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

Solder reactions between SnPb and one of the four metals, Cu, Ni, Au, and Pd have been reviewed on the basis of the available data of morphology, thermodynamics, and kinetics. The reactions on both bulk and thin film forms of these metals have been considered and compared. Also the two kinds of reactions, above and below the melting point of the solder, have been considered and compared. The rate of intermetallic compound formation in wetting reactions between the molten solder and the metals is three to four orders of magnitude faster than those between the solid state solder and the metals. The rate is controlled by the morphology of intermetallic compound formation. In the wetting reaction between molten SnPb and Cu or Ni, the intermetallic compound formation has a scallop-type morphology, but in solid state aging, it has a layer-type morphology. There are channels between the scallops, which allow rapid diffusion and rapid rate of compound formation. In the layer-type morphology, the compound layer itself becomes a diffusion barrier to slow down the reaction. Similar morphological changes occur between SnPb and Au or Pd. The stability of scallop-type morphology in wetting reaction and layer-type morphology in solid state aging have been explained by minimization of surface and interfacial energies. The unusually high rate of scallop-type intermetallic compound formation has been explained by the gain of rate of free energy change rather than free energy change. Also included in the review is the use of a stack of thin films as under-bump-metallization, such as Cr/Cu/Au, Al/Ni(V)/Cu, and Cu/Ni alloyed thin films.

Tin–lead (SnPb) solder reaction in flip chip technology

The characterization of lead-free solders, especially after isothermal aging, is very important in order to accurately predict the reliability of solder joints. However, due to lack of experimental testing standards and the high homologous temperature of solder alloys (Th > 0.5Tm even at room temperature), there are very large discrepancies in both the tensile and creep properties provided in current databases for both lead-free and Sn–Pb solder alloys. Some recent researches show that the room temperature aging has significant effects on mechanical properties of solders. This paper is intended to review all available data in the field and give rise to the possible factors including room temperature effects which causes the large discrepancies of data. This review of the research literatures has documented the dramatic changes that occur in the constitutive and failure behavior of solder materials and solder joint interfaces during isothermal aging. However, these effects have been largely ignored in most previous studies involving solder material characterization or finite element predictions of solder joint reliability during thermal cycling. It is widely acknowledged that the large discrepancies in measured solder mechanical properties from one study to another arise due to differences in the microstructures of the tested samples. This problem is exacerbated by the aging issue, as it is clear that the microstructure and material behavior of the samples used in even a single investigation are moving targets that change rapidly even at room temperature. Furthermore, the effects of aging on solder behavior must be better understood so that more accurate viscoplastic constitutive equations can be developed for SnPb and SAC solders. Without such well-defined relationship, it is doubtful that finite element reliability predictions can ever reach their full potential.

A review of mechanical properties of lead-free solders for electronic packaging

The current national consensus standard for laser safety in the United States is the American National Standard for Safe Use of Lasers (ANSI Z136.1). Over the past few years, a comprehensive rewrite of this standard has been conducted. The updated version of the standard (Z136.1-2000) incorporates a wealth of new bioeffects data and establishes a number of new maximum permissible exposure (MPE) limits for laser safety. The updated standard also includes new procedures for the computation of MPE values, which must be understood by health physicists, laser safety officers, and others in the field of occupational safety. Here we present the first in a series of tutorial articles to clarify laser safety analysis procedures under this new standard. This article deals with the proper application of three rules for determining the appropriate MPE values for repetitively pulsed lasers or repeated exposures from laser beams.

A procedure for multiple-pulse maximum permissible exposure determination under the Z136.1-2000 American National Standard for Safe Use of Lasers

A review of lead-free solders for electronics applications

Solder ball shear strength is determined for ball grid array (BGA) packages in the as-received condition and after solder reflow preconditioning (exposure to multiple solder reflow profiles). Immediately after reflow, the solder shear strength is considerably higher than the solder shear strength measured on packages in the as-received condition. When the shear strength is monitored as a function of the time after solder reflow preconditioning, it is found to decrease significantly with time. Microhardness tests made in parallel with the shear measurements, provide quantitative confirmation of a degradation in mechanical properties of the solder over time. The observed changes in mechanical properties are the result of room temperature annealing, or age-softening of the solder. Cross sectioning and scanning electron microscopy are used to identify and track microstructural changes in the solder balls with room temperature aging. The microstructural changes are correlated to the decrease in shear strength and microhardness and in some cases, to a change in failure mode. These results are discussed in terms of accurate interpretation of shear test data and suggestions are made for test procedures that can minimize data variations induced by solder aging effects.

The influence of room temperature aging on ball shear strength and microstructure of area array solder balls

Thermal fatigue is a major source of failure of solder joints in surface mount electronic components and it is critically important in high reliability applications such as telecommunication, military, and aeronautics. The electronic packaging industry has seen an increase in the number of Pb-free solder alloy choices beyond the common near-eutectic Sn-Ag-Cu alloys first established as replacements for eutectic SnPb. This paper discusses the results from Pb-free solder joint reliability programs sponsored by two industry consortia. The characteristic life in accelerated thermal cycling is reported for 12 different Pb-free solder alloys and a SnPb control in 9 different accelerated thermal cycling test profiles in terms of the effects of component type, accelerated thermal cycling profile and dwell time. Microstructural analysis on assembled and failed samples was performed to investigate the effect of initial microstructure and its evolution during accelerated thermal cycling test. A significant finding from the study is that the beneficial effect of Ag on accelerated thermal cycling reliability (measured by characteristic lifetime) diminishes as the severity of the accelerated thermal cycling, defined by greater ΔT, higher peak temperature, and longer dwell time increases. The results also indicate that all the Pb-free solders are more reliable in accelerated thermal cycling than the SnPb alloy they have replaced. Suggestions are made for future work, particularly with respect to the continued evolution of alloy development for emerging application requirements and the value of using advanced analytical methods to provide a better understanding of the effect of microstructure and its evolution on accelerated thermal cycling performance.

https://link.springer.com/content/pdf/10.1007%2Fs11837-015-1595-1.pdf

Thermal Fatigue Evaluation of Pb-Free Solder Joints: Results, Lessons Learned, and Future Trends

We investigated the growth of intermetallic compounds in Cu/Ni/Au/PbSn solder joints. The substrates that we investigated had been Au plated by one of two different techniques. The Au finish thicknesses ranged from 0.25 to 2.6 /spl mu/m. After solder renew, structural examinations using optical and electron microscopy of cross-sectioned solder joints revealed the growth of Ni/sub 3/Sn/sub 4/ at the solder/Ni interface after reflow. Solder joints with thicker layers of Au annealed in Ar gas at a temperature of 150/spl deg/C for up to 450 h, displayed an appreciable growth of Au/sub 0.5/Ni/sub 0.5/Sn/sub 4/ at the Ni/sub 3/Sn/sub 4//solder interface. Previous investigators correlated growth of a Au-Sn alloy with the degradation of the mechanical properties of the solder joint. The determination of the stoichiometry of the Au/sub 0.5/Ni/sub 0.5/Sn/sub 4/ phase provides some understanding of why this phase grew at the Ni/sub 3/Sn/sub 4//solder interface, as Sn, Au and Ni are all readily available at this interface. The growth of this ternary alloy is also consistent with trends observed in the kinetics of formation of solder alloys.

Solder metallization interdiffusion in microelectronic interconnects

With increasing focus on BGA technology as a high performance package, there is an urgent need to develop industry-wide standards for package quality and reliability. A significant factor in BGA package quality is the robustness of solder ball attachment to the package. Currently, ball shear testing is used to assess solder joint integrity. The solder ball failure mechanism can not be determined from shear force data alone, but is critical to understanding solder joint quality. A failure mode analysis, along with a fundamental understanding of the shear test process, is needed to assess solder joint quality. A common pad finish on BGA packages and PWB substrates is Ni/Au, using either electrolytic or electroless deposition. There are reports of brittle fracture in BGA and PWB interconnects with both types of Ni/Au surface finishes. During post-assembly test and manufacturing, the solder to pad interconnection has been shown to separate under certain conditions. In this study, ball shear testing is performed in conjunction with thermal preconditioning to evaluate brittle fractures on a thermally enhanced BGA with electroless Ni/Au bond pads and on a plastic BGA with electrolytic Ni/Au bond pads. Cross sectioning and fractography reveal interfacial failure in both package types. However, metallography and SEM with EDX confirm two distinct interfacial failure mechanisms. The failure analysis is interpreted in terms of the implications for package quality and performance. Although this evaluation is performed on BGA packages, the failure analysis results are applicable to brittle fractures on PWBs using a Ni/Au surface finish.

Shear testing and failure mode analysis for evaluation of BGA ball attachment

A field programmable gate array assembly ( 100, 200, 300 ) offers the unique functionality typically reserved for custom ICs and application specific integrated circuits (ASICs) with the flexibility of a programmable gate array. This is accomplished by modifying a package for a programmable IC ( 102 ), such as a programmable gate array, to electrically and mechanically couple to another IC ( 104 ). The preferred electrical and mechanical coupling occurs by stacking the IC on the programmable IC.

Richard J. Coyle

Papers

The influence of room temperature aging on ball shear strength and microstructure of area array solder balls

Thermal Fatigue Evaluation of Pb-Free Solder Joints: Results, Lessons Learned, and Future Trends

Solder metallization interdiffusion in microelectronic interconnects

Shear testing and failure mode analysis for evaluation of BGA ball attachment

Field programmable gate array assembly