Bob Landman, President Life Senior Member, IEEE IEEE Power & Energy/Reliability Societies IEEE Standards Association H&L Instruments, LLC www.hlinstruments.com/ ==================================== http://www.electroiq.com/index/display/smt-article-display/6648914340/s-articles/s-smt/s-volume-23/s-issue-5/s-columns/s-smt-advisory/s-challenges-of_lead-free.html This article is from Surface Mount Technology | Challenges of Lead-free Electronics Lead exemptions are going away in environmental legislation. Lead substitutes for die bumps in packages, and electromigration with lead-free assemblies are challenges prompting vigorous research. I recently attended the Annual Meeting of the Metals, Minerals and Materials Society (TMS). For more than 10 years, the Electronic Packaging and Materials Committee of TMS has sponsored sessions on lead-free research, and for the past 5 years there has also been a Sunday workshop, "State of the Art -- Lead-free Technology." The 2009 session was attended by almost 150 people. The challenges we face are many. From the regulatory point of view, the limitations defined under the European Union's (EU) Restrictions on Hazardous Substances (RoHS) and the EU's End-of-Life Vehicle (ELV) Directive currently are under review. Exemption 15 of RoHS allows leaded solder for flip chip connections within the package. This is frequently a high-lead solder (>85%) covered under Exemption 8, but can also be eutectic tin/lead solder. Under RoHS, this exemption expires at the end of 2014, but under ELV, the exemption expires in 2010 unless it is renewed. While there are several system-based approaches to removal of lead, there is no universal, reliable substitute for all applications. Potential replacements for the leaded bumps are: Gold (Au) stud bumps attached by thermal compression or thermosonic bonding for small die; Tin/silver (SnAg) on copper (Cu) pillars has successfully been implemented by Intel for finer-pitch die; Hitachi is pursuing a composite nano-solder of Cu and Sn powder; Some die attach alloys under study are lead/tin/silver (PbSnAg), cadmium/zinc (CdZn), AuSn, gold/germanium (AuGe), aluminum/silicon (AlSi), and ZnAl; Anisotropic conductive adhesives are a less expensive option but conductivity and lifetime might be compromised. While each of these materials fills a niche application, the bottom line is that there is no robust highly reliable solution available today to meet the ELV deadline of January 1, 2011 for under-hood applications. Electromigration of lead-free alloys was another hot topic at the TMS annual meeting and conference. This failure mode has gained importance as we move to high-density interconnects with fine-pitch ball grid arrays (BGA) and chipscale packages (CSPs) because the current density through the solder ball increases as ball size decreases. For example, a 50-µm ball with 0.2-A current will have a current density of 104 µA/cm2. Electromigration involves the movement of atoms in a metallic conductor due to the electron wind caused by high current density. When the BGA under bump metallization (UBM) is the cathode, current crowding occurs at the point where the trace enters the solder joint. Here, current density is highest at the component side of the BGA, causing an electron flow that depletes the cathode, creating pancake voids. The voids further reduce the contact area at the UBM, increasing the current density and the temperature of the solder ball, ultimately leading to failure. At the same time, the copper consumption at the anode creates intermetallic compounds (IMC) that migrate to the anode, thickening this area. Compressive stresses at the board interconnect are increased due to this IMC migration. These forces create Sn "hillocks" and even Sn "whiskers," according to some authors. Whisker growth is somewhat slowed by the consumption of Sn by intermetallic formation with Cu. The common test system used by several researchers to study this phenomenon consists of a fine-pitch BGA cross-sectioned to expose the center of the balls in a given row. The samples are exposed to current density of 104 A/cm2 for a period of time, then the current is removed, SEM analysis performed, and changes in morphology documented. Electromigration through the solder ball causes a temperature increase and, when device temperature is not controlled, this accelerates joint degradation, ultimately leading to failure. When electromigration is monitored under conditions where the device temperature remains constant, the time to failure is extended. SnAgCu (SAC) solders with a lower silver content, such as SAC 105, have an increased propensity for electromigration, even though they are desirable for handheld products due to improved drop test performance. Small amounts of Zn in the solder balls or in the UBM can also help to stabilize the solder joints, reducing electromigration. It has been show that when nickel (Ni) is present in the UBM, the rate of electromigration deceases, while the presence of Cu increases the migration and reduces the time to failure. It was pointed out at the conference that RF devices of 77 GHz and higher are of concern because there is a skin effect that enhances the electromigration phenomenon. Designers need to consider the potential for this failure mode as they create next-generation products. One other areas of note at the conference was the use of various doping agents to create SACX alloys for specific applications. Alloying agents discussed include Ni, Zn, bismuth (Bi), manganese (Mn), Al, and iron (Fe). The October issue of the Journal of Electronic Materials is devoted to papers from the Annual Meeting of the Metals, Minerals and Materials Society. Laura J. Turbini, Ph.D., is an SMT Advisory Board Member, an adjunct faculty member at the Universities of Toronto and Waterloo, and Chemistry Lab Manager and Principal Scientist at Research in Motion. She also serves on the Board of Directors at the SMTA. Contact her at (519) 888-7465, ext. 77744; lturbini@xxxxxxxx This article is from Surface Mount Technology
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