Directive Decoder

The most significant impact of RoHS has been the restriction of lead in solders. This has caused changes to the design of electrical equipment, mainly because there is no lead-free “drop-in” replacement for tin-lead solder. The use of lead-free solder does not in itself require changes to design but RoHS is causing design changes for various reasons. For example:
• Many components have become obsolete, in particular older microprocessors and memory chips
• The higher reflow temperature required using lead-free solder can damage many types of components, especially larger electrolytic capacitors and some types of connectors. If the component’s specified maximum reflow temperature is below the temperature that will be used, then an alternative will be required. Sometimes it may be possible to use through-hole components or hand solder to minimise heat damage but occasionally, complete redesigns will be required.
• Higher temperatures cause damage to components and laminates and it is wise to minimise this. Board layout can help as well as avoiding having to solder very large components and small heat sensitive components at the same time. Higher Tg laminates may help to avoid distortion and delamination problems at the higher soldering temperatures.
• Lead-free solderable coatings are usually pure tin but tin-bismuth and tin-copper are also used. All of these can be susceptible to tin whiskers which can cause short circuits. Although failures that can be attributed to tin whiskers are rare, there are published strategies for minimising the risk and one of the most comprehensive is from iNEMI. Design can help to minimise the risk; for example, do not use custom tin plated parts unless this is unavoidable and also avoid fine pitch component terminations.
• The long-term reliability of lead-free soldered electronics is uncertain due to a lack of field data – lead-free solders are too new. Research indicates that lead-free solders are more susceptible to thermal fatigue (usually due to cyclic temperature changes) than tin-lead when solder joints are subjected to high levels of strain. At low strain levels lead-free appears to be superior. Equipment design can minimise joint strain, for example by avoiding large chip components, leadless ceramic components and J-lead ICs with alloy 42 lead-frames. Components with flexible “gull-wing” terminations and small chip resistors etc, pose a lower risk

RoHS is having an impact on other materials too. EMC performance requires good conductivity between panels and this is maintained with passivation coatings that traditionally have been based on hexavalent chromium. These can no longer be used but alternatives exist for most applications. These may be inadequate for equipment used in more hostile environments however and so alternative designs which can be utilised to maintain EMC performance may be required.

Design engineers no longer have to simply consider the technical performance of components that they intend to use. They also need to ensure that they comply with legislation such as RoHS and can withstand the reflow temperature. RoHS substances can occur in surprising and unexpected places. Yellow labels may contain hexavalent chromium as a pigment and inks on components may contain lead as driers. Unfortunately, reliance on supplier’s claims of compliance may not be enough, and the reliability of the supplier must be ascertained first. If in doubt, carry out further checks.

Suppliers such as Farnell have comprehensive measures in place to ensure that if they say that a part is RoHS compliant then it will be, but not all suppliers have such rigorous procedures.

Impact on the design engineer:
• Must avoid RoHS substances
• Lead-free soldering is hotter so need to consider maximum temperature of components as well as its RoHS compliance.
• PCB design does not need to change significantly but if a RoHS compliant version of a component is not available, re-design will be necessary
• Easier to design with RoHS compliant components now for new products even if these are out of scope of RoHS. Consider how design affects long-term reliability issues.

Review of EU-RoHS:
The European Commission is currently reviewing the RoHS directive. This review will cover all aspects including the scope - should categories 8 and 9 be included? Should more hazardous substances be added? All existing exemptions will be reviewed and definitions and enforcement will also be examined.

It is likely that Categories 8 and 9 will be included (by 2012) and changes to the WEEE directive’s scope may also affect RoHS. It is hard to see how other substances could be restricted. Many very toxic chemicals are used in manufacturing processes (solvents, sealant components, etc.) but most are not present in the final product. The few toxic substances that are present such as arsenic and beryllium are used because there are no alternatives for the niche applications in which they are used. However a thorough search will be made to determine if any can be added. Hopefully many of the areas which are unclear (fixed installations, put onto the market, spare parts, etc.) will be clarified.

Directive Decoder