RoHS’ July 1, 2006 deadline will soon be here and will affect all products sold into the Euro-pean market.

RoHS, consisting of environmental regulations drafted by the European Union, prohibits lead in most electronics, among other things. In addition to lowering the processing temperatures, lead introduces “creep” into the plating finish, which allows expansion and contraction of the finish, severely reducing the incidence of tin whiskers.

The cause of tin whiskers on post-plate leadframe finishes has never been resolved, but there are steps that can be taken to minimize whisker growth.

With matte tin currently the leading post-plate, leadframe-plating finish of choice, tin whiskers may be a problem with these finishes.

At one point, some companies tried using a barrier metal of nickel beneath the tin over-plate to prevent the copper leadframe from forming intermetallics with the matte tin and causing whiskers. Results, however, were generally not promising and detecting cracked nickel beneath the tin surface was difficult.

What has worked best begins with choosing a leadframe that has the least propensity to whisker (alloy 42 is more prone to whiskering than copper).

Leadframes with the smoothest surfaces will be less likely to whisker, because the smooth surface reduces compressive stress. Etched leadframes can have smoother surfaces than stamped, and thus may have fewer whiskers.

Upgraded plating baths with a lesser propensity to whisker are being introduced by plating suppliers.

One vendor’s new plating bath is a fine-grain matte tin that is “wormy” in appearance, and doesn’t have a flat surface from which whiskers can form.

Another vendor is offering a formulation for matte-tin plating, that is designed to minimize whiskers because it has a low carbon content and a 1- to 2-micron fine-grain structure with a single 2-2-0 orientation. Additionally, the plating finish does not leave deposits on the contact probes of the final test sockets or wear them down prematurely.

Annealing or baking finished packaged parts to minimize stress for one hour at 150°C is considered a necessary step for minimizing whisker growth.

Preplated leadframes with nickel/ palladium/gold is the next most popular plating finish, and is generally preferred by companies without the existing plating line infrastructure, such as fabless companies.

The absence of tin guarantees this form of plating to be whisker-free. A look at the cost chart of plating costs, broken down to that of a single SO leadframe, shows this option to be cost-effective.

Matte tin as a plating finish is currently slightly more expensive than tin/lead options due to the cost to purchase reflow ovens for annealing, but otherwise the equipment is the same for matte tin and lead/tin plating options.

Low volumes may result in slightly higher prices for the plating additives, which will be comparable to prices for tin/lead as volumes increase.

It is important that lead-free solutions not only comply with RoHS, but also not further harm the environment, are manufacturable, and be practical in the long term for today’s and tomorrow’s applications.