interface between the bond pad and the bond ball. Aluminum smear adjacent to the bond ball leads to an initial crevice. Due to a galvanic corrosion process an oxygen deficit in the crevice might occur. Numerical simulations show that in the absence of oxygen the anodic oxidation process will become dominant, which leads to an acidification of the crevice, and finally to an enrichment in the chloride concentration. These conditions will highly accelerate the corrosion process of the intermetallic layer, and thus a growth of the initial crevice, and eventually to complete ball lift during reliability tests.

The Paradoxical Role of Sulphur in Molding Compounds: Influence on High Temperature Reliability of Cu-Al Wirebond Interconnects

This paper describes the role of sulphur on corrosion phenomena at the interface between the Cu ball and Al bondpad. Sulphur is usually linked to the adhesion promotor which is used to improve package robustness and second bond reliability, but can have other detrimental effects. A variety of techniques such as (in-situ) HTSL, chemical analysis of sulphur and performing a DOE with compound variants was used to demonstrate that the presence of significant levels of sulphur in the molding compounds can accelerate Cu-Al interfacial corrosion. The presence of a Pd coating on the wire retards this inter-metal corrosion but can lead to other chemical reactions at the side of the Cu ball.

Modeling electrochemical migration through plastic microelectronics encapsulations

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.