Authored By:
Eric Bastow, Timothy Jensen
Indium Corporation, Clinton, NY, USA
Summary
The advent of the EU's RoHS law has encouraged a significant amount of research to find an alloy, for electronic assembly, that will satisfy RoHS's lead-free requirement and have optimum process ability and field reliability. The resulting research, much of it lead by iNEMI, resulted in the near eutectic tin-silver-copper alloy SAC387 (Sn95.5Ag3.8Cu0.7) as an initial favorite to fill this need in the early 2000s. By 2004 or so, many people were using SAC305, partially because of its greater resistance to tombstoning. It appeared that SAC305 would become the de-facto lead-free standard alloy for RoHS compliant electronic assembly. However, with the dramatic increase in silver prices in the last few years, SAC105, having 2% less silver was being evaluated and used for its obvious cost savings.
Reliability testing of SAC105 also showed that although it did not perform as well as SAC305 in thermal fatigue cycle testing, it was better than SAC305 in drop shock tests. The explosive growth of mobile phone sales, over 1 billion per year, made SAC105's superior drop shock performance attractive for these and other portable devices.
In addition to research relating to SAC305 and SAC105, much work has been performed on the study of the effects of small quantities (<0.1%) of alloying metals on lead-free alloys' process ability and reliability performance. These "dopants" can dramatically affect an alloy's performance.
All of the above work has resulted in what many are calling lead-free alloy proliferation as more and more alloys are being considered for implementation. This proliferation drives up solder paste cost as manufacturers cannot achieve economies of scale. In addition, with so many alloys to consider, it is difficult for researchers to develop extensive data bases of process and reliability performance.
This paper is an overview of this lead-free alloy proliferation and an outlook on how alloy convergence might occur
Conclusions
The combination of the move to lead free solders, the miniaturization of solder joints, the explosive growth of portable electronics, and the high price of silver has resulted in a proliferation of lead-free solders to meet these varying needs. Undoubtedly, individual companies seeking competitive advantages in the marketplace have further driven alloy proliferation. Additionally, the discovery that dopant levels of certain metals in lead-free alloys can dramatically improve certain properties further exaerbates teh proliferation problem.
Groups such as iNEMI and IPC are working to help minimize the proliferation problem as alloy proliferation makes costs high and minimizes the amount of reliability data on any specific alloy. Although these efforts are encouraging, the vested interests of some companies may somewhat thawrt this effort.
However, we still see a certain type of convergence on the horizon. It could be said that even today, three lead-free solder "platforms" exist: SAC305, SAC105 and SnCu0.7. Each of the platforms may have certain dopants added, which are still being investigated, to optimize performance. We believe that these three platforms will continue to exist, as they each have benefits that support their use. The matter of dopants in each of the platforms is harder to predict. We expect that research will determine optimum dopants and eventually the marketplace will recognize the better dopant combinations and winnow out the poor ones. Unfortunately , this winnowing process may take some time.
Initially Published in the SMTA Proceedings
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