SMT - IPC Conference Raises New Lead-free Reliability Concerns

(April 13, 2007) BOSTON — The IPC/JEDEC "Global Conference on Lead-free Reliability and Reliability Testing for RoHS Lead-free Electronics," April 10–11 in Boston, demonstrated that, with more in-depth and long-term testing, the industry still does not agree on some fundamental questions about lead-free, such as the true life expectancy of a high-reliability product manufactured with lead-free solder, and the nature of tin whiskers. Certain common themes emerged at the conference: lead-free solders are improving with enhancements to the metallurgy of bulk solder; and component surface finishes greatly affect the success of lead-free reflow. Some problematic elements of the lead-free assembly process, such as BGA warpage, cannot be solved in the assembly facility, and require further R&D and control from component manufacturers.

Most presentations focused on the brittle nature of lead-free, compared to more ductile properties of leaded solder. Vibration studies, sheer and ball-drop tests, and rework cases focused on this aspect. Some results — such as Tom Woodrow's conclusion that Z-axis vibration severely impacts the center of a PCBA more so than side regions — pointed to design considerations. Lead-free solder joints on large, rigid components, such as BGAs, are less likely to fracture in extreme environments when they are placed off to the side of a PCB. Other studies concluded that bulk solder properties can be improved by adjusting silver contents.

Brian Lewis, Ph.D., of Cookson Electronics Assembly Materials, demonstrated increased reliability of new lead-free formulations, compared to initial products. Metals such as bismuth (Bi), nickel (Ni), and phosphorus can affect bulk solder properties as much as silver (Ag), copper (Cu), and other "majority" metals, noted Lewis.

Tin whiskers are still a hot discussion topic at many conferences on lead-free, and at the IPC/JEDEC show was no different. We learned about "wormy" tin whiskers, which, despite being relatively long, are bent and twisted back so much that their growth does not cause bridging. This raised the question: "Is this an 'acceptable' tin whisker?" Lead whiskers were also seen — 0.5-mil "stubby" whiskers illustrated in a scanning electron microscope (SEM) image. These discoveries open more avenues for research.

Several take-away messages emerged from the gathering of experts at the IPC/JEDEC conference. Reliability is still an unknown, because lead-free manufacturing has not been proven in the field as long as leaded solders have, and accelerated life testing and simulations do not tell the whole story. As Nihal Sinnadurai stated it, "people assume that one person's work [in testing] will apply to their product, but how wrong that is." For reliability to improve in the lead-free realm, companies need to concentrate on applicable, accurate testing; control outsourcing so that reliability is not compromised with faulty assembly practices; and consider the "big picture" of climate, vibration, and importance of the end-product, so that available R&D and process-control resources are allocated to their best use, and end products continue to become more reliable and better understood.

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