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Technological Aspects of the RoHS Directive
December 31, 1969 |Estimated reading time: 4 minutes
Exempt Industries: RoHS-5 Compliance and Mixed Technology
By Lev Shapiro
It is well known that the European Union's (EU's) RoHS Directive, which takes effect on July 1, 2006, limits the use of six materials: lead, mercury, cadmium, hexavalent chromium, polybrominated biphenyls, and polybrominated diphenyl ethers in new electrical and electronic equipment. Of these six regulated materials, the removal of lead is causing the heaviest burden on the electronics industry -- forcing a number of process, raw materials, components, PCB, and equipment changes. It also is creating another large problem -- high temperatures required for soldering.
Electronic industries that must implement the requirements of the RoHS Directive do not have a long history of lead-free technology, enough experience, or concrete data regarding long-term reliability issues. Due to their end-market application, the reliability of new technology is a significant concern for high-reliability products. It is also a strong argument for why the RoHS legislation offers a number of technological exemptions, and exempt industries, including aerospace, defense, medical, and telecommunications. Therefore, point 7 in the Annex of the RoHS Directive defines the following exemption for lead: "...lead in solders for servers, storage and storage array systems, network infrastructure equipment for switching, signalling, transmission as well as network management for telecommunication."
Lead in solders is only related to so-called 2nd level interconnects that contain "participants" in the interconnect by the attachment of the device/component to PCB:
platings (finishes) of terminals (pads) of SMT and thru-hole components, plating of pads and holes on PCBs, solder paste or wave solder.As a result of this exemption, new definitions have evolved -- RoHS-5 compliance and RoHS-5 components, which reflect five of six restricted materials, i.e. that product meets the RoHS material-content requirements for all substances except lead (Pb). These terms are not defined in the RoHS or WEEE regulations. This popular slang definition allows the use of such components for markets and applications that are exempt from lead restrictions in solders, and where they continue to use the established low-temperature tin/lead assembly in manufacturing lines. Caution is recommended when using the RoHS-5 definition so as to avoid misinterpretations. If we again consider the three "participants" of 2nd level interconnects (component platings, solder material, and PCB plating), this is clear for solder material that may be tin/lead and not lead-free. However, it is not always clear for permitted lead presence in components and PCBs.
For example, consider a jack module RJ-45 shielded and PCB mount, which is used in many telecommunications products. A shield in such parts usually is tin/lead-plated, and might be RoHS-5-compliant, if only two pins of that shielding that are grounded to PCB have lead plating instead of the entire shielding surface. A similar situation exists for PCBs where lead is exempt only for plating pads and holes, but not for other applications.
During the lead-free conversion process, component manufacturers decided to shut down much production of leaded parts because exempt industries represent a niche market. Statistics show that this niche market covers about 25% of the total electronics market. Accelerated obsolescence of tin/lead parts by manufacturers who do not want to support both tin/lead and lead-free production lines is anticipated.
Under these circumstances, exempt industries are forced make their products RoHS-compliant using backward compatibility of lead-free components. This is possible for leadframe parts and not for BGA lead-free packages that are not backward compatible. Such an approach brings exempted and other industries to mixed technology, where two types of components (lead-free and tin/lead parts) must coexist within the same bill of materials (BOM). This also is a new technology without history and experience that lacks long-term reliability data. The irony it that no one can confirm that mixed technology will be more reliable than completely lead-free technology.
Components with Bi (bismuth) in the plating may cause additional problems and dangers with regard to mixed technology. Such platings are popular in the Japanese market, and some Japanese component manufacturers prefer to add Bi (usually from 2-4%) to the tin (Sn) as substitute for lead (Pb) in the role of whisker prevention. But using SnBi-plated components with tin/lead solder can cause the ternary formation of tin/lead/bismuth (SnPbBi) with a melting point of 135°C. This is a high-reliability risk for the solder joints.
Considering tin whiskers in components with pure-tin plating, it is important to mention that the electronics industry (and exempted markets) should solve the problem of reducing the tin whiskers risk in lead-free parts, particularly for long-life products. In March 2006, JEDEC Standard JESD201 "Environmental Acceptance Requirements for Tin Whisker Susceptibility of Tin and Tin Alloy Surface Finishes" was issued (www.jedec.org/download/search/jesd201.pdf). Recommendations of this standard, developed by the Tin Whisker User Group of the International Electronic Manufacturing Initiative (iNEMI) include three aspects of whisker prevention: mitigation practices, process control, and verification testing.
Lev Shapiro, consultant, Component Master Ltd., may be contacted via e-mail: lev@compmaster.co.il.