You wont find the term ‘homogeneous’ in the RoHS Directive. In fact, it doesn’t appear in the related WEEE Directive or even the ‘Guide to the implementation of directives based on the new approach and the global approach’ (commonly referred to as the ‘blue book’). So where did it come from?

The expression first appeared in a Commission stakeholder document of December 2003. It was used in the context of proposing maximum concentration values for RoHS restricted substances:

"A maximum concentration value of 0.1% by weight in homogeneous materials for lead, mercury, hexavalent chromium, polybrominated biphenyls (PBB) and polybrominated diphenyl ethers (PBDE) and of 0.01% weight in homogeneous materials for cadmium shall be tolerated. Homogeneous material means a unit that can not be mechanically disjointed in single materials."

At the time, this document didn’t help a great deal, as no one fully understood how to correctly interpret ‘homogeneous’ or ‘mechanically disjointed’. Lead (Pb) was restricted to 0.1%, but 0.1% of what?

The latest interpretation of homogeneous from the UK government and Commission suggests the term ‘homogeneous’ to be understood as "of uniform composition throughout". So examples of "homogeneous materials" would be individual types of plastics, ceramics, glass, metals, alloys, paper, board, resins and coatings.

They go on to suggest the term “mechanically disjointed” means that the materials can, at least in principle, be separated by mechanical actions such as unscrewing, cutting, crushing, grinding and abrasive processes.

So how do these expressions actually apply in the real world of electronics?

Take an electrical cable, for example. It has a metal core, a plastic insulation and perhaps tinned terminations. The cable as a whole is not homogenous as it can obviously be separated by some of the methods described above. The metal core, even though it may be an alloy of more than one element, is still considered homogeneous, as mechanical disjointing cannot separate these alloys. The same applies to the plastic insulation. So the metal conductor, the plastic insulation and the solder used to tin the terminations are each homogeneous elements and must all comply individually with the requirements of the RoHS Directive. Moreover, if the conductor received some form of passivation treatment, any residual passive film would also need to comply.

Using this same methodology, a semiconductor is not homogeneous as it consists of, amongst other things, die, die attach, a lead frame and a housing. In essence, the legislation applies to the lowest common denominator of an item of uniform composition.

Maximum concentration values, or MCV’s, define the maximum amount of the individual restricted substance that RoHS directive will tolerate in each homogeneous material (see above). Whilst the text of the legislation defines the six substances to be restricted, it doesn’t actually specify what the MCV’s might be. The Directive clearly requires you to restrict (not remove) six substances - but to what level?

During August 2005, Commission Decision 2005/618/EC was published which amended article 5.1(a) of the RoHS Directive. This decision states:

"For the purposes of Article 5(1)(a), a maximum concentration value of 0,1 % by weight in homogeneous materials for lead, mercury, hexavalent chromium, polybrominated biphenyls (PBB) and polybrominated diphenyl ethers (PBDE) and of 0,01 % by weight in homogeneous materials for cadmium shall be tolerated."

The term ‘put on the market’ has probably caused the most confusion surrounding the legislation. The expression comes from Article 4.1 of the RoHS Directive, which states:

"Member States shall ensure that, from 1 July 2006, new
electrical and electronic equipment put on the market does not contain lead, mercury, cadmium, hexavalent chromium, polybrominated biphenyls (PBB) or polybrominated diphenyl ethers (PBDE). National measures restricting or prohibiting the use of these substances in electrical and electronic equipment which were adopted in line with Community legislation before the adoption of this Directive may be maintained until 1 July 2006."

The ‘market’ refers to the European single market. However, as EU Directives apply to the European Economic Area (EEA), Iceland, Liechtenstein and Norway also fall within its scope.

So what does putting (or placing) a product on the market actually entail? Guidance from the European Commission suggests

"... a product is placed on the Community market when it is made available for the first time. This is considered to take place when a product is transferred from the stage of manufacture with the intention of distribution or use on the Community market."

The term ‘transfer’ is key in interpreting this expression. In essence, the transfer of a product takes place when it passes from the manufacturer (or his representative) to the importer, distributor or end user. There doesn’t need to be a financial transaction for this to occur – product that is donated or given away is still considered to have been placed on the market.

A product is not considered to have been placed on the market when it is transferred to a manufacturer for further assembly, packaging or labelling, is held by customs, is to be exported to a third country or is to be displayed at trade fairs, exhibitions or demonstrations.

So, if a device enters the EU, but requires a little final assembly or packaging then it has not yet been 'put on the market'. In addition, product is not classed as been placed on the market when it is

"... in the stocks of the manufacturer, or the authorised representative established in the Community, where the product is not yet made available ..."

Have you made the product available for sale? Could a customer buy it? If not, then again, one could argue that it has not been placed on the market.

Finally, RoHS does not apply exclusively to new products. The definition above states that a product is placed on the Community market when it is made available for the first time. So a product originally sold outside of the EU, which subsequently enters the market secondhand after the 1st July 2006 (whether bought or free issued) would also fall within the scope of the legislation, regardless of the fact it is 'used'.