User:Laurasmithhp/Tin Silver Copper

Overview

Tin-silver-copper (SnAgCu), also known as SACs, is a lead (Pb) free alloy commonly used in solder. The tin-silver-copper alloy is deemed the prevailing alloy system that will replace tin-lead because it is near eutectic, with adequate thermal fatigue properties, strength and wettability. Lead free solder (Pb-free) is gaining much attention as the environmental effects of lead in industrial products is recognized, and as a result of Europe’s RoHS legislation to remove lead and other hazardous materials from electronics. At the same time, Japanese electronics companies have looked at Pb-free solder as an industrial competitive advantage.

Applications

SAC alloys are the main choice for lead-free SMT (Surface-Mount Technology) assembly in the electronics industry. SMT is a process where components of circuit assemblies are mounted directly onto the surface of a printed circuit board and soldered in place. SMT has largely replaced “through-hole technology” where components are fitted with wire leads into holes in the circuit board.

History

In 2000 there were several lead-free assemblies and chip products initiatives being driven by the Japan Electronic Industries Development Association (JEIDA) and Waste Electrical and Electronic Equipment Directive (WEEE). These initiatives resulted in looking to tin-silver and tin-silver-copper alloys being considered and tested as lead-free solder ball alternatives for array product assemblies.

In 2003 tin-silver-copper was being used as a lead-free solder. At the time people were not very happy with it because it left a dull irregular finish and it was difficult to keep the copper content under control.

In 2005 tin-silver-copper alloys became the primary choice for lead-free SMT assembly. Approximately 65% of users were utilizing tin-silver-copper and this percent was increasing. Large companies such as Intel and Sony switched from using lead solder to a tin-silver-copper alloy.

Constraints and Tradeoffs

The process requirements for SAC solders (Pb-free) 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 NEMI 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 liquidus temperature of SAC alloys is 217-220 °C, or about 34 °C higher than the melting point of the eutectic tin/lead (63/37). This requires peak temperatures in the range of 235-245°C to achieve wetting and wicking.

Some of the components susceptible to SAC assembly temperatures are electrolytic capacitors, connectors, opto-electronics, 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. NEMI has proposed a good target for development purposes would be around 260 °C.

Also, SAC solders are alloyed with a larger number of metals so there is the potential for a far wider variety of intermetallics to be present in a solder joint. These more complex compositions can result in solder joint 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 process 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 CBGA assemblies. A ceramic ball grid array (CBGA) is a ball grid array system with a ceramic substrate. The CBGA showed consistently better results in thermal cycling for Pb-free alloys. And, the findings show that Sn-Ag-Cu alloys are proportionately better in thermal fatigue as the thermal cycling range decreases. Sn-Ag-Cu performs better than Sn-Pb at the less extreme cycling conditions.

Another advantage of Sn-Ag-Cu is 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 Sn-Ag-Cu than the Sn-Pb. Also, the failure mode is changed from a partially brittle joint separation to a ductile tearing with the Sn-Ag-Cu.

Manufacturers

See Thomas Register at http://www.thomasnet.com/products/leadfree-solder-76240951-1.html