Metallurgy of low temperature Pb-free solders for electronic assembly

Abstract

Recent interest in Pb-free solders was stimulated by the introduction of (US) legislation that proposed the banning of Pb from a wide variety of uses, including electronic solders. Although there are solders that do not contain Pb, there is no obvious replacement for the eutectic and near eutectic Sn-Pb alloys that are in common industrial use today. This review surveys the literature on the metallurgy of Pb-free solders of interest for electronic assembly using organic substrates (solder liquidus temperatures between 100 and 232°C). This data (primarily on Sn-based binary eutectic alloys containing Ag, Bi, Cu, and In) provides a foundation for further study of both these alloys and more complex systems. The review is divided into three major sections: physical metallurgy (including physical properties and microstructure); mechanical properties (including tensile and shear behaviour, creep, and isothermal and thermal fatigue behaviour); and oxidation and corrosion behaviour.IMR/273

Structural and dynamic properties of liquid tin from a new modified embedded-atom method force field

“…Many alternative candidate alloys still contain tin as one of the main components. 1,2 Examples include binary alloys such as tin-copper, tin-silver, tin-gold, tin-zinc, tin-bismuth, and tin-indium 3 or even ternary alloys such as tin-silver-copper. 4 Liquid metals, including liquid tin and lithium-tin alloys [5][6][7] are currently being considered as alternative plasma-facing materials in tokamak fusion reactors for several reasons, such as the fact that liquid walls can self-replenish and they offer easier means of remote maintenance.…”

Wetting of Cu and Al by Sn-Zn and Zn-Al Eutectic Alloys

“…The RoHS and similar legislation in the USA and Japan fueled the worldwide search for Pb-free replacement to Pb-containing solders. Based on a great number of publications on phase diagrams, structure, and physical-chemical properties of alloys, a number of most promising eutectic alloys including Sn-Ag, Sn-Ag-Cu, Sn-Zn, and Zn-Al (Ref [1][2][3][4] were selected. Sn-Ag, Sn-Ag-Cu, and multicomponent alloys based on them were broadly studied (Ref 5,6) and have already used been commercially.…”

Microstructural evolution of eutectic Au-Sn solder joints

“…For a specific application, the melting point of the solder directly determines the maximum allowable temperature that a product can be exposed to in service and the maximum processing temperature that devices and substrates can withstand during processing. [1,2] Another required characteristic of an effective solder alloy is its good wetting behavior with the desired substrate. This allows for the making of a strong bond between the metals to be joined.…”

“…In a thin joint, the interfaces impose a constraint that resists this contraction; consequently, the joint itself experiences triaxial loading rather than uniaxial tensile loading. [2] Secondly, the microstructure of the solder joint, which governs its mechanical behavior, is not purely determined by the nominal composition of the solder used in a joint. This is because the microstructure of the solder joint is determined by the reaction at the interface between the solder and substrates as well as the solidification The large Ag 3 Sn phases are also uniquely found only in the joint, not in the unformed bulk solder ball.…”

“…This concern has been led to pending legislation in the EU, Japan and the US for banning the use of Pb in microelectronics. [2,[9][10][11][12][13][14][15] Recently, lead-free products in electronic industries are also used as a "green" marketing strategy. A separate reason for the use of Pb-free solders is the increased demands on the level of performance due to increased density and complexity of circuitry.…”