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SMT Perspectives and Prospects: Can Microstructure Indicate a Good Solder Joint? Part III

Editor's Note: This column originally appeared in the October 2012 issue of SMT Magazine.Last month’s column ended with this insight: “A microstructure obtained in the form of a high-quality SEM or metallographic micrograph fits well the adage: ‘A picture is worth a thousand words.’ It provides sights and insights into the state of solder joint integrity and anticipated behavior.”

Indeed, a high-quality microstructure of a solder joint provides "sights" and "insights" into the state of solder joint integrity and its anticipated behavior. How does a microstructure reflect on the mechanical properties of a solder joint formed using a specific process?

If we assess the mechanical properties of a solder joint by the commonly established techniques, the top four characteristics are: Shear strength, creep, isothermal low-cycle fatigue, and thermo-mechanical fatigue. Integration of the meaning of the four characteristics offers an overall picture of the integrity of a solder joint. For eutectic SnPb solder composition, its shear strength is improved by a very slow cooling rate that can make the microstructure approach the formation of near-equilibrium lamellar eutectic structure. On the other hand, the strength can also be enhanced by using a very fast cooling rate as a result of grain size refining. 

So, for a practitioner, the natural question is: Which one is better--a faster or slower cooling rate?

For plastic deformation under creep mode, the creep resistance depends on the operating mechanism. When the lattice or vacancy diffusion process is the predominating step, the creep resistance is often lower when the grain size of microstructure becomes finer. This is due to the increased vacancy concentration as a result of faster cooling rate. With thermal cycling, the increased fatigue resistance is often associated with the decreased size of grains (phases). Under isothermal fatigue environment, the relation of microstructure and fatigue resistance is more complex. Nonetheless, the homogeneity in microstructure is most important to low-cycle fatigue resistance.

For a lead-free SnAgCu system the heating parameter, in conjunction with the metallurgy of the surface finish, can significantly impact the resulting microstructure of a solder joint. The formation of intermetallic compounds (IMCs) exhibits the pronounced contrast between the SnPb and lead-free solders. The IMC’s properties, size, morphology, and distribution will change with heating conditions and the substrate metallurgy.

For other lead-free alloys (non-SAC), such as selected doped-SnCu and quaternary alloys (SnAgCuBi, SnAgCuIn, SnAgBiIn, etc.) currently available, the reactivity and vulnerability are often lessened.

Now, what about the final performance? The final performance (or reliability) of a solder joint is driven by the service conditions (or testing parameters) which set the rate-determining mechanism of the degradation, which either induces creep-dominant degradation or fatigue-dominant degradation (barring other extraneous failure mechanisms.)

To examine microstructural features the magnification of 100 to 5,000 times is considered a practical range; more commonly 200X to 2,000X. The characterization can utilize either optical (light) or electron microscopy. More desirable, using both techniques can reveal a higher level of information. For optical microscopy, the solder specimen has to be carefully prepared through metallographic techniques involving successive grinding and polishing with an ascending level of fineness of abrasive particles bonded on papers or used as slurry on a cloth-covered wheel. The size of abrasive particles applied can range from 20 microns to several microns and even submicrons. With the polished surface, the specimen then goes through an etching process. For soft solders the concoction of an effective etching chemistry is tricky.

In comparison, scanning electron microscopy requires relatively less sample preparation when the sectioning (cutting) of the specimen is well performed. Images from either secondary electron signals or backscattered electron signals can be readily obtained. Either image, with its distinctive features, can provide informative characteristics. When combining information from images, the microstructure and morphology of a solder joint is better understood.

Dr. Hwang will present a lecture on “Preventing Assembly Defects and Failures” at IPC APEX EXPO, February 18, 2012, in San Diego, California.

Dr. Hwang, a pioneer and longstanding contributor to SMT manufacturing since its inception as well as to the lead-free development, has helped improve production yield and solved challenging reliability issues. Among her many awards and honors, she has been inducted into the WIT International Hall of Fame, elected to the National Academy of Engineering and named an R&D Stars to Watch. Having held senior executive positions with Lockheed Martin Corporation, Sherwin Williams Co., SCM Corporation and IEM Corporation, she is currently CEO of H-Technologies Group providing business, technology and manufacturing solutions. She is a member of the U.S. Commerce Department’s Export Council, and serves on the board of Fortune 500 NYSE companies and civic and university boards. She is the author of 300+ publications and several textbooks and an international speaker and author on trade, business, education and social issues. Contact her at (216) 577-3284; e-mail JennieHwang@aol.com.