I would like to start here a discussion about RoHS, the Restriction of Hazardous Substances Directive 2002/95/EC. While its aim is to restrict hazardous materials, on the other hand some materials, such as Lead (Pb), that attend the directive can become expensive and/or of lower quality. What do designers think about it? How have you been dealing with RoHS directive for the last decade?

As I understand the directive Lead is prohibited. Thus the quality or price of Lead is not an issue other than it may save you some front end cost.

However, if you need long live not having Lead in the solder or plating can cause problems in the future. Tin can and often does grow dendrites with no external electrical field. I know this is totally contra-intutitive as we are talking about what appears to be a solid. The dendrites can and do short out close traces, parts, and connector pins. The dendrites can grow in air conditioned environments. Some early studies showed some coatings may slow the growth but not eliminate it. Thus dendrites can effect warranty cost and hurt customer relations. No question Lead is dangerous in the environment. There are some exceptions and I believe military and medical equipment are exempt. This is probably a good requirement for equipment that is obsolete every year or two and end up in land-fills.
My industry gets to use Lead has anyone found a way to keep Tin from growing dendrites?

I'm doing some high temperature work, and I'm very familiar with the tin dendrite problem. After 6 months or so of operation, they are a big problem. Big fur balls of tin that grow, as Ernie says, even in the absence of a field. In speaking with some companies that work at even higher temperatures, their solution is to solder with pure lead. Not RoHS compliant of course, but other things they are doing to the environment make that pretty much moot.