Commentary: Comply with RoHS in four easy steps

-July 11, 2006

By Duane Benson, web marketing manager, Screaming Circuits

In many a darkened hallway, just outside the development lab and two weeks prior to design-freeze, a product manager will ask, "And, of course it meets RoHS, right?”

If the answer contains any sound resembling "ummm" or  "uhhh," then read on.
As of July 2006, the European Union’s Reduction of Hazardous Substances (RoHS) Directive (2002/95/EC), which prohibits  lead and several other substances to be used in the manufacture of products shipped into or sold in Europe, went into effect. There is a host of literature detailing the exact specifications and technical implications of this new law, but not much that puts it all into concise practical terms for the design engineer developing consumer and business-targeted electronic products.
If you don't really care exactly how much Hexavelent Chromium or Polybrominated diphenyl ethers you can have in your boards; if you don't really care to wade through stacks of technical documentation, but just want to build RoHS-compliant boards, you can do so by considering four key areas.

1) Base Circuit Board. Both the substrate and board finish used on lead-free boards need to be different than for circuit boards using lead.
It's not just the lead. Lead-free solder's melting point is about 50 degrees C hotter than lead-based solder. Standard FR4 material will sometimes delaminate during the lead-free reflow process due to the higher temperature profiles required. The problem can be exacerbated if the board has to make multiple passes through the oven for double-sided SMT or for rework. You may need to be even more cautious with boards that are primarily through-hole. Early industry feedback suggests that wave soldering is more likely to cause delamination problems than a properly profiled reflow oven.
Most board fab houses can deliver acceptable boards if you request RoHS compliance. Some, however, will try to get by with just removing the lead from the finish and not changing the base pcb material. When ordering pc boards, don't just ask for a lead-free finish. Specify RoHS compliant boards and make sure the fab house uses materials that can withstand the additional heat and can make multiple passes through processing without problems.
2) Components. You must verify that all your components meet RoHS standards and do not contain any of the banned substances. Most parts suppliers can help you find RoHS equivalent components for an existing design but occasionally you will find an older part that has not been certified as RoHS compliant.
Some of the parts may be lead free and able to withstand the temperature, but just not documented as such. In that case, you may be able to use the part if you collect all the necessary documentation to prove that the part is compliant. If the part cannot be documented to comply and does not come in a RoHS variant, check other manufacturers for substitutions. As a last resort, you may need to change your design to accommodate a newer part.
Again, it is more than just the lead and other banned substances. The components must also be able to withstand the higher temperatures required in manufacturing. Most standard components will be fine, but some MEMs parts, switches, membranes, LEDs and others may not be able to survive the extra 50 degrees C required. Some can survive the temperatures, but only just barely. Make a note of any components that are heat sensitive, even if RoHS compliant, when you contact your assembly house.
3) Moisture Sensitivity. Many components will, over time, absorb small amounts of moisture. RoHS-compliant storage packaging will inhibit this and will list an expiration date. If the protective package is past the expiration date or the packaging has been opened, the part may be destroyed during reflow or wave soldering due to the rapidly expanding water vapor. Typically, such a failure would be seen as a small crack on the side of the part.
Opened or moisture-expired parts may still be used on your prototype, but they will need to be baked at the assembly house to slowly remove the excess moisture. When confirming the integrity of the moisture protective packaging, your assembly house may determine that components need to be baked prior to assembly and this may add time to the processing.
4) Ball Grid Arrays. BGAs are a special case as, by their very design, they already contain solder (the solder balls). The BGA solder balls MUST match the type of assembly. While many types of non-RoHS components will solder fine with either leaded or unleaded solders, even if they take the board out of compliance, BGAs will probably not. You can't mix leaded and lead-free BGAs either. They all have to be one or the other.
If a leaded BGA is soldered at the higher no-lead temperatures, the solder may overheat, the flux may burn off and the solder joint may end up brittle or cracked. Of course, the solder balls on a lead-free BGA may not melt at all at the lower leaded reflow temperatures, leaving it not secured mechanically and not conducting electrically. This may become an issue even if you don't want to design for RoHS if your part manufacturer starts to only supply no-lead BGAs.
Details get in the way

There has been much written about RoHS compliance, specifications and legal implications, but for the vast majority of design engineers, the plethora of detail is now getting in the way. It's a matter of condensing all of the information into practical terms that can be easily integrated into the product development process.
The RoHS directive has certainly added an additional layer of complexity to the design process, but by looking at these four key areas you can develop and produce quality product with minimal additional effort.

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