Part 11: Lead-free System Reliability - Power of Metallurgy Continued


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The performance of lead-free solders strictly follows solder metallurgy. Here, we look at the core values at play for two common lead-free solder alloys: tin/silver/copper (Sn/Ag/Cu – SAC) and tin/copper (Sn/Cu).

In my last column on Lead-free System Reliability – Power of Metallurgy, I laid out the principles and practice of lead-free solder performance. It was also stated that the performance of lead-free solder alloys follows the solder metallurgy religiously, which offers confidence in anticipating the performance trends of solder alloys. This is invaluable, since anticipating performance trends saves research money and shortens time-to-market. The correlation between solder metallurgy and solder joint performance is vividly illustrated by the real-world merits and issues of two widely used lead-free solder systems: SAC and SnCu. Let's look at the core values of the solder metallurgy and take an example to demonstrate the core values.

Core Values of Solder Metallurgy

Lead-free solder joint reliability hinges on core values of solder metallurgy through key metallurgical features, functionally and structurally: primary phase, second phase, grain boundary (phase boundary), in-grain strengthening, and dispersion strengthening.

The individual roles of these metallurgical features in the microstructure, their relative volume distribution in the microstructure, their interplay, and their responses to the external environmental parameters (such as extreme temperature excursion, mechanical shock) determine the physical and mechanical behavior of the solder joint, thus the solder joint integrity and reliability.

Take the alloy system evolution from Sn to SnAg to SnAgCu to SnAgCuNi to SnAgCu+X, Y, Z as an illustration. As discussed in my books Modern Solder Technology for Competitive Electronics Manufacturing, McGraw-Hill, 1996 and Environment-Friendly Electronics – Lead Free Technology, Electrochemical Publications LTD U.K., 2001, Sn is a weak material incapable of bearing sudden load or cyclic strains that are inherent in the service conditions of electronics' solder joints.

When incorporating Ag into the Sn base, an intermetallic compound, Ag3Sn, is formed, which may be in the manner of wide primary bands that separates Sn matrix and/or as the second phase Ag3Sn precipitates at grain boundaries. The new phase adds hardness and strength to the Sn base.

If proceeding to add Cu to this SnAg system, Sn grain matrix is strengthened by Ag3Sn precipitates and Cu6Sn5 precipitates (Environment-Friendly Electronics – Lead Free Technology, p.241-242, Electrochemical Publications, LTD, U.K., 2001). The additional Cu6Sn5 precipitates can also be located at Sn-Sn grain boundaries.

From here, the SnAgCu system can be modified by other elements. Which element to choose depends on the anticipated metallurgical reactions between the elements (Sn, Ag, Cu, and X). It also depends on what end results to target at and what the likely failure mode to mitigate. If the target is to increase the strength at high temperature, it takes different element(s) from that if the target is to alleviate the surface cracks during the solder joint formation on the production floor as commonly occurred.

When adding Ni in a small amount (less than 1.0% wt) to SAC (creating SnAgCuNi), the microstructure is expected to comprise Ag3Sn and CuyNizSn second phases in the primary Sn matrix, and likely the fine, stable second phases of Ag3Sn and CuyNizSn at the Sn-Sn grain boundaries. The microstructure may vary with the Ag content that determines the thickness of Ag3Sn bands. Ni addition SnAgCu should increase the solder joint strength over SAC in the elevated temperature range; however, it may not be advantageous during the very low temperature excursion. By adding Ni or other doping elements, based on the nature of this metallurgical mechanism, it is not expected to lower the melting temperature of the SnAgCu near-eutectic compositions (such as SAC 405, SAC305). It takes an entirely different metallurgical mechanism to lower the melting temperature of an alloy.

To fortify the point, some publications of lead-free reliability test results will be selected and assessed in relation to solder metallurgy in my upcoming Webinar series (For more info, visit smtonline.com).

Conclusion

The most rewarding experience in the lead-free world for the last ten years is that the actual performance of each solder alloy is in congruence with the teaching and principles of solder metallurgy, regardless of which solder composition is selected or for which product type that solder is used. This applies to the good results, as well as to the not-so-good ones (defects or deficiencies observed on the production floor or in field service). Hardly any exception has been observed – it is the power of metallurgy.  SMT

APPEARANCES

Dr. Hwang will offer a comprehensive webinar series on solder joint reliability in 2010. She will also deliver in-person lectures at SMT Hybrid & Packaging Conference, Nuremberg, Germany, June 8, 2010.

Jennie S. Hwang, Ph.D., an SMT Advisory Board member, is inducted to the WIT International Hall of Fame, elected to the National Academy of Engineering, and named an R&D-Stars-to-Watch. Having held executive positions with Lockheed Martin Corp., Sherwin Williams Co., SCM Corp, IEM Corp., she is a principal of H-Technologies Group providing business and manufacturing solutions. She serves on corporate, civic and university boards and is a member of the U.S. Commerce Department's Export Council. In addition to 300+ publications, she is an international speaker and author on trade, business and educational issues. Contact her at (216) 577-3284; jenniehwnag@aol.com.

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