Although lead has traditionally been used in numerous industrial applications, current regulations have mandated the elimination and/or phase out of lead in most commercial products. These mandates have stimulated new product development based upon lead-free technologies.
Soldering applications, particularly in electronics and vehicle manufacturing, have been heavily impacted by the ban on lead. Numerous alternatives to traditional lead-based solders have been developed, the Sn/Ag/Cu (SAC) system being among the most widely used, but many have exhibited drawbacks that can make them unsuitable for use in certain applications. For example, SAC solder can be unsuitable for extreme environments such as those found in automotive, military, and space vehicles, where long life and reliability are of significant importance. Furthermore, SAC solder has a significantly higher eutectic melting point (m.p. of ˜217° C.) than does traditional Sn/Pb solder (m.p. of 183° C. for 63/37 Sn/Pb or 188° C. for 60/40 Sn/Pb), thus limiting its use to substrates that are capable of withstanding its relatively high processing temperature. The same is also true for many other lead-free solder replacements. The need for high performance, thermally stable substrates for use in conjunction with SAC and other lead solder replacements can significantly impact the cost of consumer products relative to those in which lower quality substrates can be used. Another limitation of SAC solder is that its high tin content makes it prone to tin whisker formation, which can increase the risk of electrical shorting.
Several compositions containing nanoparticles have also been proposed as replacements for traditional lead-based solders. Metal nanoparticles, particularly those that are about 20 nm or less in size, can exhibit a significant melting point depression over that of the corresponding bulk metal, thereby allowing the nanoparticles to be liquefied at temperatures that are often comparable to those of traditional lead-based and lead-free solder materials. Copper nanoparticles, in particular, have been extensively studied as an alternative solder material. Although metal nanoparticles having a widely dispersed size range can be desirable in some instances, it can be more favorable in some applications for the metal nanoparticles to have a narrow, more controlled size range. Although some success has been realized in this regard by using tailored combinations of surfactants during metal nanoparticle growth, scalable processes for reliably producing bulk quantities of metal nanoparticles in a targeted size range are not yet well developed. Without being bound by any theory or mechanism, it is believed that in most metal nanoparticle syntheses, nanoparticle nucleation and growth are competing processes that take place concurrently. That is, as new metal nanoparticles are being formed by a nucleation process, other nanoparticles continue to grow, thereby resulting in a wide nanoparticle size distribution.
In addition to soldering applications, metal nanoparticles have been proposed for use in a number of other fields including, but not limited to, communication, electronic, and medical uses. Production of bulk quantities of metal nanoparticles having a narrow and desired size range remains a challenge for implementing many of these contemplated uses of metal nanoparticles.
In view of the foregoing, scalable processes for producing metal nanoparticles that address current issues relating to nanoparticle size disparity would represent a substantial advance in the art. The present invention satisfies the foregoing need and provides related advantages as well.