Pb-free standards update: several specs have already been updated, and others are undergoing revisions.
The European Union's RoHS Directive restricting use of six hazardous substances in electrical and electronics equipment will be upon us in 10 months. Lead, in the form of SnPb solder, one of the most basic and well-studied of the six materials impacted by RoHS, is the subject of this article. Changing a solder alloy composition requires additional material and process changes to ensure the manufacture of reliable products. Standardization is essential to understanding and implementing RoHS and other Pb-free restrictions. This column discusses some of the key standards being revised or developed to clarify requirements for RoHS-compliant materials and processes.
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Components. Higher processing temperatures of Pb-free alloys require component suppliers to reclassify moisture sensitive devices as covered in J-STD-020, Moisture/Reflow Classification for Non-hermetic Solid State Surface Mount Devices. J-STD-020 revision C specifies that the temperature for moisture sensitivity level testing be selected according to thickness, volume and mass of the component, and also raises the maximum MSL testing temperature to 260[degrees]C (from 250[degrees]C).
J-STD-002, Solderability Tests for Component Leads, Terminations, Lugs, Terminals and Wires, is being updated to include testing of Pb-free component termination finishes.
Many component manufacturers have changed component termination finishes from various SnPb alloys to pure tin and high (>95%) tin coatings. These coatings raise potential concerns about tin whiskers, a subject under active study by a number of researchers. To make comparisons between studies, a standardized accelerated tin whisker test is needed. Manufacturers also need test methods for evaluating devices with tin finishes and acceptance criteria. IPC, iNEMI and JEDEC are cooperating to develop methodologies for tin whisker test standardization. In May, JEDEC released JESD22A121, Test Methods for Measuring Tin Whisker Growth on Tin and Tin Alloy Surface Finishes. This standard specifies test conditions for accelerating tin whisker growth and defines how whiskers should be inspected. JEDEC, again with contributions from iNEMI and IPC, is writing a second standard: JESD 201, environmental Acceptance Requirements for Tin Whisker Susceptibility of Tin and Tin Alloy Surface Finishes. This acceptance specification will be used with the test methods.
Finally, IPC, working jointly with JEDEC and with input from iNEMI, is developing a document titled Current Tin Whiskers Theory and Mitigation Practices Guideline. This guide will help tie all that is known about tin whiskers. The user will be able to go to a single information repository that contains the two standards described above, plus this guideline document. This guideline will be revised as further data come available. The belief is that, with enough data being gathered, the word "mitigation" will change to "elimination" and the title of "guideline" will convert to "specification." However, the ultimate goal of a specification is not a short-term one.
Assembly materials qualification. IPC is revising a number of standards related to assembly materials. Revisions are underway on J-STD-004, Requirements for Soldering Fluxes. IPC-HDBK-005, Guide to Solder Paste Assessment, supports J-STD-005, Requirements for Soldering Pastes, and incorporates details of Pb-free materials. Revision of J-STD-005 is beginning and will include additional Pb-free data. Release is nearing of the latest revision for J-STD-006, Requirements for Electronic Grade Solder Alloys and Fluxed and Non-Fluxed Solid Solders, in which Pb-free alloys are addressed.
Assembly standards. When are IPC standards going to address workmanship requirements for Pb-free? Now. IPC-A-610D and IPC J-STD-001D, released this year, contain requirements for Pb-free solder connections. At the basic level, there are no differences in the solder wetting and fillet requirements. However, some differences in appearance exist. Often, acceptable Pb-free and SnPb connections exhibit similar appearances but Pb-free alloys are more likely to have surface roughness (grainy or dull), cooling lines and potentially different wetting coverage. Pb-free often exhibits a greater contact angle, but again, the 90[degrees] angle requirement for Pb-free is no different than that for SnPb.
Unique requirements acceptable only to Pb-free include fillet lifting--separation of the bottom of the solder and the top of the land--and hot tearing or shrink holes. BGA voiding may also increase, although data do not yet correlate connection failure to voiding.
Pb-free labeling. Existing IPC and JEDEC labeling standards for identification of Pb-free components and assembled boards are being combined. The current labeling standards are JESD97, Standard for Marking, Symbols and Labeling for Pb-free Assemblies and Components, and IPC-1066, Standard for Pb-free Labeling. IPC-1066 not only tracks JESD97, but also includes labeling requirements for halogen-free and conformal coatings.
Thomas D. (Tom) Newton is director, PCB programs, standards and technology, David W. Bergman is vice president of standards technology and international relations, and Jack Crawford is director, certification and assembly technology, at IPC (ipc.org); email@example.com.
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|Title Annotation:||LEAD-FREE WATCH|
|Date:||Sep 1, 2005|
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