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By Meredith Courtemanche, assistant editor HERNDON, Va. The International Electronics Manufacturing Initiative's (iNEMI's) High-reliability RoHS Task Force has published guidelines regarding assembly processes and reliability requirements for RoHS 5 and RoHS 6 sub-assembly modules related to high-reliability electronics. The task force consists of electronic product manufacturers (OEMs and EMS providers) whose products are characterized by long service life and high-reliability requirements. "Subassemblies are an area of concern because they may contain material(s) or have processes that don't meet the needs of high-reliability products, such as whisker-prone termination and body finishes; or may have mounting balls (like a BGA) that are SAC and not compatible with the tin/lead assembly processes that will continue to be used by manufacturers producing exempted high-reliability products," explained Task Force co-chair Mike Davisson, distributed materials engineering manager, Agilent Technologies.
Under current RoHS exemptions for high-reliability electronics, assemblies can contain lead and still be RoHS-compliant. RoHS 5 modules are RoHS-compliant, though they contain lead, while RoHS 6 products and modules are wholly compliant, containing no lead. iNEMI authored the guidelines for these sub-assemblies due to "confusion and inconsistency relative to the acceptance" of these components, according to the published guidelines. "These sub-components play key roles in a system's reliability. In telecom systems, dropped 'data pipes' lose a vast number of connections to the telecom/Internet infrastructureentire countries and regions in more extreme cases. Reliability is very critical," warns co-chair Joe Smetana, principal engineer, advanced technology, Alcatel. Manufacturers taking a lead exemption will require tin/lead-compatible components for their products, and may also use sub-assemblies (hard disk drives and power modules) that might not be lead-free.
Using both tin/lead and lead-free components causes reliability concerns due to differences in processing temperatures and materials. If RoHS 6 sub-assemblies are used in RoHS 5 products, the RoHS 6 sub-assemblies require thorough qualification to ensure that they meet the higher-reliability requirements of RoHS 5 products, says Task Force co-chair Thilo Sack, principal engineer, corporate technology, Celestica Inc. In some situations, solders and processes should not be mixed. For example, only BGAs with tin/lead solder balls should be used in a tin/lead assembly process, and vice versa ¿ only SAC solder balls in a SAC process.
The iNEMI consortium works with government, universities, and other funding agencies to set priorities for industry needs and R&D initiatives. Many companies, such as Agilent Technologies, Inc.; Intel Corporation; Lucent Technologies; and Delphi Electronics & Safety support iNEMI recommendations regarding RoHS 5 and RoHS 6 modules. Smetana explains that "module suppliers sometimes have a lack of understanding of field conditions and/or lifetime requirements and, furthermore, they don't have the same field liability as the OEMs." iNEMI's guidelines are aimed at lessening this disparity in research. Further guidelines and requirements include a suggestion that lead-free tin finishes on leaded and discrete components be used only when the finishes include suitable tin whisker mitigation practices, and have passed Class-2 level tin whisker acceptance testing requirements. The IPC/JEDEC joint publication JP002, Current Tin Whiskers Theory and Mitigation Practices Guideline and JEDEC standard JESD-201, Environmental Acceptance Requirements for Tin Whisker Susceptibility of Tin and Tin Alloy Surface Finishes define these standards. For sub-assembly modules meeting RoHS 5 or 6 requirements, plated leads or pins that will be attached to the printed circuit board assembly (PCBA) should be backward-compatible with tin/lead assembly processes.
The report stresses the importance of understanding the "solder hierarchy" of modules and the "implications of potential multiple reflows of components." iNEMI Guidelines for RoHS 5 and RoHS 6 modules touch on topics such as moisture-sensitivity level (MSL) ratings for modules should be based on the worst-case MSL-rated component used (for both RoHS 5 and 6 sub-assemblies). The MSL rating provided for tin/lead and SAC assemblies should apply to the module. Modules should meet temperature requirements of IPC/JEDEC J-STD-020 for tin/lead and/or SAC assembly process. To clarify, "the temperature- and moisture-sensitivity levels of the J-STD-020 MSL requirements accurately reflect the component thermal durability requirements demanded by lead-free soldering processes." Therefore, modules and internal components mandate this strict specification, says the report. For RoHS 6 sub-assembly modules, printed wiring boards (PWBs) internal to the module should be manufactured using materials compatible with high-temperature, lead-free soldering. The Task Force adds that PWB surface finishes also must be compatible with the lead-free solder alloy when using a higher-temperature reflow soldering process.
The guidelines suggest future research in the industry on SAC solder joint reliability for long-lifetime, high-reliability devices in the field, including Kirkendall voids, pad-crater cracks, and thermal-cycle reliability issues. "In general, the long-term reliability models for RoHS materials and processes are not available and are, in fact, unknown at this time. The high-reliability market has electronic assemblies that are far more complex than the consumer market, which leads to more unknowns in the new assembly technologies for RoHS," says Agilent's Davisson. Other publications that provide RoHS guidelines include iNEMI's lead-free manufacturing requirements for high-complexity, thermally challenging electronic assemblies (2/16/06), and recommendations to electronics industry component supply base (8/19/05). Visit iNEMI online at www.iNEMI.org for more information.