Soldering On Regardless
The Restriction of Hazardous Substances Directive (RoHS)

As of July 1 any European Union (EU) electronics technician reaching for his solder better come up grasping the lead-free variety -- or he'll be breaking the law. RoHS seeks to eliminate antimony, zinc, bismuth and lead (Pb) from electronics.

RoHS, together with another EU directive called WEEE (Waste of Electrical and Electronic Equipment), is designed to reduce the heavy metals in old electronics dumped in landfills. The problems that manufacturers now have pale in comparison to those that could be faced by avionics techs endeavoring to find suitably qualified RoHS components to marry up with existing surface mount technology. RoHS has been a long while coming, but it is now here and brings with it the specter of reduced or dubious reliability for aircraft avionics. Finding the right flux and solder for a particular task used to be straightforward. Henceforth the suitability might be directly related to durability - and less to solderability. Apart from new soldering irons, fluxes, rosins, pastes, temperatures and techniques, the technician will now need to avoid latent mistakes. These might not become immediately apparent. It's acknowledged that quality control by visual inspection will be infinitely more difficult. Lead-free solders are more sensitive to the effects of a dirty soldering iron. Higher soldering temperatures can oxidize the tip if it isn't cleaned and coated. Solder joints made with lead-free alloys will have a grainy dull finish. This contrasts with the bright, shiny finish associated with good joints made of lead solder. The difference in appearance will make it much more difficult for inspections to distinguish good from bad joints. But the anticipated and known problems don't stop there.

Lead-free solder alloys, such as tin-silver-copper, have a melting point of 217˚C, much higher than tin-lead alloys. Heat-sensitive components will be exposed to much higher temperatures for a longer time because the new solders have poorer wetting qualities -- thus the necessary capillary action for joint formation is delayed. The likelihood of generating solder "icicles" is much higher, especially if the soldering iron lingers after a joint forms. In service, avionics faults could increase because of mechanical vibration or shock. Why? A lead-free solder junction will always fail earlier than a tin-lead joint. There are other unique Pb-free issues worth noting.

"Tin whiskers" are filament growths emanating from tin-based plating on components and connectors. They can grow, bridge gaps and cause shorts. In the field, lead-free whisker-growth associated failures have arisen in less than two years. These are seen in component spacings of <1mm (with="">80˚C. The higher Pb-free solder reflow temperatures have also greatly increased the incidence of conductive anodic filaments (CAF) -- copper filaments that migrate along the fiberglass/epoxy interface and cause internal electrical shorts between adjacent conductors. CAF is the scourge of high-density board designs and has thus given rise to a costly specialist proprietary design specification. Avionics has reaped the whirlwind with the advent of lead-free solder complexities.

The elimination of lead-free paint has always made sense. Similarly the desire to stop heavy metals ending up in land-fill is laudable. However, when you examine the critical downside of intricate and interdependent avionics systems exposed to the natural shocks and vibrations of flight, you have to wonder whether a special case couldn't have been made for aerospace avionics to remain "leaded." Surely they could be collected for safe disposal or recycling at the end of their service lives? An optimist might see the prospect of coping with a predicted diminished reliability as a challenge. A pessimist might see it as an unnecessary cost or even as a flight safety hazard. As airliners become more automation-reliant, anything that serves to reduce reliability, dependability and maintainability should be resisted on flight safety grounds.

-- By John Sampson