The electronic file here in the Knowledge Base has been updated accordingly. For anyone interested in contacting Frank Liotine directly, he can be reached at 352-259-3607 or fliotine1@netzero.com.">

Journal of SMT Article

LEAD-FREE SOLDERING FOR SMT COMPONENTS: A REVIEW OF PROCESSING AND RELIABILITY ISSUES

Authors: Frank Liotine, Jr., PE
Company: SMT Engineering Co.
Date Published: 10/1/2004   Volume: 17-4

Abstract: The worldwide move in electronics to replace lead-based solders with solders using non-lead based materials poses technical issues from processing to long-term reliability. The metallurgy of present electronic tin-lead solders is well understood. Significant history of the present technology exists from a processing and reliability standpoint. However, no direct replacement exists; hence the need to develop a good understanding of the various versions of non-leaded solders, their metallurgical properties, and reliability considerations. Printed wiring boards (PWB) and other substrates will likely switch to plating technologies using tin, such as matte tin, instead of hot air leveled tin-lead over tin plate. Components, such as ball grid arrays (BGA), chip carriers with castellations or J-lead terminations, passive chip components, and other packages will also move away from leaded solders and lead-based plating. Noteworthy is the fact that many components are already produced using tin and various other metals, such as gold over nickel, the latter typically used in higher reliability applications; hence, the component manufacturers are already in the no-lead loop for the most part. Given that no direct conventional solder replacement exists, the likelihood of possible processing and/or reliability concerns will be presented to design, quality, and manufacturing engineers.

Only recently has information been published on a variety of topics relative to lead-free soldering. Articles can now be found in the literature on types of lead solder substitutes and their processing requirements, some data published relative to reliability (fatigue resistance via thermal cycling), and some modeling information. However, very few papers have addressed all the issues in a comprehensive approach, most likely due to the fact that this is a new technology, particularly in the United States, and due to the lack of long-term experience on the subject. For example, reliability estimations have been presented empirically using HALT/HASS testing. Unfortunately, given the nature of existing tin-lead solders, it is known that strain sensitive compositions behave differently as stress is applied and as stress/strain relaxation occurs relative to cycle time. Other factors come into play also. Consequently, assuming any of the substitutes are similar in nature to the metallurgy of the existing tin-lead, a long term, comprehensive series of studies including every aspect of the application of this technology, e.g. effect of rework, needs to be established.

This paper discusses some of the results, empirical and theoretical, of various studies. A comparison to existing theoretical and empirical data will be presented, and a brief survey of studies in progress will be mentioned. The objective of the dissertation is to develop a comparative relationship between existing reliability relationships between tin-lead and non-leaded solders, as well as to address several of the process concerns with this new technology as compared to the metallurgy of the existing composition. It is anticipated that concerns may present themselves relative to non-lead given plating changes, rework methods, and temperatures necessary to reflow lead solder substitutes, especially in SMT applications.

Key words: solder joint reliability, lead-free solder, high stress environment, surface mount technology, electronic packaging, military specifications, space, substrate, circuit card, reliability model, SMT, high density packaging, process technology.

Errata: 1) In the printed issue at the bottom right of page 18, last paragraph, tenth line from the bottom of the page, "and solder paste at percentages of [Sn(4.85)Ag(0.59)Cu]", should be Sn(3.86)Ag(0.60)Cu. In the same paragraph, second line from the bottom, "will be [Sn(14.34)Ag(0.93)Cu]" should be Sn(3.85)Ag(0.53)Cu. The electronic file here in the Knowledge Base has been updated accordingly.

For anyone interested in contacting Frank Liotine directly, he can be reached at 352-259-3607 or fliotine1@netzero.com.



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