RoHS Part II: Lighten Up on Heavy Metals

Q: Do you expect to change lifetime specifications because of tin whiskering?

A: The feeling is that product MTBF specs will not change.

Part I of this series on the impact of Restrictions on Hazardous Substances (RoHS) regulation explored how machine vision vendors are adapting their operations, including procedures and product lines, to accommodate RoHS regulations. This article will look at the technical challenges facing the machine vision industry as a result of RoHS regulations.

Although there is not a single list of hazardous substances (see ‘‘RoHS In The Box’‘ sidebar), the majority of global regulations banning imports or exports is related to four heavy metals and two toxic flame retardants, namely: Lead (Pb); Mercury (Hg); Hexavalent Chromium (Cr(VI)); Cadmium (Cd); Polybrominated Biphenyl Flame Retardants (PBB); Polybrominated Diphenyl Ether flame retardants (PBDE). Out of these six substances, lead is by far the most problematic for the machine vision industry.

Although lead discussions surrounding RoHS typically center on low-lead, high-temperature solders, lead is actually used in many parts of the machine vision product line. ‘‘As an example, even the shipping box has to be RoHS compliant,’‘ explains Matrox Imaging's Product Line Manager Pierantonio Boriero. ‘‘You need to certify that the ink on the box has acceptable levels of lead. You really have to go down to the granular level to make sure of compliance.’‘

Additionally, there are a number of RoHS exemptions that exist, and in some cases, components that are not RoHS compliant, but with exemptions. Examples of components that contain restricted substances but are not banned are lead-glass CRTs, and ceramic capacitors that have lead as part of their dielectric formulation.

Whiskers, Popcorn, Wetting
After you get past lead in cable insulation, ink, glass, and many other places one wouldn’t expect to find it, the remaining question plaguing vision suppliers is: How will high-temperature, low-lead tin solder affect my products?

Eutectic solder typically contains 63% tin and 37% lead, considerably more than the 0.1% allowable under a majority of RoHS regulations. This solder melts at 183ºC, which is just low enough not to compromise the hermetic packages, plastics, and other materials that typically make up an electronic board. Leadless solders, of which there are many, use a variety of elements to replace the lead, lowering the temperature at which the solder turns from solid to liquid for attaching components. Increasing the amount of tin and adding other rare earth materials such as silver and copper reduces the toxicity of the solder, but raises the melting temperature above 200ºC. Higher temperature solders are typically more expensive, more difficult to spread, and require a re-engineering of the soldering/electronics manufacturing process because of the high temperatures, among other problems. Staged heating systems typically are the answer so that sensitive components can be protected from long exposures to high-temperature solder processes, which complicate the manufacturing process and requires additional floor space for production.

‘‘As recently as 5 or 10 years ago, most people took solder for granted. RoHS is forcing everyone to look at the process and really understand what's going on,’‘ notes Daniel Vuk, Matrox Imaging's (Dorval, Quebec, Canada) Engineering Hardware Manager. This relates to the tin whisker problem.

Pure tin has a habit of growing crystals perpendicular to the surface over time. When the whisker grows between two isolated conductors, a short can occur, leading to the ‘‘tin whisker’‘ problem often cited as the biggest challenge facing RoHS compliancy. The lack of long-term studies on the effects of tin whiskers was cited by the U.S. Air Force in an airworthiness study as the reason that the armed forces did not want to include RoHS products in airborne vehicles. Since then, the other branches of the U.S. Department of Defense (DoD) have publicly said they want no part of RoHS. Unfortunately, in a global marketplace, captive markets are prohibitively expensive – even for the DoD.

Although the majority of the electronics industries, including the machine vision industry, have adopted tin solders as the way forward, high temperatures and whiskers are not the only problem plaguing this solder material. Dendrites are structures that grow on pure tin along the surface of the solder joint instead of away from it, potentially resulting in additional resistance and shorts. Popcorning can occur if any water vapor is trapped inside an IC package and subjected to fast temperature ramps; the water vapor can cause the component to explode. Solders wet, or flow differently at high temperatures, potentially leading to wetting imbalances and ‘‘tombstoning’‘ effects where one edge of an IC is lifted higher than the other and the result is stress and torque on the part, potentially shortening MTBF.

Although staged heating during solder reflow helps to limit the time that sensitive components are subject to elevated temperatures, not all components react the same way to the process. According to JAI's Product Manager, Steve Kinney, manufacturers have to be careful of fine-pitch IC packages. ‘‘Fine pitch ball grid arrays (BGA) are tough because of higher temperature processes. There are questions about brittle columns – they take the heat, but just barely,’‘ Kinney explains.

Making Everyone Happy
As illustrated in Part I of this series on RoHS, companies are taking different approaches to complying with RoHS standards despite a growing list of exemptions and product quality concerns from customers. But regardless of whether a machine vision company sees RoHS as a global, regional, or industry-specific issue, RoHS will continue to impact the global electronics supply chain.

The question of whether to buy RoHS compliant components or not may soon be a moot issue, as the world's largest discreet component suppliers seek to maximize economies of scale for their product lines by pursuing RoHS compliancy. Even companies who purchase RoHS components with the intention of using eutectic lead solder may still have to face the questions of whiskers and dendrites. ‘‘There are still some questions about whether a RoHS compliant component with tin-flashing on the leads can grow a whisker above the eutectic solder reflow line [where the pure tin is exposed],’‘ explains JAI's Kinney.

JAI, for instance, plans to use a combination of RoHS components and lead solder for military systems that simply will not accept RoHS compliant products. ‘‘We're going to use gold plated printed circuit boards and a lead solder process along with the same components used in our RoHS-compliant products,’‘ Kinney continued. ‘‘Wherever possible, these components will be tin-nickel flash over copper to minimize any whisker issues, but some tin flash over copper may need to be used when tin-nickel is not available. If customers are still concerned, we'll offer conformal coatings, but those aren't cheap.’‘

As Matrox's Vuk points out, at the very least, the playing field is level for everyone in the machine vision industry. ‘‘For all intents and purposes, RoHS is a new way of operating that adds new requirements to the equation. The good news is we’re all in the same boat. We have to do our due diligence and our suppliers have to do their due diligence, to make sure that the components that they supply us no longer contain lead,’‘ Vuk concludes.