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RoHS in Four Steps
December 31, 1969 |Estimated reading time: 4 minutes
By Duane Benson
As of this month, the RoHS Directive (2002/95/EC) restricts the use of lead and several other substances in the manufacture of electronics products shipped into the European Union (EU). There is a host of literature detailing exact specifications and technical implications of this directive, but not much puts it all into concise, practical terms for design engineers developing consumer and business electronic products.
If you aren't concerned with how much hexavelent chromium or polybrominated diphenyl ethers (PBDEs) you can have in your boards, or if you don't want to wade through stacks of technical documentation to build RoHS-compliant boards, you can do so by considering four key areas.
1. Base circuit boards: The substrate and board finishes used on lead-free boards must be different than that used for lead-based circuit boards. It's not just the lead. The melting point for lead-free solder is about 50°C hotter than lead-based solder. Standard FR-4 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 rework. You may need to be more cautious with boards that are primarily thru-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 ones that meet RoHS compliance. Some will try to simply remove the lead from the finish, and not change the base PCB material. When ordering PCBs, it is wise not to simply 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 multiple passes without problems.
2. Components: You must verify that all of your components meet RoHS standards, and do not contain any banned substances. Most parts suppliers can help you find RoHS-equivalent components for an existing design. However, you occasionally will find an older part that has not been certified as RoHS-compliant. Some parts may be lead-free and capable of withstanding the elevated temperatures, but not documented as such. In such a case, you may be able to use the part if you collect all the necessary documentation that proves compliance. If the part cannot be documented as compliant, and is not available in a RoHS-compliant version, check other manufacturers for substitutions. As a last resort, you may need to change your design to accommodate a newer part.
It should be stressed that compliance involves more than just lead and other banned substances. 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, light-emitting diodes (LEDs), and others may not be able to withstand the additional 50°C. Some can survive the temperatures barely. Make a note of any components that are heat-sensitive, even if RoHS-compliant, when you contact your assembly house.
3. Moisture sensitivity: Over time, many components will absorb small amounts of moisture. RoHS-compliant storage packaging inhibits this. It also lists 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 rapidly expanding water vapor. Typically, such 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 may need to be pre-baked at the assembly house to slowly remove excess moisture. When confirming the integrity of the moisture-protective packaging, your assembly house may determine that components should be baked prior to assembly; this may increase processing times.
4. BGAs: By their design, BGAs already contain solder. BGA solder balls must match the assembly type. While many types of non-RoHS components will solder well with lead-based or lead-free solder even if they take the board out of compliance BGAs will probably not. You cannot mix lead-based and lead-free BGAs. They have to be one or the other.
If a leaded BGA is soldered at higher lead-free temperatures, the solder may overheat, the flux may burn off, and the solder joint may become brittle or cracked. The solder balls on a lead-free BGA may not melt at all at the lower tin/lead reflow temperatures, leaving it mechanically unsecured and electrically non-conductive. This may become an issue if your parts manufacturer supplies lead-free BGAs only.
ConclusionThere has been much written about RoHS compliance, specifications, and legal implications. But for the vast majority of design engineers, it's a matter of condensing this information into practical terms that can be integrated easily into the product-development process. The RoHS directive has added an additional layer of complexity to the design process, but by looking at these four key areas, you can develop and produce quality products with minimal additional effort.
Duane Benson, marketing manager, Screaming Circuits, may be contacted at (866) 784-5887; e-mail: dbenson@screamingcircuits.com.