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Part 8: Lead-free Reliability for Harsh Environment Electronics
December 31, 1969 |Estimated reading time: 4 minutes
By Jennie S. Hwang, Ph.D.
Over the last two years, I have offered a series of seven parts to cover the important aspects of lead-free reliability and its testing and assessment. This article is the eighth part to continue this series with the theme: "Back to Basics Embellished by Historical Perspectives." Usually, a writer leaves the conclusion to the end, making the story a cliffhanger. Yet, let me, just this time, deviate from that tact by stating the conclusion first.
Back to Basics"Lead-free Reliability for Harsh Environment Electronics" is the ultimate manifestation of back to basics. It cannot be emphasized enough that solder joint behavior during its service life or while in its formative stage during production follows principles of science and metallurgy. The fundamentals are even more crucial when the required performance level increases, such as for harsh environment electronics. Test results that were conducted under a variety of testing parameters by many laboratories, albeit with apparent variations, all are in congruence with the fundamental principles and metallurgical engineering, discussion of which is beyond the scope of this article. The disparity of some results can well be elucidated by the variations in design specifications, material selections, process effects and/or testing parameters. More importantly, production floor observations and real-world product performance attest to the interplay of intrinsic metallurgy and external environment.
This is an omnipotent outcome meeting the fundamental principles provides the necessary confidence in and comfort with any new system.
Let's look at some key junctures of the history and evolution of lead-free research, manufacturing, and implementation.
Historical PerspectivesBefore late 1980s, there were scattered lead-free electronics studies by individuals, corporations, and institutions. However, a concerted effort started around 1989, when lead reduction was included in the U.S. Mantech program as one of subset objectives to fulfill the advancement of military electronics in cost and performance. I was at the time in a position to get engaged in the task. Basic research and product development were faithfully and diligently carried out without legislative mandate.
In mid 1990s, a consortium initiated a program to evaluate and compare several known and new lead-free solder alloys. As a result, three alloys were selected and recommended to the industry: tin/silver (Sn/Ag) eutectic, tin/bismuth (Sn/Bi) eutectic, and tin/silver/bismuth (Sn3.5Ag4.0Bi). As the time went on, it became evident that none of these three alloys could deliver the performance and applicability suitable for all product requirements of the electronics industry.
When the enactment of the EU's RoHS directive was becoming clear in late 1990s, the broad industry participation then commenced, focusing on unifying around one lead-free solder alloy, which is the natural and understandable desire. This led to the widely deployed evaluation on tin/silver/copper (Sn/Ag/Cu SAC) system, and the abundant supply of SAC solders from most solder producers.
Starting in 1998, with Japan pioneering the lead-free manufacturing movement, other continents and countries have been gradually and steadily moving into manufacturing lead-free electronics, largely propelled by the EU's RoHS and then emerging China RoHS directives.
After several years in actual production, some production defects or presumptive defects associated with SAC system surfaced. Back to the basics, as one example, the recurring solder joint surface cracks/fissures is not a surprise due to the anticipated metallurgical phases and intrinsic microstructure during heating and cooling excursions. In this case, manufacturers reserve the rights to hand over the verdict (accept or reject) of such occurrence, pending the product performance criteria, internal workmanship standards, and other subjective factors. However, when such a phenomenon is a concern, changes and modifications to the solder alloy are in order.
While some problematic outcome were well predicted based on the fundamental metallurgical knowledge and practical manfactuing know-how governing solder and solder joint, it is not elegant simply to say "I told you so." For the past two or three years, the fact is that along with meeting other requirements, most solder suppliers have scurried to introduce new or modified solder alloys rather than SAC to mitigate some defective or undesirable results that have been occurring on the production floor and in the field.
EpitomeIn a nutshell, we, the industry as a whole, have tried hard to standardize one alloy for the lead-free world, and understandably so. For lead-free solder alloys, what actually has occurred and continues to evolve is the initial convergence followed with intermediary divergence, then eventual consolidation. The reality perhaps points to the application-specific and performance-focused practice as it should be.
APPEARANCES:Dr. Hwang will deliver lectures on lead-free reliability for harsh environment electronics and BGA/CSP/WLP reliability at SMT/Hybrid/Packaging Microelectronics Conference, June 1, in Nuremberg, Germany; and at National Electronics Week, June 17 in London.
Jennie S. Hwang, Ph.D., an SMT Advisory Board member, is elected to the National Academy of Engineering, inducted to the WIT International Hall of Fame, and named an R&D-Stars-to-Watch. During the 28-year SMT manufacturing establishment, she has helped improve SMT production yield and solved challenging reliability issues. 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.