Lead-free standards update: several specs have already been updated, and others are undergoing revisions.

Ed.: This column can be found in its entirety at www.pcdandm.com.

THE EUROPEAN UNION'S Rolls Directive restricting the 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 lead-free restrictions.

This column discusses some of the key standards being revised or developed to clarify requirements for RoHS-compliant materials and processes.

Components. Higher processing temperatures of lead-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 lead-free component termination finishes.

PCB design. Many companies ask how IPC design standards will address lead-free soldering, particularly in the area of SMT land patterns. The design standards are not changing, in any fashion, because of lead-flee requirements. Land patterns in the newly released IPC-7351 (superseding IPC-SM-782) meet requirements for surface-mount assembly with either SnPb or lead-free solders. When IPC-7351 design rules are applied, lead-free SMT assembly does not differ from eutectic SnPb soldering, and dimensional modifications are not required.

Bare boards. For PCB surface finishes, fabricators have many options, and nearly all will work for lead-free assemblies. Choice of finish is dependent on the application and preference. Hot air, lead-free solder leveling (HASL), immersion fin, immersion silver, organic solderability preservative (OSP) and electroless nickel/immersion gold (ENIG) have all been used successfully in lead-free assemblies. IPC has published two finish standards, IPC-4552 for specifying ENIG and IPC4553 to define immersion silver. A third specification, IPC-4554, addressing immersion tin is nearing the final draft, and the fourth standard in the series, IPC-4555 (OSP), is underway. J-STD-003A, Solderability Tests for Printed Boards, is being updated to include testing for lead-free board finishes.

IPC's PCB laminate standards also address lead-free soldering. Again, the higher processing temperatures and the potential impact on the plated through-holes and vias are of concern. Historically, glass transition temperature (Tg) has been used to study thermal robustness. Tg can be evaluated per test methods in IPC-TM-650. The requirements for a specific laminate are called out in the individual specification sheets within IPC-4101, Specification for Base Materials for Rigid and Multilayer Printed Boards.

Electrical properties. Changes in PCB materials necessary to improve thermal resistance can alter electrical properties. However subtle the polymeric changes may be for lead-free assembly, designs that require the maximum electrical performance of traditional epoxy materials will be affected, particularly in high-speed designs. Permittivity or dielectric constant (Dk or [epsilon]r) as well as dissipation factor or loss tangent (DF or tan [sigma]) testing should be run on any new laminate materials designed for lead-free applications to quantify the need for electrical design adjustments. IPC-4101 provides requirements for the electrical properties of laminates in its specification sheets.

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 lead-free materials. Revision of J-STD-005 is beginning and will include additional lead-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 lead-free alloys are addressed.

Assembly standards. When are IPC standards going to address workmanship requirements for lead-free? Answer: Now. IPC-A-610D and IPC J-STD-001D, released this year, contain requirements for lead-free solder connections. At the basic level, there are no differences in the solder connection wetting and fillet requirements. However, some differences in appearance exist. Often, acceptable lead-free and SnPb connections exhibit similar appearances (FIGURES 1 and 2) but lead-free alloys are more likely to have surface roughness (grainy or dull), cooling lines and potentially different wetting coverage. Lead-free often exhibits a greater contact angle, but again, the 90[degrees] angle requirement for lead-free is no different than that for SnPb.

Lead-free labeling. Existing IPC and JEDEC labeling standards for identification of lead-free components and assembled boards are being combined. The current labeling standards are JESD97, Standard for Marking, Symbols and Labeling for Lead-free Assemblies and Components, and IPC-1066, Standard for Lead-free Labeling. IPC-1066 not only tracks JESD97, but also includes labeling requirements for halogen-free and conformal coatings. PCD&M

THOMAS D. (TOM) NEWTON ([e-mail]) 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.