For lead–free solder alloys, based on considerations for toxicity, cost and availability, manufacturability, and metallurgical, chemical, physical and mechanical properties, the worldwide industry has converged on to the Sn–Ag–Cu (SAC) eutectic alloy, based on earlier recommendation by iNEMI. More recently, IPC has specifically recommended the Sn–3.0Ag–0.Cu alloy composition. Recent fluctuations in the metal market further increased the sensitivity of the cost of the solder alloys to their Ag content. For wave soldering applications, the Sn–0.7Cu eutectic alloy has been an option for cost sensitive applications. For solder balls for area array packages (BGA and CSP), Sn–1.0Ag–0.5Cu has been introduced to increase the solder joint compliance, thereby helping reduce failures under mechanical shock (such as in drop tests).

Meanwhile, there have been many efforts to modify the SAC alloy to further improve its process performance and mechanical reliability. One of my patents with my colleagues at Ford (US5,863,493, filed 1996) teaches the addition of 0.1–2% Ni to modify the SAC lead–free solder. Another patent (EP0847829B1, priority date 1996) teaches the addition of less than 1% of Ce (and optionally Au, Bi, In, Sb, and Zn); This may well be the earliest teaching about Ce addition to SAC–based lead–free solders, to the best of my knowledge. Interestingly, the claims in the patent only refer to "an electrical solder composition," without specifying whether it refers to the original solder alloy or the solder joint produced, or how the alloying additions may be introduced. U.S. patent #5,833,921 (filed 1997) teaches the addition of 0.1–3% Ce (and optionally Ni, Co, Te, Se, Au, Cs, Si, Mg, Ca, and/or others) to Sn–Bi–Sb alloy for low temperature lead–free applications.

Recently, additions to SAC alloys (particularly using Ce, Co, and/or Ni as alloying elements) have received greater attention in many parts of the world (including some of the most important geographies for electronics manufacturing and consumption). A number of papers at conferences this year (such as ECTC) reported very interesting results from investigations by a number of large companies on the performance of lead–free solder joints which are made of SAC with alloying additions. Further research will undoubtedly help further reveal the potential benefits of these approaches. One interesting point is that, due to the small amount of the alloying additions, their impact on cost and melting temperature (therefore the soldering temperature) is not expected to be very significant, thereby easing their adoption by the industry around the infrastructure that has already been built up to accommodate the key characteristics of the SAC alloy.

At the same time, there have been efforts in the industry to develop lead–free solder alloys with a soldering temperature lower than is required by SAC. For example, U.S. patent #6,416,597 (Shangguan and Gordon, priority date 2000) teaches a method to protect solder powders of Sn–Zn eutectic alloy (melting temperature around 198oC) from oxidation in order to extend the shelf life of no–clean solder paste. It is expected that, while SAC–based alloys will remain the predominant alloy for lead–free solders for some time to come (based on considerations for the economy of scale and the industry infrastructure put in place over the past few years), active efforts will continue to be made by the industry to develop lower temperature lead–free solder alloys, in part to help overcome the difficulties that have been encountered in converting to lead–free for large complex boards typical of telecom infrastructure products currently under exemption by EU RoHS (which will be under periodic review by EU).

If I have to bet on what would follow lead–free, I would certainly put some of my money on nano–solder. Although still at its nascent stage, nano–solder offers many attractions, including process temperature, mechanical performance, miniaturization, among others. Significant work remains to be done to develop processes so that the potentials offered by nano–solder can be fully leveraged.

This column appeared in Global SMT & Packaging magazine issue 7.1 – January 2007.

Keywords : Dongkai Shangguan, Shangguan, lead-free, RoHS, SAC, Sn-Ag-Cu

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