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Part 10: Lead-free System Reliability Power of Metallurgy
December 31, 1969 |Estimated reading time: 4 minutes
By Jennie S. Hwang, Ph.D.
On this global lead-free electronics manufacturing's tenth anniversary (plus and minus a year or so), it is timely to reflect what have progressed and what is on the horizon. If I have to mention one thing that stands out, that is the power of metallurgy. How so?
The industry has evolved in such a way that we have generally accepted two developmental routes in terms of lead-free solder joint alloys. Whether it is the best way or not is a separate discussion. These two material routes are the tin/silver/copper (Sn/Ag/Cu SAC) system and the SnCu system.
PrinciplesThe basic physical and mechanical properties of both SAC and SnCu can be readily measured and predicted. A natural question to pose: are these basic material properties able to exhibit the true performance in an actual product, which has its own design, structure, and performance parameters? This is a fair question. Certainly, the external parameters that a product encounters in the field do affect performance results. Even just under the laboratory testing, data can vary with the testing parameters. Nonetheless, two principles shall not be violated.
First, a solder joint cannot deliver performance beyond its intrinsic material properties, such as under fatigue environment or under mechanical shock. In other words, an inferior material cannot deliver superior performance. Second, the test results that fall outside the realm of fundamental principles shall point to the need of troubleshooting. In such situations, an effort should be directed to understand the causes instead of accepting the results spontaneously.
In lead-free as in leaded systems, system properties follow the metallurgy religiously. The principles can be delineated into several key areas. This leads to the fundamental metal strengthening approaches in relation to the anticipated common damaging mechanisms, which are discussed in the books Modern Solder Technology for Competitive Electronics Manufacturing (McGraw-Hill, 1996) and Environment-Friendly Electronics Lead Free Technology, (Electrochemical Publications LTD, U.K., 2001).
Practice Relying on these principles, one can design solder alloys starting from the least number of elements Note that the least number of elements in a solder alloy is not equal to the simplest metallurgy. In a SAC system, the solder's microstructure develops when adding Cu to SnAg or adding Ag to SnCu. The microstructure and the expected metallurgical phases correlate to the given performance. When the performance does not measure up to the required parameters, further modifications by additional "elements" can alter the properties as shown in following schemes.
SnAg → SnAgCu → SnAgCuNi → SnAgCu+x,y,z and other systems
SnCu → SnCu+Ni → SnCu+Ni +x → SnCu+Ni +x,y,z and other systems
Which elements to use? This all depends on what end results to target at and what are the likely failures to mitigate.
The likely failure processes and failure modes of solder joints include interfacial, near-interfacial, bulk, inter-phase, intra-phase, voids-induced, surface-crack-induced, and others. The failure mechanisms can proceed through ductile, brittle, and ductile-brittle fracture. It is generally understood that fatigue failure is often caused by dislocation slip and the localization of plastic deformation. It is also generally understood that the plastic deformation follows the power-law dislocation climb-controlling mechanism at high-stress/low-temperature conditions. At low stress region and high temperature, the grain boundary sliding becomes a rate-controlling process. Therefore, to strengthen the overall performance of a tin-based alloy that is to be subject to stress or cyclic strain as a result of external conditions (temperature or mechanical) and/or in-circuit power dissipation and power on/off, several approaches can be considered.
Approaches that can potentially hinder this damaging phenomenon fall in four types: microscopic incorporation of non-alloying dopant; microstructural strengthening; alloying strengthening; and macroscopic blend of selected fillers. These approaches involve both process and material factors.
Real World PerformanceThe most rewarding experience that has been happening in the lead-free world for the last ten years is that the actual performance of a specific solder alloy is in congruence with the teaching and principles of metallurgy, regardless which solder composition is selected and used in which type of products. This applies to the good performance, as well as to the not-so-good performance, such as certain defects or deficiencies observed on the production floor or in the field. Hardly any exception has been observed. It is the power of metallurgy!
APPEARANCES:Dr. Hwang will deliver lectures on "Lead-free Reliability How to Alleviate Failures" and "Interactive Discussion on Lead-free Electronics" at APEX on Monday, March 30.
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. Since the inception of SMT manufacturing, she has helped improve SMT manufacturing yield and solved challenging reliability issues. Having held executive positions with Lockheed Martin Corp., Sherwin Williams Co., SCM Corp, IEM Corp., she is currently a principal of H-Technologies Group providing business 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. In addition to technical publications, she is an international speaker and author on trade, business, education, and social issues. Contact her at (216) 839-1000; JennieHwang@aol.com.
Read Part 9: Lead-free System Reliability Avoid Likely FailuresPart 8: Lead-free Reliability for Harsh Environment ElectronicsPart 7: Lead-free Reliability for Harsh- environment ElectronicsPart 6: Lead-free Reliability for Harsh-environment ElectronicsPart 5: Lead-free Reliability for Harsh-environment ElectronicsPart 4: How to Test and Assess Lead-free ReliabilityPart 3: Testing and Assessing Lead-free ReliabilityPart 2: Testing and Assessing Lead-free ReliabilityPart 1: Testing and Assessing Lead-free Reliability