Solder is widely used to connect chips to their packaging substrates in flip chip technology as well as in surface mount technology. At present, the electronic packaging industry is actively searching for Pb-free solders due to environmental concern of Pb-based solders. Concerning the reliability of Pb-free solders, some electronic companies are reluctant to adopt them into their high-end products. Hence, a review of the reliability behavior of Pb-free solders is timely. We use the format of “case study” to review six reliability problems of Pb-free solders in electronic packaging technology. We conducted analysis of these cases on the basis of thermodynamic driving force, time-dependent kinetic processes, and morphology and microstructure changes. We made a direct comparison to the similar problem in SnPb solder whenever it is available. Specifically, we reviewed: (1) interfacial reactions between Pb-free solder and thick metalliztion of bond-pad on the substrate-side, (2) interfacial reactions between Pb-free solder and thin-film under-bump metallization on the chip-side, (3) the growth of a layered intermetallic compound (IMC) by ripening in solid state aging of solder joints, (4) a long range interaction between chip-side and substrate-side metallizations across a solder joint, (5) electromigration in flip chip solder joints, and finally (6) Sn whisker growth on Pb-free finish on Cu leadframe. Perhaps, these cases may serve as helpful references to the understanding of other reliability behaviors of Pb-free solders.

Six cases of reliability study of Pb-free solder joints in electronic packaging technology

Differential scanning calorimetry (DSC) and isothermal calorimetry have been applied extensively to the analysis of light metals, especially Al based alloys. Isothermal calorimetry and differential scanning calorimetry are used for analysis of solid state reactions, such as precipitation, homogenisation, devitrivication and recrystallisation; and solid–liquid reactions, such as incipient melting and solidification, are studied by differential scanning calorimetry. In producing repeatable calorimetry data on Al alloys, sample preparation, reproducibility and baseline drift need to be considered in detail. Calorimetry can be used effectively to study the different solid state reactions and solid–liquid reactions that occur during the main processing steps of Al based alloys (solidification, homogenisation, precipitation). Also, devitrivication of amorphous and ultrafine grained Al based powders and flakes can be studied effectively. Quantitative analysis of the kinetics of reactions is assessed through reviewing the interrelation between activation energy analysis methods, equivalent time approaches, impingement parameter approaches, mean field models for precipitation, the Johnson–Mehl–Avrami–Kolmogorov model, as well as novel models which have not yet found application in calorimetry. Differential scanning calorimetry has occasionally been used in attempts to measure the volume fractions of phases present in Al based alloys, and attempts at determining volume fractions of intermetallic phases in commercial alloys and amounts of devitrified phase in glasses are reviewed. The requirements for the validity of these quantitative applications are also reviewed.

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Analysis of aluminium based alloys by calorimetry: quantitative analysis of reactions and reaction kinetics

In the early 1970s in Japan, the United States and France it was found that additions of nitrogen into aluminum oxide resulted in new spinel-like phases. At about the same time there was much increased interest in oxynitrides, stimulated by Professor K. Jack in the UK and Y. Oyama in Japan. Following these activities a major research program in this area was initiated at the Army Materials and Mechanics Research Center in Watertown, Massachusetts in 1974. These efforts resulted in the first complete Al2O3–AlN phase equilibrium diagram and a process to reactively sinter to nearly full density, translucent aluminum oxynitride spinel ceramic, which was named AlON. Subsequently, the Raytheon Company further developed AlON into a highly transparent material (ALON™) with many applications including transparent armor and EM domes and windows, among others—the technology was recently transferred to the Surmet Corporation. This paper will review the early history, phase equilibrium, crystal chemistry, and properties of this material, along with more recent work in our laboratory on transient liquid phase sintering and new data on lattice parameter measurements. In addition, recent results of collaborative work on AlON's dynamic mechanical properties using plate impact, Kolsky bar and edge-on impact (EoI) experimental techniques, including preliminary modeling at the microstructural scale of AlON in the EoI test, will be presented.

AlON: A brief history of its emergence and evolution

The Al–B–Nb–Ti system: IV. Experimental study and thermodynamic re-evaluation of the binary Al–Nb and ternary Al–Nb–Ti systems

The Al-B-Nb-Ti system

Experimental and thermodynamic assessment of the FeZr system

The CuFeNi system was reassessed taking into account new experimental data in particular in the region of the ordered phase (Cu,Ni) 3 Fe. Its thermodynamic description was assessed using a four sub-lattice model, while a random substitutional solution model was used for the disordered solution phases.

Thermodynamic assessment of the CuFeNi system

The binary Al-Ca, Ca-Fe, Ca-Si systems and the ternary Al-Ca-Fe, Al -Fe-Si, Ca-Fe-Si systems were thermodynamically assessed. Experimental determination of the enthalpy of fusion of the ternary compounds Si2Al2Ca, Si7Al8Fe5 and Si2Al3Fe, as well as invariant temperatures in the Al-Ca-Si and Al-Fe-Si systems were derived from DTA measurements. A set of thermodynamic parameters consistent with the phase diagram, mainly in the Si-rich region, is obtained.

Contribution to the experimental and thermodynamic assessment of the AlCaFeSi system—I. AlCaFe, AlCaSi, AlFeSi and CaFeSi systems

The Gibbs energy of formation of the solution and compounds phases in the Fe-Nb system were derived from an optimization procedure using all the available experimental thermodynamic and phase diagram data. The thermodynamic description of the stoichiometric and non-stoichiometric compounds was made using two, three or four sublattice models, while a Redlich-Kister polynomial was used for the solution phases.

Thermodynamic assessment of the Fe-Nb system

The AlN-Al2O3 system has been reassessed due to new experimental data published in particular near the Al2O3 rich part and relative to the φ′ and δ phases. The Gibbs energy of formation of the solution and compound phases in the AlN-Al2O3 binary section were derived from an optimisation procedure using all the available experimental thermodynamic and phase diagram data. The thermodynamic description of the ordered compounds was made using the sub-lattice model, while a Redlich-Kister polynomial was used for the solution phases.

C. Servant

Papers

Experimental and thermodynamic assessment of the FeZr system

Thermodynamic assessment of the CuFeNi system

Contribution to the experimental and thermodynamic assessment of the AlCaFeSi system—I. AlCaFe, AlCaSi, AlFeSi and CaFeSi systems

Thermodynamic assessment of the Fe-Nb system

Thermodynamic reassessment of the AlN-Al2O3 system