Lead-free solder mechanical properties

The lead-free solders are mostly based on Sn-containing binary and ternary alloys. Among them, the Sn-Ag system is one of the earliest commercially available lead-free solders and has been recommended for general-purpose use as a substitute for Sn-Pb eutectic solder. Addition of nano-particles of second phase helps in improving thermo-mechanical properties such as melting temperature, mechanical strength, mechanical fatigue resistance, creep resistance and solder-joint reliability. 

PEEK is a semi-crystalline polymer, insoluble in all common solvents and can be used at temperatures of up to 300 °C. This is a particular benefit when it is used in applications which involve the use of lead-free solder. The polymer has excellent chemical and mechanical stability. Victrex has achieved success in the Asian market with PEEK, which has been selected by Shod, a Japanese capacitor and miniature rechargeable battery manufacturer and distributor, to replace PPS resin in all the resin mould cases of its PetitCap aluminium electrolytic capacitors. PEEK-HT (high temperature polymer), which maintains its physical and mechanical properties at temperatures 30 °C higher than standard grades, has been introduced by Victrex. 

A more comprehensive list of mechanical material properties of lead-free solders and other materials used in electronic packages can be found in Reference 22. 

M. Fine, Physical Basis for Mechanical Properties of Solders, Handbook of Lead-Free Solder Technology for Microelectronic Assemblies, Marcel-Dekkar, 2003, p 211-237 Referencing H. Maroovi, et al., J. Electron. Mater., Vol 26, 1997, p 783-790... 

A. Schubert, et al., Thermal Mechanical Properties and Creep Deformation of Lead-Containing and Lead-Free Solders, 2001 International Symposium on Advanced Packaging, Materials Proc., 2001, p 129-134... 

L. Xiao, J. Liu, Z. Lai, L. Ye, and A. Tholen, Characterization of Mechanical Properties of Bulk Lead-Free Solders, Sixth International Symposium on Advanced Packaging Materials, 2000, p 145-151... 

Visual inspection, refined over the years to identify defects and faults that have been correlated to mechanical property behavior and reliability in the field, is a very important aspect of the electronic assembly process. Accordingly, a set of visual inspection criteria has been identified and generally practiced across the industry for eutectic Sn-Pb solder joints. However, the mechanical properties, visual appearance, and solder joint geometry of lead-free solders, in combination with lead-free and lead-containing terminations, are markedly different from those of PbASn, as listed in Table 28. Therefore, new visual inspection guidelines must be developed [19]. 

The lead-free solder alloys attracting the greatest attention are the tin-rich alloys based on the tin-silver (Sn-Ag) and tin-copper (Sn-Cu) systems and their ternary alloys, Sn-Ag-Cu [3-5]. Numerous studies have been reported on Sn-Ag-Cu near-ternary alloys investigating processing and mechanical property behavior, and also their solidification characteristics [6-9]. 

Ninomiya, R. Miyake, K. Matsunaga, J. Microstructure and mechanical properties of new lead free solder. Proc InterPACK 97 Kohala Coast, HI, ASME New York, 1997 1329-1333. 

Incorporation of dispersoids has been pursued to improve the mechanical and, particularly, thermomechanical behavior of solders and their service-temjjerature capability without significantly altering the processing parameters [16-19]. This chapter addresses the role of dispersoids, their requirements, methods to incorporate them in a solder, and their influence on the microstructure and properties. Prior studies have dealt with several aspects of dispersoids in leadbearing solders, which will be briefly reviewed as they provide the basis for developments in lead-free solder systems. 

Given the wide selection of lead-free solder alloys with higher processing temperatures than eutectic Sn-Pb solder, a choice required a solder that had equivalent or better mechanical properties, adequate availability, and one with suitable soldering characteristics. The focus was... 

The NCMS project developed a large data set of the mechanical properties of lead-free solder alloys. For three Pb-free solder alloys and eutectic Sn Pb, these include creep data, constitutive equations, and microstructural data, as well as detailed descriptions of test methods, sample geometries, and material data for the components and boards used in the experimental testing, as listed in Table 15. Partial datasets for 75 additional solder alloys were generated and are included in the NCMS CDROM [2]. 

JWES is investigating the standardization of lead-free solder evaluation methods. It is necessary to standardize test methods related to the evaluation of material properties such as melting, solidification, mechanical properties, and wetting properties for comparison of Pb-free alloys. New standard test methods are expected to be established by 2002. JEITA plans to establish comparable standards for the evaluation of components and soldered parts as well. 

To establish the properties of lead-free solders relative to the Sn-Pb eutectic alloy with respect to their mechanical and thermomechanical performance. 

Physical properties of the alloys were measured, mainly at NMRC. Tables 28-31 list selected physical and mechanical properties of eutectic Sn-Pb and lead-free solders, such as coefficient of thermal expansion (CTE), elasticity, yield stress, and plastic behavior. Tables 28-31 illustrate a significant point about lead-free solders as compared with Sn Pb. Eutectic Sn-Pb is more ductile at low temperatures, while the lead-free solders are much more ductile at temperatures of 100°C and greater. The effect of Bi in hardening the base Sn-3.8Ag-0.7Cu alloy is noted in Table 29. (Data in Tables 30 and 31 should only be used to compare temperature trends because the specific values obtained are dependent on test conditions, therefore the data reported in the hterature may vary.)... 

It is well known that the microstructure of a solder alloy, like any material, has a very significant effect on its mechanical properties. The higher the application temperature in proportion to a material s melting point, the more rapid is the microstructural change, and hence the impact on solder joint reliability. Successfully migrating onto and implementing lead-free solders requires better understanding of how these solder alloys interact with the termination and lead materials... 

McKormack, M. Jin, S. New Pb-free solders. J. Electron. Mater. 1995, 23 (7), 635 40. McKormack, M. Jin, S. Improved mechanical properties in new, lead-free solder alloys. Journal of Electronic Materials 1994, 23 (8), 715-720. 

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