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Are You Ready for Lead-free Solder Paste?
December 31, 1969 |Estimated reading time: 7 minutes
Evaluating and selecting lead-free solder paste.
Brian D. Bauer
Michael P. O`Neill
Any lingering doubts about the global nature of the electronics assembly marketplace have been put to rest by the topic of lead-free processing. Although the United States currently has no restrictions on lead use in electronics assemblies, other markets are in the process of implementing them. As a result, manufacturers of both surface mount assemblies and hybrid circuits must meet the restrictions to remain competitive.
In Japan, several major electronics manufacturers - including Matsushita (Panasonic), Sony, Toshiba and Hitachi - have undertaken a voluntary initiative to eliminate lead by 2001, in advance of legislation proposed by the Japanese Ministry of International Trade and Industry (MITI) to mandate recycling of products containing lead. The European Community has drafted similar legislation that is expected to take effect by 2004. To minimize the need for recycling programs for electronics products, European manufacturers are already in the process of removing lead from their assembly processes. They are running pilot programs and, in at least one instance, are already running production on a lead-free assembly line.
If these markets were self-contained entities, there might not be any pressure on U.S.-based manufacturers to switch to lead-free processing. However, European and Japanese consumers represent major markets for products manufactured all over the world. For American companies with assembly facilities in the United States, South America and Southeast Asia, products assembled in those locations will face a significant competitive disadvantage without a "green" label signifying lead-free processing. Even in the United States, the green label is likely to be perceived as a positive factor by consumers concerned about environmental issues, regardless of legislation limiting lead usage.
Lead content in electronic assemblies comes from a variety of sources: components with either lead-tinned leads or solder spheres, printed circuit boards (PCB) with hot-air solder-leveling (HASL) finishes and solder paste based on the traditional tin/lead alloy. In general, component manufacturers and board fabricators are working toward eliminating lead in their products with a growing sense of urgency.
Solder paste manufacturers have been working since the early 1990s to develop lead-free alloys. Suppliers have made significant time and money investments in internal research programs, and some have participated in industry-wide consortia to evaluate alloy performance. A National Center for Manufacturing Sciences (NCMS) study involving more than 200 major manufacturers, researchers and suppliers was conducted over the course of several years. Recently, a National Electronics Manufacturing Initiative (NEMI) task force was formed to develop a lead-free strategy and evaluation criteria.
In the NCMS study, numerous lead-free alloys were reviewed. Most consisted primarily of tin combined with one or more of the following elements: silver, bismuth, copper, indium or antimony. The primary difference between all lead-free alloys and traditional tin/lead alloys is a higher reflow temperature. While tin/lead solder melts at 183°C, lead-free alloys melt at temperatures that range from 200° to 230°C. Because many components, particularly aluminum-electrolytic capacitors, cannot withstand temperatures above 240°C without being damaged, the temperature profile lower limit can be raised, but not the upper limit.
The result is a significantly smaller reflow process window. Therefore, a modern forced-convection oven with tight control over temperature variations (and preferably configured for nitrogen) is required for lead-free reflow. In the long run, component redesign may resolve this problem; indeed, recent IPC roadmapping has called for the development of components that can withstand temperatures as high as 260°C.
While no exact drop-in replacement for eutectic solder has yet been identified, a tin/silver/copper alloy (95.5Sn/4.0Ag/ 0.5Cu) is a leading candidate. This formulation, which melts at 217° to 219°C, provides effective reflow without endangering temperature-sensitive components. It also offers significant process-related benefits: fully compatible with existing screen printing, placement and reflow technologies; forms highly reliable solder joints; has excellent wetting ability and tack time; is patent-free; and does not interact with lead that may still be present in components and boards.
Production-level runs using solder paste based on this alloy have been completed successfully at 225°C without causing damage to active devices such as application-specific integrated circuits (ASIC) and ball grid arrays (BGA). Studies of the assemblies reflowed in these runs have indicated that paste based on the tin/silver/copper alloy performs at least as well as, and in some cases better than, tin/lead solder.
The tin/silver/copper alloy printability, tack time and work life were found to be equal to those used in lead-based formulations. Even though the components and FR-4 substrates used in the run contained lead, the alloy did not form a brittle intermetallic phase, a characteristic found in some other lead-free alloys. End-of-run testing found no manufacturing defects that were attributable to the lead-free solder paste.
To determine long-term solder joint stability, an extensive series of tests covering copper dissolution kinetics, isothermal aging and creep deformation was conducted. The results of these tests indicated that the alloy produced solder joints that lasted at least as long, and sometimes longer than, eutectic solder joints.1
These findings hold particular interest for the automotive industry, in which there is an ongoing effort to move as many electronic assemblies as possible out of the passenger compartment and into the engine compartment. The under-the-hood environment, however, is a hostile one, with low air flow and operating temperatures sometimes exceeding 170°C. To function in this environment, hybrid circuit materials have experienced rising specifications, from 125° to 150°C, and are expected to increase as this trend continues. Thermal cycling specifications also have become more rigorous, increasing in range from -40° to 125°C, to -40° to 150°C, and sometimes to as much as -50° to 150°C.
Whether used in surface mount assemblies or hybrid circuits, lead-free solder paste provides the improved fatigue resistance, lack of creep deformation and higher operating temperature tolerance required for these demanding applications. With this combination of functional as well as environmental benefits, the tin/silver/copper alloy is undergoing extensive evaluation among suppliers of electronics assemblies to the automotive industry.
An additional benefit of the alloy is its availability. Because it is not patented, it can be supplied by a number of solder paste manufacturers. In addition, because no licensing fees are required for a non-patented product, the alloy also should be more cost-effective. For the electronics manufacturer looking to replace tin/lead solder with a lead-free product, availability and cost are among the first criteria to consider. Others include performance reports, industry-wide studies, individual reliability testing and supplier support. In other words, homework needs to be done before an appropriate selection can be made.
Each solder paste manufacturer offering a lead-free product should be able to supply data documenting the results of its internal research program. In addition to offering lead-free products - solder paste, bar, wire or spheres - the supplier should provide the proper flux, training and process support. A flux that has been specially formulated for use at higher temperatures is critical to successful lead-free reflow. With a conventional flux, the active elements can burn off before the liquidus temperature is reached, rendering the flux ineffective for lead-free processing. Manufacturers should therefore ensure that the supplier can provide an entire lead-free system, including a flux designed for high-temperature activity that passes IPC and Bellcore requirements.
Training may be needed for workers who conduct post-reflow inspection because the wetting angles of lead-free solder joints can vary in appearance from those formed by tin/lead solder (Figure 1), even though the functionality is equal or better. A supplier familiar with the difference between the two types of solder joints should be able to provide this information. Once inspectors learn what a successful lead-free solder joint should look like, previously established inspection procedures may be used.
Process support may include a variety of services, from advising the manufacturer on the most appropriate reflow profile to conducting a process audit designed to optimize assembly line productivity. Because there often may be overlapping use of surface mount assembly and hybrid circuit applications, it can be particularly helpful to the manufacturer to work with a supplier experienced in both areas of interconnection technology. The combination of wide-ranging expertise and comprehensive laboratory resources contributes to ensuring a successful transition to lead-free assembly. SMT
REFERENCE
1 Angela Grusd, "Integrity of Solder Joints from Lead-free Solder Paste," NEPCON West, 1999.
BRIAN D. BAUER is business manager, surface mount materials, and MICHAEL P. O`NEILL is director of marketing and sales, at Heraeus Inc., Cermalloy Div., 24 Union Hill Road, West Conshohocken, PA 19428; (610) 825-6050; Fax: (610) 825-7061; Web site: www.4hcd.com.
Figure 1. BGA spheres made of traditional Sn63/Pb37 solder
and the lead-free Sn/Ag/Cu alloy are virtually identical in appearance.