Is the supply chain ready for RoHS? Common materials and the availability of RoHS-compliant versions.

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Manufacturers have made significant investments in new processes that will eliminate substances banned under the EU's RoHS Directive--especially lead. However, they must also be assured of a strong supply chain that can provide sufficient replacement materials.

Many systems elements will need to be RoHS-compliant, including cables, connectors, plastics and enclosure sheet metal. This article highlights some commonly used materials and the availability of RoHS-compliant versions.

Printed circuit boards. Manufacturers are concerned about the compatibility of PCB materials with Pb-free soldering temperatures of 240 to 260[degrees]C. Compatibility is critical for large, thick boards used in high-end computer systems. Some high-temperature materials are becoming available in production quantities, but availability will be constrained through 2006.

All major PCB manufacturers currently offer some version of a RoHS-compliant board finish. RoHS-compliant OSPs are commonly available, but require a high degree of process control for high-yield soldering. Easy-to-solder immersion tin and silver finishes are readily available.

Cable insulation materials. Cadmium, hexavalent chromium and lead compounds are often found in polyvinylchloride, which is used for cable insulation. These heavy metal compounds are also used in pigments to mark cable insulation material, and mercury compounds have been used for curing control. Major cable suppliers are implementing substitute materials, such as titanium, for RoHS-compliant cables.

Connector materials. SnPb plating is commonly used on electrical connectors. A pure tin finish meets RoHS requirements, but lead removal increases the potential for tin whisker formation and growth, which can cause problems in high-reliability products. The connector industry is generally moving to SnNi plating, which mitigates tin whiskers. (1) No supply problems should arise.

Electrical components. Plated-through-hole devices typically use SnPb plating over a copper-plated steel wire. Pure tin plating is usually of little concern for PTH devices because of the relatively wide (>1 mm) spacing between adjacent leads. Wire wound resistors, some capacitors and some LEDs are available with PTH connections. Obtaining RoHS-compliant PTH connection devices by the end of 2005 should not be a problem.

SMT lead-frame manufacturers have historically used SnPb as a finish coating, and many intend to use a pure tin replacement finish. Tin whiskers are a major reliability concern for components with tight lead-frame spacing (<1 mm) and high-reliability system manufacturers are requiring suppliers to implement one of several whisker mitigation strategies (e.g., annealing or nickel underlay) for lead-frame components with tight spacing. (2) With the exception of some legacy products, RoHS-compliant SMDs should be available.

BGAs with high lead content (>85%) are RoHS-exempt for reliability purposes. However, eutectic SnPb BGAs will have to remove lead. All BGA component manufacturers will provide Pb-free BGAs by the end of this year.

Server and storage products are exempted from RoHS until 2008 and can continue to use SnPb solders. However, availability of SnPb BGA components will be problematic because most manufacturers are switching production to lead-free BGAs for high-volume commodities that are not exempt. Mixing lead-free solder balls with SnPb solder paste causes reliability degradation. When SnPb BGA balls are no longer available, manufacturers must decide whether to use lead-free solder attach or stay with SnPb solder.

Sheet and bulk metal. All electronic hardware needs a mechanical enclosure, meaning that sheet steel must be RoHS-compliant. Additional internal mechanical hardware, such as fasteners and metal heat sinks, use sheet steel and must also be compliant.

Sheet steel is typically purchased with a coating, most commonly hexavalent chromium. Since RoHS regulations require hexavalent chromium concentrations in the coating be <1000 ppm, most system designers have eliminated it as a steel finishing process.

When looking at RoHS compliance, consider each of the six banned substances. Remember that restrictions apply to more than the electronics; they include other parts of the product, such as enclosures and accessories.

References

(1.) G. T. Galyon and R. Gedney, "Avoiding Tin Whisker Reliability Problems," Circuits Assembly, August 2004, pp. 26-32.

(2.) Recommendations on Lead-Free Finishes for Components Used in High-Reliability Products (updated May 2005), iNEMI Tin Whisker User Group. http://thor.inemi.org/webdownload/projects/ese/tin_whiskers/User_Group_mitigation_May05.pdf.

George Galyon is senior technical staff member at IBM (ibm.com) and chairs the iNEMI Tin Whisker Modeling Project; [e-mail]. Glenn (Woody) Sellers is senior engineer with IBM. Ron Gedney consults on iNEMI's Pb-free and tin whisker projects.