Tin-silver-copper ( Sn- Ag- Cu, also known as SAC), is a

lead-free The Restriction of Hazardous Substances Directive 2002/95/EC (RoHS 1), short for Directive on the restriction of the use of certain hazardous substances in electrical and electronic equipment, was adopted in February 2003 by the European Unio ...

( Pb-free)

alloy An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductility, ...

commonly used for electronic

solder Solder (; NA: ) is a fusible metal alloy used to create a permanent bond between metal workpieces. Solder is melted in order to wet the parts of the joint, where it adheres to and connects the pieces after cooling. Metals or alloys suitable ...

. It is the main choice for lead-free

surface-mount technology Surface-mount technology (SMT), originally called planar mounting, is a method in which the electrical components are mounted directly onto the surface of a printed circuit board (PCB). An electrical component mounted in this manner is referred ...

(SMT) assembly in the industry,Peter Biocca
Lead-free SMT Soldering Defects: How to Prevent Them
mirror
Lead-Free Defects in Reflow Soldering – How to Prevent Them
emsnow, Feb 17, 2005 as it is near eutectic, with adequate

thermal fatigue Thermo-mechanical fatigue (short TMF) is the overlay of a cyclical mechanical loading, that leads to fatigue of a material, with a cyclical thermal loading. Thermo-mechanical fatigue is an important point that needs to be considered, when constructi ...

properties, strength, and wettability. Lead-free solder is gaining much attention as the environmental effects of lead in industrial products is recognized, and as a result of Europe's

RoHS The Restriction of Hazardous Substances Directive 2002/95/EC (RoHS 1), short for Directive on the restriction of the use of certain hazardous substances in electrical and electronic equipment, was adopted in February 2003 by the European Unio ...

legislation to remove lead and other hazardous materials from electronics. Japanese electronics companies have also looked at Pb-free solder for its industrial advantages. Typical alloys are 3–4%

silver Silver is a chemical element with the Symbol (chemistry), symbol Ag (from the Latin ', derived from the Proto-Indo-European wikt:Reconstruction:Proto-Indo-European/h₂erǵ-, ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, whi ...

, 0.5–0.7%

copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...

, and the balance (95%+)

tin Tin is a chemical element with the symbol Sn (from la, stannum) and atomic number 50. Tin is a silvery-coloured metal. Tin is soft enough to be cut with little force and a bar of tin can be bent by hand with little effort. When bent, t ...

. For example, the common "SAC305" solder is 3.0% silver and 0.5% copper. Cheaper alternatives with less silver are used in some applications, such as SAC105 and SAC0307 (0.3% silver, 0.7% copper), at the expense of a somewhat higher melting point.

History

In 2000, there were several lead-free assemblies and chip products initiatives being driven by the

Japan Electronic Industries Development Association The (Formerly ) was an industry research, development, and standards body for electronics in Japan. It was merged with EIAJ to form JEITA on November 1, 2000. JEIDA was similar to SEMATECH of the US, ECMA of Europe. JEIDA developed a number o ...

(JEIDA) and

Waste Electrical and Electronic Equipment Directive The Waste Electrical and Electronic Equipment Directive (WEEE Directive) is the European Community Directive 2012/19/EU on waste electrical and electronic equipment (WEEE) which, together with the RoHS Directive 2011/65/EU, became European Law ...

(WEEE). These initiatives resulted in tin-silver-copper alloys being considered and tested as lead-free

solder ball In integrated circuit packaging, a solder ball, also a solder bump (ofter referred to simply as "ball" or "bumps") is a ball of solder that provides the contact between the chip package and the printed circuit board, as well as between stacked pa ...

alternatives for array product assemblies. In 2003, tin-silver-copper was being used as a lead-free solder. However, its performance was criticized because it left a dull, irregular finish and it was difficult to keep the copper content under control. In 2005, tin-silver-copper alloys constituted approximately 65% of lead-free alloys used in the industry and this percentage has been increasing. Large companies such as

Sony , commonly stylized as SONY, is a Japanese multinational conglomerate corporation headquartered in Minato, Tokyo, Japan. As a major technology company, it operates as one of the world's largest manufacturers of consumer and professional ...

and

Intel Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California. It is the world's largest semiconductor chip manufacturer by revenue, and is one of the developers of the x86 seri ...

switched from using lead-containing solder to a tin-silver-copper alloy.

Constraints and tradeoffs

The process requirements for (Pb-free) SAC solders and Sn-Pb solders are different both materially and logistically for electronic assembly. In addition, the reliability of Sn-Pb solders is well established, while SAC solders are still undergoing study, (though much work has been done to justify the use of SAC solders, such as the iNEMI Lead Free Solder Project). One important difference is that Pb-free soldering requires higher temperatures and increased process control to achieve the same results as that of the tin-lead method. The

melting point The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends ...

of SAC alloys is 217–220°C, or about 34°C higher than the

of the eutectic tin-lead (63/37) alloy. This requires peak temperatures in the range of 235–245°C to achieve

wetting Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. This happens in presence of a gaseous phase or another liquid phase not miscible with th ...

and wicking. Some of the components susceptible to SAC assembly temperatures are

electrolytic capacitors An electrolytic capacitor is a polarized capacitor whose anode or positive plate is made of a metal that forms an insulating oxide layer through anodization. This oxide layer acts as the dielectric of the capacitor. A solid, liquid, or gel ...

, connectors,

opto-electronics Optoelectronics (or optronics) is the study and application of electronics, electronic devices and systems that find, detect and control light, usually considered a sub-field of photonics. In this context, ''light'' often includes invisible form ...

, and older style plastic components. However, a number of companies have started offering 260 °C compatible components to meet the requirements of Pb-free solders. iNEMI has proposed that a good target for development purposes would be around 260°C.Lead-free Solder Assembly: Impact and Opportunity
Edwin Bradley, Motorola Also, SAC solders are alloyed with a larger number of metals so there is the potential for a far wider variety of

intermetallics An intermetallic (also called an intermetallic compound, intermetallic alloy, ordered intermetallic alloy, and a long-range-ordered alloy) is a type of metallic alloy that forms an ordered solid-state compound between two or more metallic elemen ...

to be present in a solder joint. These more complex compositions can result in

solder joint Soldering (; ) is a process in which two or more items are joined by melting and putting a filler metal (solder) into the joint, the filler metal having a lower melting point than the adjoining metal. Unlike welding, soldering does not involv ...

microstructures that are not as thoroughly studied as current tin-lead solder microstructures. These concerns are magnified by the unintentional use of lead-free solders in either processes designed solely for tin-lead solders or environments where material interactions are poorly understood. For example, the reworking of a tin-lead solder joint with Pb-free solder. These mixed-finish possibilities could negatively impact the solder's reliability.

Advantages

SAC solders have outperformed high-Pb solders C4 joints in ceramic

ball grid array A ball grid array (BGA) is a type of surface-mount packaging (a chip carrier) used for integrated circuits. BGA packages are used to permanently mount devices such as microprocessors. A BGA can provide more interconnection pins than can be p ...

(CBGA) systems, which are ball-grid arrays with a ceramic substrate.PCB Glossary
/ref> The CBGA showed consistently better results in thermal cycling for Pb-free alloys. The findings also show that SAC alloys are proportionately better in

as the thermal cycling range decreases. SAC performs better than Sn-Pb at the less extreme cycling conditions. Another advantage of SAC is that it appears to be more resistant to gold embrittlement than Sn-Pb. In test results, the strength of the joints is substantially higher for the SAC alloys than the Sn-Pb alloy. Also, the failure mode is changed from a partially brittle joint separation to a ductile tearing with the SAC.

References

Tin alloys Fusible alloys Brazing and soldering {{Update, date=July 2016