RoHS beyond the fear factor

There is more to RoHS than compliance and exemption, writes Terry McManus of Tekdata Interconnections.

Anyone in the electronics industry should be aware of the passing of the July 2006 deadline for the implementation of the European RoHS directive on the use of hazardous substances, and the ancillary WEEE regulations covering disposal of waste.

The last few months have seen companies working feverishly to establish whether they are covered by the directive, or exempt from its provisions: and, if they do need to comply, how to go about it.

But it is becoming increasingly clear that exemption and compliance are not the only issues.

Just as manufacturers who move to compliance need to maximise the yields of new manufacturing processes, so those who are staying with established technologies will have to deal with issues such as finding reliable and verifiable sources of exempt components and materials.

This may not be as easy as it sounds: "non-RoHS" components with terminations made with lead are already commanding a price premium.

The EU regulations attempt to reduce the environmental impact of disposing of products at the end of their lifetime, by restricting the use of six substances at the manufacturing stage: lead, mercury, cadmium, hexavalent chromium, polybrominated biphenyls and polybrominated diphenyl ethers.

A complete ban on these substances is, of course, impractical.

The Directive therefore sets out limits on the weight-for-weight%age of the substances usable in any end product.

They also specify a list of industries - such as telecommunications infrastructure and the military - which do not need to comply, and a corresponding raft of specific end products.

For example, CRT manufacturers can continue to use glass that contains lead.

Although no-one seems terribly certain where the list of substances originated, there is little doubt that the one that has caused most consternation and debate for the electronics industry is lead, largely because of its formerly ubiquitous presence in solder processes and materials.

Eliminating lead throws up a series of difficulties.

For a start, alternative metal alloys used for soldering behave differently to conventional solders.

This means requalification and in some cases complete redesign, if the physical layout of a PCB is not conducive to good soldering results using the new process.

Much of the problem is caused by the fact that new alloys have a higher melting point than traditional tin-lead.

In some cases this may require investment in new production machinery able to hit the required temperatures.

There remains an increased possibility of consequent heat damage to components.

Nor are circuit board materials immune to this effect: even if they are not physically damaged, production engineers need to establish the materials' mechanical and thermal stability under the new conditions.

Military, aerospace and telecommunications manufacturers secured RoHS exemption partly on the basis that, as a relatively new technology, insufficient evidence existed about the long-term reliability of lead-free soldering.

Although many of these fears have recently been allayed as a fund of process expertise and reliability data has built up, many experts point out that there is no getting away from the fact that pure tin joints are mechanically less strong than their traditional tin-lead predecessors.

Concerns also remain about a number of phenomena associated with the use of pure tin solder coatings.

In particular, they tend to form tin whiskers, filamentous growths that appear spontaneously, and are prone to breaking off and causing short circuits between exposed contacts.

Pure tin is also prone to solder-balling and intermetallic formation: and component tombstoning, caused by unequal stresses within soldered joints, can also be a challenge during reflow of lead-free components.

It might seem that RoHS exemption is a ready solution to all of these problems.

But even exempt manufacturers need to be cautious.

We have already noted that non-RoHS components are in short supply and therefore more expensive.

Then there is the problem of differentiating between lead-based and lead-free parts.

Component manufacturers and distributors will undoubtedly be supplying both types, and the simple practical problem of telling them apart will only worsen as compliance becomes the norm rather than the exception.

There will be a specific challenge presented to companies which supply into both RoHS compliant businesses and into sectors which are exempt.

They will need to establish robust controls to segregate materials and processes in order to eliminate the risk of contamination.

For the exempt manufacturer, there may eventually be little alternative in the effort to ensure acceptable quality than to test the metallurgy of all incoming components.

The two types are visually virtually indistinguishable, and problems are likely to show up only when field failures occur.

Looking even further down the line, some providers are certain to discontinue the supply of lead-based components as volumes go down, leaving OEMs to rely on last-time buys, the grey market, or their own stocks of components.

Perhaps the final challenge is the human one.

In the light of RoHS, people have to change their behaviour and the processes they use; at the inspection stage, solder joints will look different, and people will need to learn about the change.

The good news is that many of the problems appear not to be as severe as was first thought.

The industry has now overcome the fear factor, and applied itself to getting on with the job in hand.

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