This article is part of the TechXchange: RoHS Design Issues and Challenging Resistors
What you’ll learn:
- RoHS, also known as Directive 2002/95/EC, originated in the European Union and restricts the use of specific hazardous materials found in electrical and electronic products (known as EEE).
- The RoHS Directive allows for exemptions from its restrictions, under certain conditions defined in article 5(1), adapting the Annexes to scientific and technical progress.
- It’s important to understand exemptions and their effective duration. Industry regularly applies for the renewal of exemptions or for additional applications to be exempted from the Directive's requirements.
RoHS stands for Restriction of Hazardous Substances. Also known as Directive 2002/95/EC, RoHS originated in the European Union and restricts the use of specific hazardous materials found in electrical and electronic products (known as EEE). All applicable products in the EU market after July 2006 must pass RoHS compliance.
The substances banned under RoHS include lead (Pb), mercury (Hg), cadmium (Cd), hexavalent chromium (Cr6), polybrominated biphenyls (PBB), polybrominated diphenyl ethers (PBDE), and four different phthalates (DEHP, BBP, BBP, DIBP). The restricted materials are hazardous to the environment, pollute landfills, and are dangerous in terms of occupational exposure during manufacturing and recycling. Exceptions are the maximum permitted concentrations in non-exempt products be 0.1% or 1000 ppm (except for cadmium, which is limited to 0.01% or 100 ppm) by weight.
The restrictions are on each homogeneous material in the product. That means the limits don’t apply to the weight of the finished product, or even to a component, but to any single material that could (theoretically) be separated mechanically—for example, the sheath on a cable or the tinning on a component lead.
RoHS Exemption 7c-1
The RoHS Directive allows for exemptions from its restrictions, under certain conditions defined in article 5(1), adapting the Annexes to scientific and technical progress. Seven exemption groups have been approved for the use of lead in certain applications under EU RoHS Annex III for a few more years. Exemption 7c-1 has the greatest impact on electric and electronic components industry, which exempts lead in glass or ceramics as stated below:
"Electrical and electronic components containing lead in a glass or ceramic other than dielectric ceramic in capacitors, e.g., piezo electronic devices, or in a glass or ceramic matrix compound.”
It’s important to understand exemptions and their effective duration. Industry regularly applies for the renewal of exemptions or for additional applications to be exempted from the Directive's requirements.
Each request must be evaluated, and when appropriate, an exemption is granted. Historically, lead-based exemptions have been extended, but it’s never a guarantee and staying vigilant on the RoHS site is critical.
RoHS Today
The EU is expected to take the industry recommendations and extend this exemption for lead containing glass for three more years from the original expiration date in 2021. In the past, lead has been an essential element in the dielectric glass used on nearly all thick-film chip resistors, which comprise the bulk of the resistors used globally in all markets.
Lead in the dielectric glass is critical to stabilize the laser calibration trim. It’s worth noting that the lead in this glass isn’t free lead; it’s chemically bound to the thick-film dielectric material itself. Therefore, the potential for lead contamination due to the lead content in this material is highly questionable. However, the worldwide electronics industry has committed to removing lead from all components and materials used in manufacturing regardless of the true potential risk in each individual usage case.
Lead-Free Thick-Film Chip Resistor Evolution
Some manufacturers have had fully RoHS-compliant chip resistors for many years. Some early designs simply removed the lead containing dielectric layer completely. Still, these designs were never successful as these designs had substantial instability with respect to voltage, parasitic capacitance and inductance, and pulse performance.
Later designs utilized more expensive dielectrics or may have required multiple dielectric layers to be applied prior to calibration trim. Cost for these early fully compliant series was typically 5X to 10X the cost of the equivalent general-purpose chip resistor.
Resistor Options for Completely Lead-Free Design Requirements
Most reputable manufacturers of thick-film chip resistors have completely lead-free options for general-purpose thick film chip resistors. The current demand for this type of part is very small, though, which leads to several issues that must be addressed before a complete market changeover to totally lead-free thick film chips.
First, the performance of these lead-free options is simply not as uniform across all manufacturers as is the case currently with thick-film chips that have the 7C-1 exemption. Design engineers and end users can be fairly confident that if they’re using a reputable manufacturer of these resistors, the performance will be consistent and predictable.
For fully lead-free chip resistors, performance differences still exist between materials from different thick-film suppliers, let alone differing methods on implementation of these new materials. As volumes increase, the performance differences will be less noticeable as material suppliers and resistor manufacturers make the expected continuous improvements to materials, designs, and processes.
Another major obstacle for fully lead-free thick-film chip resistors is cost. With low volume, the new lead-free dielectric materials will be far more costly until economies of scale can be reached for current dielectrics. It will take a leap of faith by the electronics industry to make the move to these higher-cost components in the short term, with the expectation that the cost of these series will eventually come down.
This short-term cost impact will be felt in every market. However, it will obviously be magnified in markets such as cell phones, laptops, and other consumer handheld electronics, as well as the automotive industry, as these markets consume a massive amount of chip resistors.
For other specialty thick-film resistors, the completely lead-free options are much more limited. Many manufacturers don’t have thick-film, fully RoHS-compliant options currently available for anti-surge/pulse withstanding, anti-sulfur, high voltage, current-sensing, precision, or high-frequency requirements.
Although general-purpose thick-film resistors are the bulk of the global thick-film chip-resistor market need, many designs will utilize one or more of these specialty resistors. It’s critical to have fully lead-free versions of these specialty thick-film resistors before the market moves away from options that require the 7C-1 exemption.
It should be noted that for certain applications, there are non-thick-film resistor options available to the design engineer. General-purpose thin-film chip resistors, with tolerance and TCR similar to thick-film chip resistors, are an option at minimal increased cost. Precision requirements may use thin-film nichrome or tantalum-nitride chip resistors. Thin-film resistors don’t require a dielectric to stabilize the calibration trim area and therefore are, and have been, fully RoHS-compliant for some time.
For current-sensing applications, foil on ceramic carrier, thin-film/metal-film, and all metal sense resistors offer electrically superior performance options, but at increased component cost.
Summary
Fully lead-free requirements for thick-film chip resistors will inevitably increase in the next few years. For electronics manufacturers looking to make the switch currently, there are options for general-purpose types. However, finding stock on any of them will be difficult.
Furthermore, for specialty thick-film requirements, very few options are available from any manufacturer, and finding stock will be next to impossible. Some of these specialty applications may utilize other technologies, such as thin-film/metal-film, metal element, and wirewound, as a bridge until fully RoHS-compliant thick-film options are developed and stock becomes available.
Read more articles in the TechXchange: RoHS Design Issues