Pb-Free Sn-Ag-Cu-Al or Sn-Cu-Al Solder

Abstract

A solder alloy includes Sn, optional Ag, Cu, and Al wherein the alloy composition is controlled to provide a strong, impact-and thermal aging-resistant solder joint that has beneficial microstructural features and is substantially devoid of Ag₃Sn blades.

Description

[0001]This application claims benefits and priority of U.S. provisional application Ser. No. 61 / 343,135 filed Apr. 23, 2010, the disclosure of which is incorporated herein by reference.CONTRACTUAL ORIGIN OF THE INVENTION[0002]This invention was made with government support under Contract No. DE-ACO2-07CH11358 awarded by the U.S. Department of Energy. The government has certain rights in the invention.FIELD OF THE INVENTION[0003]The present invention provides a Pb-free solder alloy (Sn—Ag—Cu—Al or Sn—Cu—Al) that displays reliable joint solidification control to provide a strong, impact- and thermal aging-resistant solder joint having beneficial microstructural features and substantially devoid of Ag3Sn blades and that is useful for joining electronic assemblies and electrical contacts and to substitute for Pb-containing solders in all surface mount solder assembly operations, including solder paste reflow and ball grid array joints.BACKGROUND OF THE INVENTION[0004]The global electron...

AI Technical Summary

Benefits of technology

[0023]Solder joints made with the Sn-optional Ag—Cu—Al solder alloy may need to accommodate some minor addition of Pb due to reflow and mixing with Sn—Pb component lead plating during reflow assembly of solder joints. Slight contamination by such small Pb levels is not expected to raise the beneficial (about 217° C.) melting point of the Sn—Ag—Cu—AI solder alloys of the invention and may even help improve the wettability during joint formation. This type of Pb-tolerant behavior is an advantage over competing Sn—Ag—Bi (Pb-free) solders that run the risk of generating an extremely low melting Sn—Pb—Bi ternary eutectic, if alloyed with Sn—Pb component platings. It is expected that the global supply of “legacy” electronic components with Sn—Pb solder plating will continue to diminish and eventually vanish during the current transition to full Pb-free electronic soldering, but this possibility must be tolerated in any new Pb-free solders that are proposed.

[0024]The Sn—Ag—Cu—Al solder alloy exhibits a reduced melting point (solidus or melting temperature) of about 217° C. as compared to the melting points of Sn—Ag binary eutectic solder (221° C.), and the Sn—Cu binary eutectic solder (227° C.) and a narrow liquid-solid mushy zone with a liquidus temperature not exceeding about 5° C., preferably 3° C., above the solidus temperature for solderability. This significantly reduced melting point is a great advantage for solder assembly of electronic circuits and electrical systems. In the type of solder paste reflow and ball grid array (BGA) applications that are envisioned for use with the Sn—Ag—Cu—Al solder, every single degree of reduced reflow temperature is a precious advantage for reducing damage to temperature sensitive electronic components and to the circuit board material, itself. In fact, a reason that SAC solder came into broad use as a Pb-free alternative to Sn—Pb solder is that the minimum reflow temperature of SAC solder for most applications, about 240° C., is just below the threshold for significant damage of one of the most popular circuit board materials, a fiberglass / epoxy composite, i.e., FR-4. Thus, the Sn—Ag—Cu—Al solder alloy pursuant to the present invention should permit a more comfortable margin for preventing thermal damage of most components and common circuit board materials.

Problems solved by technology

The unusually high undercooling of the SAC solder joints was associated with the difficulty of nucleating Sn solidification, as a pro-eutectic phase.

Especially during slow cooling, e.g., in ball grid array (BGA) joints where cooling rates are less than 0.2° C. / s, increased undercooling of the joints also can promote formation of undesirable pro-eutectic intermetallic phases, specifically Ag3Sn “blades,” that tend to coarsen radically, leading to embrittlement of as-solidified solder joints.

Because these alloys deviate increasingly from the eutectic, they exhibit a wider melting range (mushy zone) with a liquidus temperature (for SAC 105) as high as 226° C. Unfortunately, some observations of unmodified SAC 105 interfacial failure on impact loading still occurred, since occasional high undercooling still may permit Ag3Sn blade formation during slow cooling.

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