iNEMI Publishes Guidelines for Sub-<br>assemblies in High-Reliability Applications

With the current RoHS exemptions for high-reliability electronics, it is possible for electronic assemblies to contain lead (Pb) and still be RoHS-compliant. Manufacturers taking the Pb exemption will continue to require SnPb-compatible components for their products, and may also use subassemblies, such as hard disk drives and power modules, that may or may not be Pb-free. "It is very likely that manufacturers will use subassemblies that contain both tin-lead and lead-free components, which causes reliability concerns due to the differences in processing temperatures and materials," said Thilo Sack, principal engineer, corporate technology, for Celestica Inc. and co-chair of the iNEMI High-Reliability RoHS Task Force. "It is the task force's position that, if RoHS6 subassemblies are to be used in RoHS5 products, the RoHS6 subassemblies should be thoroughly qualified to ensure that they will meet the higher reliability requirements of the RoHS5 products. Furthermore, in some cases, such as BGAs that are internal to RoHS5 or RoHS6 subassemblies, 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." Some of the additional guidelines and requirements defined by the iNEMI task force include: Pb-free Sn finishes on leaded and discrete components should only be used when these finishes (1) include suitable tin whisker mitigation practices, as defined in the IPC/JEDEC joint publication JP002, Current Tin Whiskers Theory and Mitigation Practices Guideline, and (2) have passed Class 2 level tin whisker acceptance testing requirements, as outlined in JEDEC standard JESD-201, Environmental Acceptance Requirements for Tin Whisker Susceptibility of Tin and Tin Alloy Surface Finishes. For subassembly modules meeting RoHS5 or RoHS6 requirements, plated leads or pins (the module terminals) that will be attached to the printed circuit assembly should be backward compatible with SnPb assembly processes. For RoHS5 or RoHS6 subassemblies, the MSL rating of the module should be based on the worst case MSL rated component used in the module. MSL rating should be provided for SnPb and SAC assembly if applicable to the module. Additionally, the module should meet the temperature requirements of IPC/JEDEC J-STD-020 (revision C or later) for the appropriate SnPb and/or SAC assembly process. For RoHS6 subassembly modules (Pb-free module terminals, and the subassembly of the module is built with Pb-free solder and processes) PWBs internal to the module should be manufactured using materials compatible with high-temperature Pb-free soldering. PWB surface finishes also need to be compatible with the Pb-free solder alloy and higher temperature reflow soldering process used. The complete set of guidelines can be downloaded here. (PDF)