Circuits Assembly Online Magazine - RoHS and the Supply Chain

If each component can be verified, then the finished product falls into place.

Let’s take a look into the future: Somewhere in Europe, some youngsters in a department store are looking over the latest game boxes with wireless virtual reality controllers. Others are checking out PDAs or jukeboxes (integrated phone, MP3 player, video player and camera, using 2 Gb memory sticks or microdrives). The new point-and-shoot 8 MP digital cameras with wireless download are out there, too. Elsewhere in the store the latest high-tech appliances are displayed – washing machines featuring the latest intelligent software; Bluetooth-controlled window shades; domestic lighting systems controlled from the car as you approach your driveway.

Doesn’t sound like a great stretch? That is because we are only looking a year into the future. But something else is special about these products – all are RoHS-compliant. In Europe, just a year from now, any products on the shelf with any electrical or electronic content will be built from materials compliant with the Restriction of Hazardous Substances Directive.

Will there be fanfare on July 1, 2006, the critical date that RoHS legislation becomes enforceable? Probably not: Compliant products will already be established in the supply chain. In fact, many of the products available next year will differ little in material content from models currently available; most high-volume consumer items (typically portable devices using surface-mount technology) are already RoHS-compliant while many of the peripherals and white goods (using mixed technology) are still ahead of the curve in the process of conversion.

As the “RoHS date” approaches, one question is being raised more frequently: How will the legislation be implemented? No single answer exists – it will be up to each individual member state to police – and for most OEMs it is somewhat academic. With products shipping globally, they will simply need to meet the tightest specifications for markets served. And while the mechanics of legislation are not yet defined, it is a sure bet that documentation of compliance will be a prime requirement.

As a first step to ensure compliance, OEMs will require that each component is itself compliant, based on data from their suppliers. RoHS takes it one stage further, requiring that each homogenous material used in any product is itself RoHS-compliant.

What is the current definition of “homogenous material?” The RoHS directive was formulated as an adjunct to the WEEE (Waste Electrical and Electronic Equipment) directive, which would restrict hazardous materials at the source (device manufacture) so the requirements of WEEE could be better achieved. WEEE governs how finished electrical and electronic goods are ultimately disposed. When disposing of such items, it is important to know which of the components or subassemblies are “mechanically disjointable,” as they may come back for recycling already broken into their constituent parts.

(As an aside, WEEE is scheduled for introduction ahead of RoHS. Of the EU member states, Greece is currently the only one to implement on schedule, but many other states will have it in place prior to July 1, 2006.) RoHS defines a homogenous material as “a material that cannot be mechanically disjointed (in principle) into different sub-materials, i.e. solder, ceramic, plating, etc.” The directive takes “mechanically disjointed” a step further, though, meaning that materials can be, in principle, separated by unscrewing, crushing, grinding and other abrasive processes, so the current definition has been taken to mean material used in the manufacture or assembly from any individual process step.

In reality, the construction of many components results in a solid matrix (e.g., the electrodes and dielectric material of a ceramic chip) so while the overall material content can be determined by grinding and testing the residue, any restricted material content from such an analysis can only be measured in terms of the overall component weight. While the component as a whole may be within limits, to ensure that the overall content does not arise from any one homogenous material being over limit means that the individual raw materials prior to manufacture must also be assessed and certified by the manufacturer as being compliant.

So, if you are taking the overall hazardous material content of, say, a laptop, it could be calculated from the restricted material content of the casing, wires and cables, screen, printed circuit board, etc. The PCB would then include electronics components. At the component level, each part can then be separated into the homogenous (individual process) materials used in manufacture; in the case of ceramic capacitors, that would mean the different termination layers, the internal electrodes, dielectric materials and any external casing and leads as applicable. Obviously, in a single SMT 0201 or 0402 chip that’s not a lot of milligrams to be accounted for, but again, if each component in the chain can be verified, then the overall finished product content falls into place.

The RoHS directive defines six restricted substances, taken from the ELV (End of Life Vehicles) directive, five of which must be less 0.1% maximum content in any homogenous material:

Lead, mercury, chromium VI (hexavalent chromium), polybrominated biphenyls (PBBs), polybrominated diphenyl ethers (PBDEs)
For one substance, cadmium, the set limit is tighter (0.01% max.).

For many electronics components, some of the RoHS-restricted materials are still required as they are integral parts of the construction, necessary for electrical functionality of the parts. A number of material-specific exemptions in RoHS address these. In the case of SMT PCBs, the most common are trace lead above 0.1% in the dielectric material of ceramic chips (these are typically the class 1 NP0 materials having greatest temperature stability and used in high-frequency applications) and lead in glass used in electronics components (the glass frit layer of the termination). Through-hole components used in mixed-technology PCBs often have an internal lead attach that uses high-temperature solder (>85% lead content) for the joint that is also exempt.

Since the RoHS legislation was formulated, other categories for material exemption have come under review (another 22 are posted for consideration this year). As noted, the RoHS legislation was developed to support WEEE, which is concerned with contaminants returned to the environment via recycling, so many of these exemptions are safe bets. For example, the trace lead in ceramic dielectric is fired into the matrix at over 1000°C and is very unlikely to leach back into the environment.

Note, however, that no matter how “safe” an exemption may be, it is only considered a short-term measure (until 2010), so alternative materials in all categories are being researched.

Lead elimination continues to be the major focus for assemblies. While functional alternatives can be found for most other materials, removal of lead has the most profound effect on electronics manufacturing. There are three principal sources of lead in electronics assemblies: the solderable traces on the circuit card, the solderable finish on the components and the solder alloy used to connect the two (either solder paste for reflow or liquid solder for wave). Leaded solders have always been the natural interconnect choice, having an optimum combination of strength, ductility, conductivity, processability and long-term reliability. Much research has been completed over the past few years into the optimum alternative joint materials. Tin/silver/copper (SAC) is becoming the alloy of choice for reflow, as is tin/silver for wave soldering. The standard termination for most passive components is matte tin, as it is compatible with all variants of the aforementioned systems and backwards-compatible with traditional SnPb alloys; in fact, matte tin has been used as a standard termination material for many passive families for a number of years in SnPb solder processes.

The new systems will require reflow soldering at higher temperatures – typically 250°C gives optimum wetability for the joint but systems can run at up to 260°C – while wave profiles are essentially unchanged. Component leads or bottom-side surface-mount components require 260°C immersion. As with the evaluation of different solder alloys, much development has been completed in the past few years to upgrade the design of many families of SMT components to resist negative effects of higher temperature processes. In the case of plastic-encapsulated devices, many will retain the same MSL (moisture sensitivity level) ranking as with standard SnPb processes (in all cases, check suppliers data sheets to see if special handling is required).

Many differences between SnPb and Pb-free systems are still the subject of discussion and ongoing research. The new systems produce joints with a different visual appearance – more granular and less shiny – and different visual standards will be needed for PCB manufacturers. For many critical infrastructure applications, the choice is to retain traditional SnPb soldering until more data on Pb-free are available, so RoHS contains additional “application based” exemptions, which are general (military/defense, aerospace, large telecom infrastructure equipment, medical devices, etc.). Again, these exemptions last only through 2010 and will be at the discretion of the OEM; at a component level (for commercial parts) a supplier will not know what the ultimate use of any given part would be, so RoHS-compliant (backwards-compatible) parts will still be supplied into these applications, but a SnPb PCB soldering system will be retained. Such OCBs have been termed “X-Compliant” or “RoHS 5-compliant.”

For critical military or aerospace applications, all MIL-STD components will retain SnPb terminations, but there has been an increase of commercial products used in these products. With the majority of commercial parts having standardized on tin terminations for a number of years, concerns have emerged about tin whisker growth. “Whiskers” are thin hairlike crystals that can grow from a pure tin surface under certain conditions. Concerns remain that if permitted to grow, they may cause short circuits between metalized PCB traces. Typically, users will assess the risk based on their own board processing and end-application conditions.

While whiskers are a concern for certain electromechanical devices, there is still a lack of documentation for any occurrence with capacitors and resistors when soldered in place on the board. At a component level, under reflow conditions, the solder paste into which the component is placed will wet most of the termination surface. This area will not be susceptible to whisker growth, as the additional elements alloyed in the solder paste will also alloy with the tin from the termination plating. Only a minor portion of the termination area will remain coated with pure tin. Any potential whisker growth in this area is also significantly reduced by best practices such as the selection of a base material underlayer (typically nickel), special interlayer barriers or thermal treatment (often the reflow process itself acts as an annealing mechanism).

Although such actions have resulted in whisker-free assemblies in process trials, for space applications military and NASA specifications maintain a minimum 3% lead content in the termination finish to guarantee a whisker-free surface. Because of this, many commercial component families with SnPb terminations are now being reintroduced.

This is the time for manufacturers to complete the switch to Pb-free. While many components are already compliant, some families (plastic film) and some processes are still being converted and there will be ongoing technical coordination for these, as applicable.

In the meantime, throughout the supply chain, activity in part numbering and labeling is underway to ease the last stages of transition. Again, while many products are already RoHS-compliant, additional labeling is being introduced so such material can be more easily identified. In the case of products newly converting to compliance, the trend is to change part numbers to enable identification.

Most suppliers have posted data on their Websites showing which products are fully RoHS-compliant and which (if any) subsets will invoke the above exemptions. They will also have material certification on request, so if a user then wishes to receive product with tighter material limits than set by RoHS, or does not wish to recognize the exemptions (where applicable), they will be able to flag this to their supplier.

It is easy to view RoHS as a “weapon of mass disruption” but it is not possible to ignore it and too late to fight it. It is a defined goal that is necessary to meet to continue to do business all over the world.

Chris Reynolds is a product manager at AVX Corp. (avxcorp.com); creynolds@avxus.com.