Lead-free Reliability issues

Obviously, many lead-free reliability issues were not addressed in this chapter and much development work lies ahead for the industry s understanding of SAC reliability to come up to par with that of standard Sn-Pb solder. 

The recommended lead-free solder formulation is Sn-Ag-Cu for board assembly but there are other formulations such as Nickel-Palladium (NiPd), or Nickel-Palladium with Gold flash (NiPdAu). Passive components, to be compatible with a lower temperature Lead process (which is 215°C for 50/50 Tin/Lead formulations and 230°C for 40/60 formulations) and the higher lead-free process of up to 260°C, use pure matte Tin for their contacts. The use of lead in solder is partially based on several potential reliability issues. Pure Tin component leads have been shown to result in inter-metaUic migration in the termination of the electronic component and the growth of tin whiskers which could cause short circuits (which is why there is a exemption for military use (only) components). 

The issues of defining and predicting reliability of products in this new situation have absorbed a great deal of corporate resources since the EU directive was announced. The predictive models and history were based on tin-lead alloys, and new models are needed for lead-free alloys. For this edition, we have added new material and expanded existing discussions on this subject. 

Lead-Free Assembly and Long-Term Reliability Issues... 

A key issue for manufacturers is the lead-free alloy melting point of 217°C, higher than the 183°C for lead alloys. The higher melting point wiU have significant impact on manufacturing processes and potentially on component reliability. 

The mandated conversion to lead free assemblies has led to the convergence of several critical issues in the microelectronics industry. The conversion has further compounded several other factors that have been pushing the envelope in Component to Printed Wiring Board (PWB) reliability, including... 

There are three primary issues related to PWB for lead-free soldering laminate materials for reliability (to be discussed in a later section), halogen content, and surface finishes for soldera-bility. 

The issue of component mixing, or cross-contamination, warrants special concern, especially during the industry-wide transition to lead-free. If a tin-lead solder board is to be repaired (for example for warranty repair at some fiiture time) with a lead-free solder, the reliability of homogeneously mixed lead-free solder and tin-lead solder is probably not inferior to the tin-lead solder in most cases however, the temperature impact on the components (especiily plastics package parts) could be a concern. Carefol consideration must be given to the use of area array packages with lead-free balls to repair a tin-lead... 

Other component reliability issues related to lead-free solders include flip chips and wafer level CSPs with lead-free solder bumps and balls, where the higher soldering temperature and higher stiffness of the lead-free solder can adversely effect the reliability of the low-k dielectric layer on the die. Low k dielectric is needed for high speed applications, but is typically more fragile and prone to cracking. 

Managing the compatibility issues is critical to lead-free transition. These include materials compatibility (solder, components and PWB), process compatibility (reflow, wave soldering, rework, equipment, and yield), design compatibility, reliability compatibility, and business compatibility (cost, supply chain, and operations). 

M. Amagai, M. Watanabe, M. Omiya, K. Kishimoto, and T. Shibuya, Mechanical Characterization of Sn-Ag-Based Lead-Free Solders, Microelectronics Reliability, Vol 42 (Issue 6), June 2002, p 951-966... 

P. Chaleo, Solder Fatigue Reliability Issues in Lead-Free BGA Packages, SMTA Pan Pacific Microelectronics Symposium Proc., 2002,p 163-168... 

P. Chaleo, and E. Blackshear, Reliability Issues of BGA Packages Attached with Lead-Free Solder, The Pacific Rim/ASME International Electronic Packaging Technical Conf Proc., 2001 (CD-ROM)... 

As the preceding chapters have illustrated, lead-free solder interconnect reliability is an important, yet complex, subject. A significant amount of work has been carried out over the past decade, yet more challenges still remain. Some of the most pressing issues are outlined in the following. 

Significant volumes of research and development work on manufacturing issues associated with lead-free assembly have been conducted and published in the past decade by the industry, national laboratories, consortia, and academia worldwide. Reliability studies of lead-free solder interconnects, however, are still emerging. 

The last issue to consider is the optimumcooling rate needed to produce the most reliable lead-free solder joint. This, however, is another area that does not have a definitive answer. There are several formal studies that can provide guidance, but no generally accepted specification. 

Excessive solder voids can create reliability issues, especially in applications where the lead-free assembly is exposed to thermal... 

Developing a lead-free SMT process requires planning and a close-working relationship with all suppliers. Understanding component and board compatibility issues with the use of higher temperatures is essential. Avoiding certain elements, such as bismuth and lead that may impact solder joint reliability also is important. 

Martin Wickham, National Physical Laboratory, "Voiding Occurrence and Reliability Issues with Lead-free."... 

Icey issue and goal in lead-free technology is sound metallurgy and long-term joint reliability. This is critical to ensure that future generations of electronics circuits do not fail in the field. 

These are just a few issues of concern regarding the transition to lead-free solder. Many questions still remain. This is evident in the multitude of studies, experiments, workshops and conferences available focusing on reliability, inspection, rework, alloy selection, and exemptions, as well as labeling and materials declarations. As the deadline draws near, most of the kinks should be worked out. It seems most U.S. companies are taking the EU dates seriously. No one is shirking their duties, says NEDA s Martin. No one said, No big deal, they ll slip those dates. The best advice, adds Martin, is to prepare and keep your eyes on the EU s laws as each member state completes their laws. Companies can do things in anticipation, but it all trickles down and it seems that time will tell how lead-free plays out for the entire SMT supply chain. Until then, maybe we should still keep our eyes on the price of duct tape. 

The wettability and solderability of lead-free solder candidates must be of sufficient adequacy to form solder joints whose reliability is not degraded relative to eutectic Sn-Pb joints due to wettability-related issues. 

Most lead-free alloys of interest have a melt temperature that is about 40°C higher than eutectic Sn-Pb (mp = 183°C), which can have a drastic effect on the integrity, reliability, and functionality of printed wiring boards, components, and other attachments (e.g., connectors) [18]. The bulk of lead-free research has focused on identifying suitable solder candidate alloys to replace eutectic Sn-Pb. Solder pastes, wires, and bar stocks are only a part of the issue. The effects of the increase in process temperature necessary for utilizing most Pb-free solders are listed in Table 26 and discussed in the following sections. 

There are two main drivers motivating the migration to lead-free solders legislative-based issues and health concerns related to lead (Pb)-containing materials including solder. Also, there is pressure to support the growing worldwide movement to remove lead from processes and products to demonstrate environmental consciousness and maintain a competitive posture in the market place. However, assuring the reliability of solder joints made with lead-free materials is difficult due to the lack of field experience and lack of fundamental property data upon which to construct reliable service life models [2]. 

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