RoHS Revisited

The regulations game is heating up and not just with the promise of warmer weather. Unless you’ve spent the past five years in a cave, and possibly even then, you know that European Parliament directive 2002/95/EC, better known as the Restriction of the use of certain Hazardous Substances (RoHS) Directive, was adopted in February 2003 and came into force on July 1, 2006.

The RoHS Directive itself states in Article 6 that the European Commission (EC) will review the legislation this year with an eye toward determining whether or not there should be any additional substance restrictions.

So, in accordance with Article 6 the RoHS Directive is now being reviewed and the EC seems ready to shuffle the deck once more. Potentially we will see an expansion of the list of six specified banned substances (lead, mercury, cadmium, hexavalent chromium and the flame retardants PBB — polybrominated biphenyls — and PBDE — polybrominated diphenyl ether), all of which are recognized as hazardous materials that can be toxic if ingested by humans.

The EC has contracted two consulting firms to manage the review, Öko-Institut of Norway and Germany’s Fraunhofer IZM. To date 46 additional substances have been identified as candidates for regulation, although Öko-Institut has noted that they anticipate the final number will be much smaller. Among the new substances mentioned are antimony trioxide, arsenic (used in semiconductors including gallium arsenide chips and LEDs), beryllium, a big group of brominated flame retardants, cobalt, liquid crystals, nickel, and rosin (used in solder flux). The complete list may be found at http://hse-rohs.oeko.info/fileadmin/user_upload/Documents/RoHS_High_priority_substances_in_EEE.doc

Industry Response: Not So Fast

In comments to the Öko-Institut sent on January 10 of this year Fern Abrams, Director, Government Relations & Environmental Policy at IPC, a 2600-member trade association representing all facets of the electronic interconnect industry, noted: “The electronics industry has invested an enormous amount of time and resources to comply with existing RoHS substance restrictions and the full technical, social, and cost implications of the RoHS Directive’s implementation are still being discovered.

“IPC,” Abrams wrote, “urges the Commission to avoid restricting additional substances, eliminating existing technology exemptions or adding additional electronics equipment to the RoHS scope while industry, governments and the public are still facing a variety of implementation challenges. Any expansion of the RoHS scope, including removal of existing exemptions must be thoroughly reviewed for technical feasibility.”

Ms. Abrams argued that should the Öko-Institut deem additional substance bans to be absolutely necessary, “a full life cycle assessment of the substance and its substitutes must be conducted in order to ensure that the substitution does not have unintended adverse environmental and human health impacts. Until life cycle assessments are conducted proving that the environmental and human health impacts across the alternative’s life cycle are better than the substances being replaced, the Öko-Institut should not restrict any further substances under RoHS.”

The Annex to the RoHS Directive currently lists 29 applications of lead, mercury, cadmium, and hexavalent chromium exempted from the Directive’s substance restrictions. The list is reproduced as Table 1 on page 00 of this issue of ARM. Materials and components can be exempted — get ready, here comes the legalese — “if their elimination or substitution via design changes or materials and components which do not require any of the restricted materials or substances is technically or scientifically impracticable, or where the negative environmental, health and/or consumer safety impacts caused by substitution outweigh the environmental, health and/or consumer safety benefits thereof.”

Öko-Institut and Fraunhofer are in the midst of performing a detailed review of these exemptions as well as attempting to execute a clear assessment on whether a number of additional requests for exemptions are justified in line with the criteria given in Article 5 (1) (b) of the RoHS Directive.

The EC also has to decide whether to adopt recommendation made in the July 2006 Review of Directive 2002/95/EC (RoHS) Categories 8 and 9 Final Report, covering medical and monitoring and control equipment. Category 8 and 9 consumer products include smoke detectors, thermostats, medical self-test equipment (such as blood pressure monitors) and heart pacemakers. The study recommended that Category 8 and 9 products remain exempt from the RoHS directive until 2012 with the exception of:

1. In-vitro diagnostic equipment — proposed inclusion from 2016

2. Industrial test and measurement instruments — suggested inclusion in 2016 (or 2018)

3. Active implanted medical devices — permanent exclusion or delay inclusion until 2020

The study was conducted by the UK firm ERA Technology. Since the EC has not yet adopted the report’s recommendation, the exact timing of RoHS application to Category 8 and 9 products remains uncertain.

Technical Issues

As an example of the many technical issues which continue to challenge the electronics industry during its implementation of the RoHS Directive, the shift from lead bearing solder alloys to lead-free alloys has created reliability concerns within solder joints. Because the lead within the alloys provides greater ductility, the ability to be shaped without breaking, while high tin content solder joints may be stronger, the thermal stresses applied are transferred to other locations within the assembly causing failures within the board or the components. Printed circuit boards made using lead-free materials also have proven to be more susceptible to corrosion than their tin/lead counterparts.

In his keynote address at CARTS 2008, John Maxwell, Vice President of Engineering at Johanson Dielectrics, cited a number of issues with lead-free solder, starting with the fact that most lead (Pb)-free alloys melt at a higher temperature than standard tin-lead’s 183°C. This has led to concerns about the potential thermal damage to components and circuit boards during the soldering process.

Maxwell expressed concern about the long term degradation of electronics reliability caused by the substitution of Pb-free solder processes. Among issues with lead-free solder he cited:

• Solderability is reduced when using Pb-free alloys due to poor wetting and increased oxidation.

• Higher reflow temperatures increasing the risk of “popcorn” cracking of plastic molded components and de-lamination due to dramatically increased steam pressure.

• Mixing different alloys during production and rework requires that the alloys have adequate time to blend or mix.

• Increase laminate de-lamination and/or PTH (plated through hole) barrel cracking can result.

• Increased reflow/rework time increases board and component stress.

The adoption of lead-free solder has made electronics manufacturers more aware of al metallurgical phenomenon commonly referred to as “whiskers,” that can affect circuit assembly reliability. Tin whiskers are microscopic single-crystal filaments of the metal that “grow” from tin-based solders and surface finishes on printed-circuit boards, components, and connectors. In the past lead in small amounts (as little as 3%) could be added to the plating to halt this phenomenon.

If these conductive whiskers grow long enough, they can bridge adjacent connections and cause short circuits between board traces or component leads. Some companies producing a high volume of lead-free products with pure tin-plated components have observed tin whiskering in the field and have seen failures in less than 2 years.

For practical strategies to mitigate tin whiskers in RoHS compliant assemblies see the Electronic Products article “Tin Whiskers and RoHS: a Perspective” by Patrick Lavery of Vicor Corp., which can be found at:

http://www2.electronicproducts.com/Tin_whiskers_and_RoHS_a_perspective-article-vicor-rohs-sep2006-html.aspx

Also see the Electronic Products article “Controlling Tin Whiskers in Pb-free Assemblies” by Michael Hundt of STMicroelectronics, found at

http://www2.electronicproducts.com/Controlling_tin_whiskers_in_Pb-free_assemblies-article-stmicro-rohs-jul2006-html.aspx

For semiconductor companies lead-free concerns go beyond the issue of whiskering. Traditionally, tin-lead solder alloy has been used for FLI (First Level Interconnect) chip-to-substrate and 2LI substrate-to-board attachment interconnect materials. The presence of lead in tin-based solder alloys lends the solder superior thermal and mechanical characteristics for microelectronic assembly and reliability. Compared to their tin-lead counterpart, high tin content, lead-free C4 (controlled collapse chip connection, also known as solder bumps) solders possess physical properties less desirable for assembly and reliability.

The higher melting point of lead free solders increase peak reflow solder temperature from 220°C to 250°C for many applications and as much as 260°C peak for some large boards or subassemblies. In the Electronic Products article “RoHS-compliant IC Manufacturing Reliability Challenges Industry” (http://www2.electronicproducts.com/RoHS-compliant_IC_manufacturing_reliability_challenges_industry-article-altera-rohs-mar2006-html.aspx), Altera’s Hyung Lim pointed out that process technology developments on smaller packages such as TQFPs, PQFPs, and proprietary FineLine BGAs have shown that these packages can withstand the higher peak reflow temperatures of up to 260°C. Many IC manufacturers, he wrote, use higher-grade molding compounds and newer process techniques to achieve this goal, adding that with the high-speed signals now required, an increasing number of manufacturers are using large die and flip-chip ball-grid array (FCBGA) packages, which present, in Lim’s view, an additional reliability challenge.

When flip-chip BGA packages were tested at 245°C (process temperatures of 260°C yield BGA package temperatures of 245°C), Lim reported, a common die surface failure known as “die corner de-lamination” resulted with an IC moisture sensitivity level (MSL) rating of 3. MSL dictates how much time parts can be exposed to manufacturing environments. An MSL 3 rating means parts can be out of their sealed moisture proof containers for 168 hours (1 week) prior to the solder operation.

Intel’s Experience

In a recent paper entitled “Materials Technology for Environmentally Green Micro-electronic Packaging” (Intel Technology Journal, Volume 12, Issue 1, February 2008) Intel described its green initiative to provide lead-free packaging materials solutions as well the enabling of halogen-free (HF) substrates technology to eliminate the use of brominated flame retardants.

To meet the stringent integration challenges of transitioning to Pb-free-compliant packages. Intel said that its drive to “get the lead out of the package” began over five years ago when the company produced a Pb-free tin-silver-copper (SAC) solder for 2LI applications. 2LI is accomplished with solder sphere, flux, and/or paste, and it involves two reflow processes: ball attachment (BA) and surface-mount reflows for board attach.

Among the lead free 2LI reliability challenges described in the article include de-lamination with increased reflow temperature (35° C higher than Sn/Pb), lower mechanical margin in high strain rate shock testing due to increased stiffness (1.5 times stiffer than SnPb solder), creep performance difference compared to SnPb, and Sn whiskers.

The Intel authors (Mukal Revanikar et. al.) said the transition to Pb-free solders for 2LI posed severe challenges to surface-mount technology, and an equivalent effort in adapting board manufacturing and SMT processes was required. The transition affected not just the SMT pastes used in board assembly, but also wave solders and board materials.

Intel has also successfully developed substrate, FLI flux, and underfill (UF) materials technologies that are compliant with higher Pb-free processing temperatures. The company recently reached a critical milestone by eliminating lead from the FLI solders in its next-generation 45nm silicon technology roadmap products. The transition to Pb-free FLI interconnects that connect the silicon die to the substrate eliminated the last 5% of Pb remaining in the package, according to the company. Intel selected tin-silver-copper (SAC) solder metallurgy as the lead-free chip attachment material for its 45nm CPU products.

Next Up, a Meeting with RoHS Exemption Stakeholders

While everyone loves a good mystery, keeping engineers guessing as to what substances will join the EC’s verboten list or which exemptions suddenly will no longer be valid can only benefit the makers of antacids. It’s hard to predict what the EC is going to do in the next month, much less the next few years, but best current estimates are that no new regulations will take effect before 2010 at the earliest (insert sigh of relief).

In the meantime, Öko-Institut will continue its review. The consulting firm’s mandate requires it to 1) involve and consult stakeholders and then forwarding their comments to the EC Technical Adaptation Committee and 2) provide clear and unambiguous wording for the preparation of a Draft Commission Decision for those exemptions where an exemption can be justified.

To that end Öko-Institut has indicated that technical stakeholder meetings in Brussels will probably be held June 9 – 13 with regard to exemptions nos. 1-4, 5, 6, 7(a)-7(c), 8, 9a, 13, 16-20, 25, and 26 in Table 1 below.

We’ll keep you posted as developments happen.

Murray Slovick

Table 1: RoHS Exemptions

* Exemption 9a) was annulled by The European Court of Justice (ECJ) in April.** Exemption status for servers, storage, and storage array systems granted until 2010. *** Exemption was granted only until 1 July 2007.