Elsevier

Microelectronics Reliability

Volume 47, Issue 12, December 2007, Pages 1907-1920
Microelectronics Reliability

Feature extraction and damage-precursors for prognostication of lead-free electronics

https://doi.org/10.1016/j.microrel.2007.02.022Get rights and content

Abstract

Damage pre-cursors based health management and prognostication methodology has been presented for electronic systems in harsh environments. The framework has been developed based on a development of correlation between damage pre-cursors and underlying degradation mechanisms in lead-free packaging architectures. The proposed methodology eliminates the need for knowledge of prior stress histories and enables interrogation of system state using the identified damage pre-cursors.

Test vehicle includes various area-array packaging architectures subjected to single thermo-mechanical stresses including thermal cycling in the range of −40 °C to 125 °C and isothermal aging at 125 °C. Experimental data on damage pre-cursors has been presented for packaging architectures encompassing flex-substrate ball grid arrays, chip-array ball grid arrays, and plastic ball grid arrays. Examples of damage proxies include phase-growth parameter, intermetallic thickness and interfacial stress variations. Damage proxies have been correlated with residual life. The damage proxies have also been correlated with computational finite-element model predictions. Plastic and creep strain energy densities have been correlated to the identified damage proxies.

Introduction

Health management of electronic systems requires knowledge of impending failure. Presently, acquisition of mechanical system-diagnostics has been successfully achieved for automotive applications through an elaborate system of fault codes. The state-of-art health management systems focus on detection and isolation of faults and failures, and are largely reactive in nature, limiting the scope of maintenance decisions. In this paper, the focus is health management methodologies for electronic systems in various harsh environment applications, based on system-level prognostication to facilitate assessment of residual life. The reliability of electronic control and safety systems in harsh environment applications, such as automotive safety systems, can be significantly impacted through development of methodologies for monitoring the degradation and understanding damage evolution to enable avoidance of system-level failures. Challenges in implementing prognostics can be attributed to the lack of understanding of the underlying component degradation mechanisms.

In this paper, investigation of the changes of the features as well as time-evolution of physical damage, and the relationship between physical damage and feature-set has been established for thermo-mechanical stresses. Comprehensive health monitoring framework proposed in this paper will facilitate quick assessment of system state and potential for failure of critical electronic systems. In this paper, a methodology for pre-cursors based computation of residual life of electronic systems has been presented. A damage pre-cursors based residual life computation approach for various package elements has been developed, to prognosticate electronic systems prior to appearance of any macro-indicators of damage (Fig. 1). In order to implement the system-health monitoring system, precursor variables or leading indicators-of-failure have been identified for various package elements and failure mechanisms. Model-algorithms have been developed to correlate precursors with impending failure for computation of residual life. The correlations serve a basis for interrogation of damage-state and extraction of features quantifying underlying degradation. Examples of damage pre-cursors include phase-growth, intermetallic thickness, and patterns in interfacial stress distributions. Change in damage pre-cursors are sensed through a network of system-state monitors. Mathematical relationships have been developed for computation of residual life based in terms of damage proxies. Use of interfacial stress variation as a prognostic parameter has been discussed in [14], [15]. In this paper, damage proxies for lead-free interconnects are presented.

The pre-cursor based damage computation approach eliminates the need for knowledge of prior operational stresses and enables health management of deployed non-pristine electronic systems under unknown prior-loading conditions. The approach is powerful, since it reduces the demands on electronic system field-usage and deployment logistics required for acquisition of prior stress histories. Use of pre-cursors for damage computation addresses the limitation of life-prediction model based prognostication techniques, which target damage estimation for known stress histories imposed on pristine materials. Examples of life-prediction models include Paris’s Power Law [19], [20], Coffin–Manson Relationship [3], [26], [24], [17] and the S–N Diagram.

Reconstruction of operational profiles is often challenging and future operational profiles often unpredictable. In addition, it may not be always possible to characterize the operational loads under all possible scenarios (assuming they are known and can be simulated). Damage pre-cursors target fundamental understanding of underlying degradations in electronic systems, such a thermo-mechanical interconnect-fatigue, interfacial delamination of underfills, etc. Once identified for specific package elements and failure mechanisms, the pre-cursors are scalable for future package architectures and for application across a broad spectrum of designs.

Section snippets

Lead-free solder grain coarsening analytical model

Phase growth and intermetallic-growth under thermal cycling and steady-state temperature have been identified as pre-cursors for understanding progression of damage in this paper. Evolution of solder microstructure and the growth of intermetallic due to thermal fatigue have been reported previously by several researchers. Micro-structural coarsening during thermo-mechanical deformation is attributed to the generation of excess vacancies caused by the combined effect of local hydrostatic state

Experimental design and phase analysis

Components analyzed include various packaging architectures including, plastic ball-grid arrays, chip-array ball-grid arrays, tape-array ball-grid arrays, flex-substrate ball-grid arrays, and discrete resistors. Ball counts are in the range of 64–676 I/O, pitch sizes are in the range of 0.5–1 mm, and package sizes are in the range of 6–27 mm (Table 1). The boards contain six trace layers to simulate the thermal mass of a true production board, though all functional traces were run on the topmost

Determination of prognostication parameters

The average phase size “g” in the selected region is measured using Image Analysis Software. The phase growth parameter S can be expressed asS=(g)4-(g0)4where, g0 is the average phase size of solder after reflow at zero thermal cycle. The average phase growth parameter, S, changes with the cycles in thermal cycle environment and with time in steady-state temperature environment. Fig. 5, Fig. 6, Fig. 7, Fig. 8 show SEM backscattered images exhibiting examples of Ag3Sn -phase growth process in

Nonlinear finite element models

Temperature fluctuations caused by either power transients or environmental changes, along with the resulting thermal expansion mismatch between the various package materials, result in time and temperature dependent creep deformation of the solder. This phenomenon dominates solder joint fatigue. Fatigue failure is the combined phenomenon of time-dependent creep and time-independent plasticity. The creep behavior of 95.5Sn4Ag0.5Cu is modeled by Norton Steady-State Creep Constitutive Equation,ε

Intermetallic thickness as a leading indicator

In this portion of the study, the growth of the intermetallic thickness during thermal aging as leading indicator of failure has been explored. In order to investigate the correlation of interfacial intermetallic thickness growth versus thermal aging, the cycled component is been cross-sectioned at various interval of thermal aging. The aged components are sliced periodically to measure the thickness in SEM using 750× magnification. The mean thickness of IMC layers are measured using commercial

Implementation of damage pre-cursors approach

The prognostics approach presented in the paper may be implemented using sacrificial devices, which can be cross-sectioned to determine the failure progression of the assembly. It is envisioned that the sacrificial devices will be small, low cost devices, such that several of these can be conveniently located along edge of card assemblies to enable cross-sectioning or on a separate card within an electronic module or card cage.

For example, in the case of solder interconnects, chip resistors may

Summary and conclusions

A damage pre cursors based methodology for prognostication-of-electronics including assessment of residual-life, has been developed and demonstrated under single stresses of thermal cycling and steady-state temperature. The damage pre-cursors enable assessment of system damage-state significantly prior to appearance of any macro-indicators of damage. Phase growth rate and interfacial intermetallic layers growth rate have been identified as valid proxies for determination of residual life in

Acknowledgement

The research presented in this paper has been supported by Grant Number EEC-0533241 from the National Science Foundation.

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