Solder physical, mechanical properties

Solders. In spite of the wide use and development of solders for millennia, as of the mid-1990s most principal solders are lead- or tin-based alloys to which a small amount of silver, zinc, antimony, bismuth, and indium or a combination thereof are added. The principal criterion for choosing a certain solder is its melting characteristics, ie, soHdus and Hquidus temperatures and the temperature spread or pasty range between them. Other criteria are mechanical properties such as strength and creep resistance, physical properties such as electrical and thermal conductivity, and corrosion resistance. 

After having discussed the physical, chemical, and mechanical properties, we may say a few words about the soldering of aluminium. This soldering is not without difficulties, which are both of a physical and chemical nature. 

Metal finishing is the name given to a range of processes carried out to modify the surface properties of a metal, for example by the deposition of a layer of another metal or a polymer, or by formation of an oxide film. The origins of the industry lay in the desire to enhance the value of metal articles by improving their appearance, but in modern times the importance of metal finishing for purely decorative reasons has decreased. The trend is now towards surface treatments which will impart corrosion resistance or particular physical or mechanical properties to the surface (e.g. conductivity, heat or wear resistance, lubrication or solderability) and hence to make possible the use of cheaper substrate metals or plastics covered to give them essential metallic surface properties. 

Table 3.1-286 Physical and mechanical property data on Pb-Sn solders [1.309]...

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. 

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... 

The importance of understanding the physical metallurgy of the Pb-Sn alloy system stems from the microstructure-properties relationship. That is, the microstructure of a Pb-Sn alloy, or any material for that matter, determines its mechanical properties. In the case of Pb-Sn solders, this relationship is particularly critical because (1) the microstructure of these materials is sensitive to the cooling rate used in an assembly process, and (2) the microstructure can be readily altered during the course of the service life experienced by an interconnection. 

V. PHYSICAL AND MECHANICAL PROPERTIES OF LEAD-TIN SOLDER AND SOLDERED JOINTS... 

The Pb-Sn solders continue to play a critical role in the Level 1, Level 2, and Level 3 interconnections of commercial and military electronics. The widespread availability of Pb and Sn ores and the relative ease of winning the elements from these ores have contributed to their low primary metal costs. However, the real attributes of the Pb-Sn system lie in the physical and mechanical properties of these alloys ... 

This chapter will review a few promising Pb-free solders and their applications in electronic packaging in terms of their physical and mechanical properties, microstructure, wetting characteristics, interfacial reactions, thermal fatigue behaviors, and reliability concerns. The Pb-free solders discussed in this chapter include the binary systems, Bi n, Sn-Sb, Sn-Cu, Sn-Zn, Sn-In, and their ternary or quaternary compositions. 

The mechanical and physical properties of bulk eutectic Bi-Sn eutectic solder compare favorably with eutectic Sn-Pb as noted in Table 2. The mechanical properties of the solder alloys are strongly influenced by both the strain rate and their microstructure [24]. This strong strain-rate sensitivity of these materials is recognized by noting the wide ranges of tensile strength and... 

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.)... 

The effects of small Ge additions on physical and mechanical properties were also studied. It was concluded that small additions of Ge contribute to enhance both surface and interface characteristics to improve bonding and bulk materials properties. It was reported that Ge-containing solders have similar or better performance and higher reliability than eutectic Sn-Pb solders. Although these results are very promising, this solder family must be appropriately tested and exercised under actual assembly conditions in manufacturing. The interfacial microstructures and joint rehability require further investigation. 

Final properties are often characterized into physical, electrical, mechanical, and chemical properties. Most people who specify solder mask utilize the criteria in the IPC-SM-840 document when specifying desired mask performance. There are other industry and regional specifications but the IPC specification is recognized worldwide. 

The accepted method of nondestructive testing used to control the underfill process is SAM. The thin layer allows this technique to detect voids in the underfill material, which when located near the solder interconnections can be responsible for a significant loss of thermal mechanical fatigue reliability. X-ray techniques can be used to monitor the density of the underfill material, specifically, the distribution of filler material within the layer under the die. Density variations can indicate a larger distribution of underfill mechanical and physical properties, which may affect long-term reliability performance of the solder joints. Quantitative image analysis can be coupled into SAM and x-ray analysis data to provide valuable process control tools for the factory floor. 

The test methodologies, whether tension/ compression tests on bulk solders or shear tests on solder joints, have their merits for specific study objectives. Tension and compression tests on bulk solder provide critical input data for constitutive models. The finite element analysis within those models can account for geometric effects on creep deformation in an actual joint configuration (included spatially varying stress state). However, length-scale effects of small joints, or elemental contamination that alters the intrinsic mechanical and physical properties of the solder, require an entirely new set of constitutive equations because finite element analysis cannot account for these effects. Thus, in the latter circumstance, the most accurate creep data would be results obtained from tests on actual joints. 

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