Tin-silver-copper

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Tin-silver-copper ( Sn-Ag-Cu , also known as SAC), is a lead-free (Pb-free) alloy commonly used for electronic solder. It is the main choice for lead-free surface-mount technology (SMT) assembly in the industry, [1] as it is near eutectic, with adequate thermal fatigue properties, strength, and wettability. [2] 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 legislation to remove lead and other hazardous materials from electronics. Japanese electronics companies have also looked at Pb-free solder for its industrial advantages.

Contents

Typical alloys are 3–4% silver, 0.5–0.7% copper, and the balance (95%+) tin. [3] 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 (JEIDA) and Waste Electrical and Electronic Equipment Directive (WEEE). These initiatives resulted in tin-silver-copper alloys being considered and tested as lead-free solder ball alternatives for array product assemblies. [4]

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. [5] In 2005, tin-silver-copper alloys constituted approximately 65% of lead-free alloys used in the industry and this percentage has been increasing. [1] Large companies such as Sony and Intel switched from using lead-containing solder to a tin-silver-copper alloy. [6]

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 of SAC alloys is 217–220 °C, or about 34 °C higher than the melting point of the eutectic tin-lead (63/37) alloy. This requires peak temperatures in the range of 235–245 °C to achieve wetting and wicking. [1]

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. iNEMI has proposed that a good target for development purposes would be around 260 °C. [7]

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. [8] 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. [8]

Advantages

SAC solders have outperformed high-Pb solders C4 joints in ceramic ball grid array (CBGA) systems, which are ball-grid arrays with a ceramic substrate. [9] The CBGA showed consistently better results in thermal cycling for Pb-free alloys. The findings also show that SAC alloys are proportionately better in thermal fatigue 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. [7]

Related Research Articles

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<span class="mw-page-title-main">Eutectic system</span> Mixture with a lower melting point than its constituents

A eutectic system or eutectic mixture is a homogeneous mixture that has a melting point lower than those of the constituents. The lowest possible melting point over all of the mixing ratios of the constituents is called the eutectic temperature. On a phase diagram, the eutectic temperature is seen as the eutectic point.

<span class="mw-page-title-main">Ball grid array</span> Surface-mount packaging that uses an array of solder balls

A ball grid array (BGA) is a type of surface-mount packaging 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 put on a dual in-line or flat package. The whole bottom surface of the device can be used, instead of just the perimeter. The traces connecting the package's leads to the wires or balls which connect the die to package are also on average shorter than with a perimeter-only type, leading to better performance at high speeds.

<span class="mw-page-title-main">Surface-mount technology</span> Method for producing electronic circuits

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 to as a surface-mount device (SMD). In industry, this approach has largely replaced the through-hole technology construction method of fitting components, in large part because SMT allows for increased manufacturing automation which reduces cost and improves quality. It also allows for more components to fit on a given area of substrate. Both technologies can be used on the same board, with the through-hole technology often used for components not suitable for surface mounting such as large transformers and heat-sinked power semiconductors.

<span class="mw-page-title-main">Brazing</span> Metal-joining technique

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<span class="mw-page-title-main">Restriction of Hazardous Substances Directive</span> European Union directive restricting ten hazardous materials

The Restriction of Hazardous Substances Directive 2002/95/EC, 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 Union.

<span class="mw-page-title-main">Wave soldering</span>

Wave soldering is a bulk soldering process used for the manufacturing of printed circuit boards. The circuit board is passed over a pan of molten solder in which a pump produces an upwelling of solder that looks like a standing wave. As the circuit board makes contact with this wave, the components become soldered to the board. Wave soldering is used for both through-hole printed circuit assemblies, and surface mount. In the latter case, the components are glued onto the surface of a printed circuit board (PCB) by placement equipment, before being run through the molten solder wave. Wave soldering is mainly used in soldering of through hole components.

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<span class="mw-page-title-main">Reflow soldering</span> Attachment of electronic components

Reflow soldering is a process in which a solder paste is used to temporarily attach one or thousands of tiny electrical components to their contact pads, after which the entire assembly is subjected to controlled heat. The solder paste reflows in a molten state, creating permanent solder joints. Heating may be accomplished by passing the assembly through a reflow oven, under an infrared lamp, or by soldering individual joints with a hot air pencil.

<span class="mw-page-title-main">Rework (electronics)</span> Refinishing operation of an electronic printed circuit board assembly

Rework is the term for the refinishing operation or repair of an electronic printed circuit board (PCB) assembly, usually involving desoldering and re-soldering of surface-mounted electronic components (SMD). Mass processing techniques are not applicable to single device repair or replacement, and specialized manual techniques by expert personnel using appropriate equipment are required to replace defective components; area array packages such as ball grid array (BGA) devices particularly require expertise and appropriate tools. A hot air gun or hot air station is used to heat devices and melt solder, and specialised tools are used to pick up and position often tiny components.

<span class="mw-page-title-main">Selective soldering</span>

Selective soldering is the process of selectively soldering components to printed circuit boards and molded modules that could be damaged by the heat of a reflow oven or wave soldering in a traditional surface-mount technology (SMT) or through-hole technology assembly processes. This usually follows an SMT oven reflow process; parts to be selectively soldered are usually surrounded by parts that have been previously soldered in a surface-mount reflow process, and the selective-solder process must be sufficiently precise to avoid damaging them.

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<span class="mw-page-title-main">Dip soldering</span>

Dip soldering is a small-scale soldering process by which electronic components are soldered to a printed circuit board (PCB) to form an electronic assembly. The solder wets to the exposed metallic areas of the board, creating a reliable mechanical and electrical connection.

<span class="mw-page-title-main">Thermal profiling</span>

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<span class="mw-page-title-main">Soldering</span> Process of joining metal pieces with heated filler metal

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<span class="mw-page-title-main">Bismuth–indium</span>

The elements bismuth and indium have relatively low melting points when compared to other metals, and their alloy bismuth–indium (Bi–In) is classified as a fusible alloy. It has a melting point lower than the eutectic point of the tin–lead alloy. The most common application of the Bi-In alloy is as a low temperature solder, which can also contain, besides bismuth and indium, lead, cadmium, and tin.

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Solder fatigue is the mechanical degradation of solder due to deformation under cyclic loading. This can often occur at stress levels below the yield stress of solder as a result of repeated temperature fluctuations, mechanical vibrations, or mechanical loads. Techniques to evaluate solder fatigue behavior include finite element analysis and semi-analytical closed-form equations.

<span class="mw-page-title-main">Solder alloys</span>

Solder is a metallic material that is used to connect metal workpieces. The choice of specific solder alloys depends on their melting point, chemical reactivity, mechanical properties, toxicity, and other properties. Hence a wide range of solder alloys exist, and only major ones are listed below. Since early 2000s the use of lead in solder alloys is discouraged by several governmental guidelines in the European Union, Japan and other countries, such as Restriction of Hazardous Substances Directive and Waste Electrical and Electronic Equipment Directive.

References

  1. 1 2 3 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
  2. ↑ Lead-Free Solder FAQ’s Archived October 15, 2006, at the Wayback Machine
  3. ↑ Sawamura, Tadashi; Igarashi, Takeo (2005-06-29). "Difference Between Various Sn/Ag/Cu Solder Compositions" (PDF). Almit Ltd. Retrieved 2016-08-24.
  4. ↑ STATS picks pure-tin solder as best lead-free packaging solution, eetimes, Nov 24, 2000
  5. ↑ Can lead-free solder joints be good looking? (and give better sounds quality?), interconnectionworld, Dec 16, 2003
  6. ↑ "Getting the Lead Out", T. DeBonis, Intel 2007
  7. 1 2 Lead-free Solder Assembly: Impact and Opportunity, Edwin Bradley, Motorola
  8. 1 2 David Hillman; Matt Wells; Kim Cho; Rockwell Collins. "The Impact of Reflowing A Pb-free Solder Alloy Using A Tin/Lead Solder Alloy Reflow Profile On Solder Joint Integrity". Lead Free Papers. American Competitiveness Institute. S2CID   2678802.
  9. ↑ PCB Glossary
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