Lead-free solder alloys

M. Abtew et al. published a chart of the electromagnetic force (EMF) of various metal couples present in some lead-free solder alloys. As a rule, the lower the EMF, the more corrosion-resistant the alloy. Eutectic Sn-Pb, the basis of comparison, was by far the lowest value on the list at 0.010 volts. The next nearest, Sn-51In, was 0.201 volts or 20 times the voltage... 

Solder requirements are the same as for any other process. There are no ahoy composition requirements specific to laser soldering, as this soldering method is compatible with leaded or lead-free solder alloys. Even high-temperature alloys can be soldered by this technique. When single-point laser reflow is apphed, the board quality and integrity are not compromised if parameters are chosen and adequately controlled. 

Sn-8Zn-3Bi Lead-Free Solder Alloys in. NaCl Solution, 16th International Corrosion... 

Over the last decade, the industry has studied a wide range of alloys to replace the tin-lead alloy. The alloy selection has been based on the following considerations (Ref 12-15) toxicity, physical properties (melting temperature, surface tension and wettability, thermal and electrical conductivity), mechaiucal properties, mi-crostructural characteristics, electrochemical properties (corrosion, oxidation and dross formation, and compatibility with no-clean fluxes), manufacturability, cost, and availability. Yet another important consideration for selecting the lead-free solder alloy for commercial use is whether or not the alloy may be covered by any patents. Lead-free alloy selection, as weU as associated patent issues, have been described in detail in toe literature in Ref 16-20. 

There are many solder alloys that do not contain lead, and various lead-free solder alloys will continue to be used in tbe industry. Currently, the industry (Ref 17) is converging on the ter-... 

Other lead-free solder alloys which have been developed and evaluated by fee industry include Sn-Ag, Sn-Ag-Bi, Sn-Bi-In, and other ternary and quaternary alloys (Ref 24-33). The use of alloys containing indium (In) may be limited due to fee scarcity of indium and its high cost. The eutectic Sn-Ag alloy (melting temperature 221 °C, or 429 °F) was qualified for automotive applications in fee 1990s on ceramic thick film substrates and PWBs (Ref 3,12-15,34- 5). The microstructural characteristics of the Sn-Ag-Cu system under different reflow conditions and... 

For high temperature applications, the eutectic Au-20Sn alloy (melting temperature 278 °C, or 532 °F) has been used for die attach, especially for optoelectroiucs packaging. Tin-antimony (melting temperature 235 °C, or 455 °F) is another example of a high temperature lead-free solder alloy. For low temperature applications, alternatives include Sn-Zn, Sn-Zn-Bi, Sn-Bi-ln, and Sn-Bi-Ag (Ref 46 8). 

The discussions in fee following sections of this chapter, as well as fee other chapters of this book, will be focused primarily on fee SAC and tin-copper lead-free solders, as these are currently the leading lead-free solder alloy choices for fee worldwide electronics industry. There are still ongoing discussions about fee merits of these and other alloy choices nonetheless, a sig-itificant amount of industry resources have been and will continue to be devoted to building up fee infrastructure around the characteristics of these alloys. 

Early attempts to simply mix no-clean flux (developed for tin-lead) with lead-free solder alloys yielded miserable results. No-clean flux required reformulation for lead-free solder alloys in order to accommodate their unique characteristics. The chemical reaction between fee flux and fee solder alloy in fee paste affects fee rheological characteristics of fee solder paste (which is critical for printing performance). The differences in density between fee lead-free and tin-lead solder alloys mean feat fee metal loading of fee solder paste needs to be different. 

Rework for lead-free solders has been found to be more difficult, because the lead-free solder alloys typically do not wet or wick as easily as the Sn-Pb solder due to their difference in wettability. This can be easily seen with QFP packages. In spite of these differences, successful rework methods (both manual and semi-automatic) have been developed (Ref 74-75) with lead-free solders (Sn-Ag-Cu, or Sn-Ag), for many different types of components. Most of the rework equipment for tin-lead can still be used for lead-free solder. For area array packages, it is helpful to use a rework system with split vision and temperature profiling features. The soldering parameters must be adjusted to accommodate the higher melting temperature and reduced wettability of the lead-free solder. The other precautions for tin-lead rework (such as board baking) still apply to lead-free rework. 

D. Shangguan and A. Achari, Lead-Free Solder Alloys, U.S. Patent 5,429,689 (1995), and Mexico Patent 196053... 

W. Ren, Z. Qian, M. Liu, S. Liu, and D. Shangguan, Investigation of a New Lead-Free Solder Alloy Using Thin Strip Specimen, ASME Transactions Journal of Electronic Packaging, I2I (1999), p 271-274... 

Fatigue and Creep of Lead-Free Solder Alloys Fundamental Properties... 

G. Grossmann, G. Nicoletti, and U. Soler, Results of Comparative Reliability Tests on Lead-Free Solder Alloys, 52nd Electronic... 

Among the various lead-free solder alloys, the National Electronics Manufacturing Initiative (NEMI) recommends Sn-3.9Ag-0.6Cu ( 0.2%) for reflow soldering (Ref 1), while JEITA (Japan Electronics and Information Technology Industries Association) recommends Sn-3.0Ag-0.5Cu. Under most typical use conditions, Sn-Ag-Cu (SAC) alloy in general exhibits a greater resistance to creep as compared to Pb-Sn alloy and hence creeps 10 to 100 times slower (Ref 2). The microstructure, the plastic and creep behavior, and the failure mechanism in Sn-Ag-Cu solder are vastly different compared to Pb-Sn solder, and therefore, it is important to develop appropriate thermomechanical predictive models for Sn-Ag-Cu solder. 

Constitutive Models for Lead-Free Solder Alloy... 

Isotropic elastic properties, E, the Young s modulus, and t>, the Poisson s ratio can be used to represent the elastic behavior of the solder alloy. The temperature-dependent elastic behavior of two lead-free solder alloys Sn-Ag-Cu and Sn-Ag are presented in this section. For example, E is given by ... 

---