How one company made the switch.

EMS firms have been involved in the Pb-free transition for a considerable time. In fact, one might say the process has reached a level of maturity. Early adopters were faced with myriad issues such as solder paste availability, board materials, surface finishes, component terminations, parts numbering and logistics, processing temperatures, and the lack of reliability data. This level of change was daunting even for the larger EMS firms, but had a major impact on smaller firms that lacked abundant engineering resources. So how did we do it and how has the process matured?

VirTex began its transition in 2004 to be ready for the July 1, 2006, European Union deadline. To examine the process, it is necessary to break it down into each element, examine the changes, and define what has been accomplished by the industry to move from initial utilization to a more refined set of materials and processes.

Solder paste. When the process transition was started, it made sense to go from SnPb eutectic to a Pb-free eutectic (e.g., SAC 309 or SnAg3.0Cu0.9) material. Manufacturing several circuit card assemblies revealed excessive component tombstoning. A move to a lower copper content (0.7%) reduced tombstoning significantly, but it was the move to SAC 305 that provided the results we were looking for: tombstoning occurring on less than 0.5% of the subject 0402 and 0201 components. Many other companies that followed this path achieved the same results, and SAC 305 became the de facto standard for SM reflow and wave. In general, Pb-free solder materials do not wet as well as the standard SnPb compositions, but have achieved a level of acceptability. The greater use permits industry to accumulate reliability data and focus on process improvements and enhanced reliability.

Components. Several issues became obvious as various devices were switched to RoHS compliant materials. These issues included parts numbering, materials, lead finish or ball composition, and compatibility with higher temperatures associated with Pb-free assembly. In the 2004 timeframe, it was necessary to take a customer bill of materials (BoM) and manually crosscheck the parts to those that met Pb-free manufacturing requirements. Many parts, such as passives, were available in Pb-free equivalents for years and, as such, the part numbers remained the same. Other suppliers put a notification on their Web sites; still others changed their part numbering schemes. The result: a logistical nightmare. Today, most distributors have a customer-accessible database that quickly permits the uploading of a BoM and cross-referencing of Pb-free part numbers. Finally, it became necessary to take leaded parts headed toward obsolescence, remove the lead finish and recoat using Pb-free materials. VirTex accomplished this with the help of Six Sigma Corp., which stripped and refinished the parts. This action provided our customers a year to redo software and incorporate new devices into their designs.

Another issue is the ability of parts to withstand the almost 40°C increase in reflow temperatures for Pb-free processing. In many cases, components, such as connectors, were RoHS-compatible (meaning that the seven banned materials were not present in the new part), but were not compatible with the Pb-free manufacturing process. These parts would either melt or warp at the higher temperatures. Today, most datasheets provide the maximum temperature to which a component can be subjected.

Finally, as BGA or other area array packages go, so goes the assembly. If the balls on any BGA are Pb-free, then the entire assembly must be produced with Pb-free materials (e.g., solder paste, board materials, etc.) to ensure solder joint reliability. In this circumstance, it is vital for the EMS firm to verify the BGA balls are Pb-free. This can be accomplished through a simple swabbing operation or XRF (x-ray fluorescence). In many instances, customers did not realize that a Pb-free BGA required significant changes in other materials, particularly the printed wiring board, as an FR-4 board may not survive the higher reflow temperatures. The EMS company must guide the customer in order to avoid product failures.

PWB fabrication. As noted, standard FR-4 materials are not designed to handle the 250°C exposure required of Pb-free reflow. To remain competitive, PWB suppliers have switched to RoHS-compliant materials. This involved the removal of polybromide-biphenyls (PBB) or polybromide-biphenyloxides (PBBO) from the fabrication process. In 2004, this was not the case. It was vital that the EMS interact with the supplier to identify, or specify, the laminate and B-stage materials required. Most suppliers now use materials such as IS410 or Nelco-4000 to meet Pb-free requirements. As a result of the switch, users must pay attention to a new parameter, CAF (conductive anodic filament) resistance. This involves how close traces can be designed before the propensity for a dendrite to grow between the traces increases. EMS companies must obtain this information from their board suppliers to manage bare board procurement.

Surface finishes. Circa 2004, a HASL finish was typical on most FR-4 boards. RoHS mandated the removal of lead. As such, surface finishes like electroless-nickel/immersion gold (ENIG), immersion silver (ImAg), or organic surface protectant (OSP) are now among the common technologies. More recently, some fabricators are using a high temperature HASL finish, which is also viable. It is necessary to provide this information to your supplier. VirTex prefers the finishes in the order noted, as the ENIG approach provides the most uniform surface for reflow; silver does well, and OSP offers some challenges in our experience.

ImAg boards must also be properly stored so that the finish does not oxidize. We place a sheet of Daubert Cromwell anti-tarnish paper between boards and seal them in nitrogen. All other boards are either sealed in nitrogen or with desiccant to eliminate oxidation.

OSP is removed from the surface of the board during reflow, and the combination of flux in the paste and the reflow temperatures activate its removal. Because of the slow wetting of Pb-free solder pastes, VirTex has experienced issues with achieving complete OSP removal. This can result in pad opens where the material was not completely removed.

Stencil design. Spreading characteristics of leaded vs. Pb-free solder pastes are considerably different, with the newer Pb-free materials having a much slower wetting cycle. This variance in the material characteristics provides a reliability concern because of the potential for exposed copper on the board pads, particularly in pad corners and perimeters, thus increasing the potential for tombstoning or solder balling. As noted, the solder paste formulation plays heavily into the success of this process. So, too, does stencil aperture design. VirTex has found that rounded apertures behave significantly better, particularly when used with 0402 or 0201 size components. This produces a more uniform solder flow and facilitates alignment of the components. With continued component miniaturization, aperture openings and stencil alignment to board pads are critical for repeatable solder joint quality.

Manufacturing flow. The overall SMT manufacturing flow is essentially the same sequence of operations used for leaded manufacturing. However, the applied reflow temperatures, to achieve SAC 305 solder reflow at 217°C, must be increased from 240° to 252°C. These increased temperatures and slower wetting cycles require the EMS fully understand the materials submitted for reflow. Significant damage to an assembly can occur if the laminate is not compatible with higher soldering temperatures. In the case of a Pb-free BGA device and a non-RoHS board, the EMS company should suggest to its customer that the product not be assembled. The potential for the board delaminating is very high and irreparable.

Also, if reflowing a Pb-free BGA using a conventional leaded temperature of 215°C, ensure the balls on the BGA do not slump and a reliable solder joint is formed. It is imperative that the EMS verify all these packaging parameters prior to reflow.

Rework. Pb-free rework processes have been developed during the past couple years. The applied temperatures from the focused hot air or gas can be as high as 280°C. Again, the materials used must be capable of withstanding these temperature levels.

Pb-free board manufacturing has matured significantly during the past three years. Parts are now more common; board materials can handle the higher temperatures; component terminations and board finishes are compatible with the increased temperatures; solder paste formulations provide better reflow characteristics, and processes have been developed for all functions. Now we are quickly moving toward a time in which process refinements that enhance reliability will be the focus – just in time to support the new Chinese requirements.

Bibliography

• James Hall, “The Maturing of Lead-Free Assembly,” SMT, May 2007.
  
  
• Chrys Shea and Ranjit Pandher, “Optimizing Stencil Design for Lead Free SMT Processing,” SMTA International Proceedings, September 2004.
  
  
• Greg Caswell, Lead Free Manufacturing from a Contract Manufacturer’s Perspective, IPC Designers Council, February 2005.

Greg Caswell is director, engineering services, at VirTex Assembly Services (virtexassembly.com); gregc@virtexassembly.com.