Microstructural considerations for ultrafine lead free solder joints
Introduction
Electronics industries are now manufacturing products with features that are smaller, lighter, have more functionality and at a lower cost to meet ever-increasing consumer needs. The solder joints in consumer-oriented portable electronic products have now shrunk to such a microscopic scale that the microstructural features, e.g., the scale of the dendrites, the size, morphology and distribution of the intermetallic compounds (IMC) within the bulk solder, and the morphology and thickness of the interfacial IMCs, are becoming significantly important in determining the reliability of the interconnects, and thus the whole electronic device, module or system. With lead free solders, materials issues such as dissolution of the metallization metals into liquid solders are becoming more important during the reflow process, due to high tin contents and high reflow temperatures, which together play a role in controlling the formation of the IMC present both within the bulk solder and at the solder-metallization interface.
Understanding the interfacial microstructure could help to improve the reliability of the micro solder joints. The formation of the interfacial IMCs indicates good wetting and bonding properties, however, excessive IMCs present at the interface may degrade the mechanical properties of the whole joint [1]. Extensive experimental research has been reported in the literature for interfacial reactions between Pb-free solders and different metallizations (e.g., [2]), however, there appear to have been few investigations of the size and geometry effects on the microstructure of solder joints, and in particular, issues such as whether ultrafine solder joints (<100 μm) contain the same interfacial microstructure, e.g., thickness and morphology of the interfacial IMCs, as large as few mm in size. In comparison to the mature SnPb solders, there appears to be less understanding of the materials behaviours of the Pb-free solders in the literature. Fortunately, the efforts in the development of thermodynamic descriptions for systems that are relevant to solders have resulted in the availability of several thermodynamic databases including those being produced by the major COST531 European collaborative program on lead free solders [3], [4], [5], [6], which provide an opportunity to explore the interdependence of the material compositions, processing and microstructure without the need for as many costly and time consuming experimental investigations.
The objectives of this paper are therefore to use combined thermodynamic-kinetic modelling, in parallel with carefully designed experimental work, to investigate the materials issues in the transition to Pb-free solders and joint miniaturization. An emphasis is put on the differences of the microstructure present within different sizes of solder bumps and their interfaces. A novel computational interface between a thermodynamic calculation software package, MTDATA [7], and a high-level scientific computing software MATLAB [8] (applicable to COMSOL Multiphysics [9]), is also introduced in this paper, which greatly extends the capabilities of both MTDATA and MATLAB and provides a powerful methodology for combined thermodynamic and kinetic modelling.
Section snippets
Experimental and modelling techniques
Three different bumping processes, i.e., solder dipping (SD), stencil printing followed by reflow (SPR), and electroplating of Sn followed by reflow (EPR) were used in this study to investigate the solder bump size and geometry effect on the microstructure of the solder bumps when a liquid solder is in contact with a Cu or ENIG pad. Four sizes of circular Cu pads, 1000 μm, 500 μm, 250 μm and 100 μm diameter respectively, were fabricated on a PCB. Two sizes of ENIG pads, 1500 μm and ∼80 μm in
Thermodynamic microstructure modelling
Thermodynamic modelling allows the prediction of the possible stable phases and their proportions as a function of alloy composition and the processing conditions. Thermodynamic calculations in MTDATA were performed under both “equilibrium” (representative of slow cooling) and “Scheil” conditions. The Scheil calculation does not allow for any diffusion in the solid, and assumes complete diffusion in the liquid, corresponding to the worst case of segregation during cooling, and is therefore more
Dissolution of metallizations into liquid solder
Dissolution is the first step in the interactions between metallizations and the liquid Sn-based solders, and as such understanding the kinetics of this process is of importance for accurate prediction of the solder-pad interactions, and hence the growth kinetics of the IMCs.
Fig. 4 presents the predicted dissolution kinetics of two sizes of Cu pads, with diameters 0.1 mm and 1 mm, respectively into liquid Sn, Sn–3.5Ag, and Sn–3.8Ag–0.7Cu solders (liquid solder volume is ∼1.32 × 106 mm3) at 240 °C
Microstructure of ultrafine lead free solder bumps
The bulk microstructure of the solder bumps presents useful information on the solidification mechanisms of the solder alloys. To obtain a general impression of the as-solidified microstructure across a cross-section of a solder bump, optical microscopy using differential interference contrast (DIC) is a good choice. The phases present in a Sn–Ag–Cu ternary alloy can be easily discriminated through their general appearances when examining the as-solidified microstructure. Fig. 8 presents an
A novel platform for microstructure-based FE reliability modelling
In many different materials processing simulations, it is necessary to obtain thermodynamic data for a system at equilibrium or in a metastable state [24], [25]. Therefore, being able to freely access the functions of thermodynamic software is of practical importance. This is especially the case in the field of combined thermodynamic-kinetic modelling. Tanaka et al. [26], for example, used a static linking to ChemApp [27] to calculate the surface tension of Sn–Bi alloys. In recent literature,
Conclusions
- 1.
Thermodynamic calculations using MTDATA and a solders database have been shown to be a useful tool for understanding the microstructure of multicomponent Pb-free solders and the interactions of Pb-free solders with Sn–37Pb eutectic solder.
- 2.
The dissolution kinetics of the conductor metal into liquid solder is dependent on both the solder compositions and pad sizes. The dissolution of Cu is faster in Sn–3.5Ag than in pure Sn.
- 3.
Solder bump size can influence the as-soldered microstructure. In
Acknowledgements
The authors acknowledge financial support from the UK’s EPSRC-IMCRC at Loughborough University. The authors acknowledge the support of the National Physical Laboratory (NPL), particularly Alan Dinsdale and Hugh Davies, for the provision of the MTDATA software, the solder database and technical support. One of the authors, Z. Huang, also acknowledges E. Jung and the Fraunhofer IZM Berlin (Germany) for providing a Visiting Researcher position and access to their experimental facilities during the
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