Steve Berry and Sandra Winkler Contributing Editors

After years of griping, people in the industry are ready to comply with the lead-free initiatives and want to know the most cost-effective and reliable method of lead-free compliance. (Lead-free is defined as less than 0.1% lead.)

Worldwide, over 20 million pounds of tin-lead solder are consumed annually. Lead is found in the flip-chip bumps, solder paste, and plating on leadframe devices, PWB land pads and through-holes and on cable harnesses. Lead adds solderability, making it possible to fuse all the elements of the electronics together into a working whole.

Lead performs the function of lowering the temperature requirements for the reflow operation. It also increases the flexibility of the material during expansion and contraction-known as creep-which prevents cracking. In plating operations, the presence of lead in the electrodeposit alters the diffusion behavior of the tin in a way that inhibits tin whisker growth.

Minimizing Whiskers

The inclusion of lead in a plating bath is the best-known method of minimizing tin whiskers, yet lead is the most hazardous in plating operations, because the lead is mobile.

Tin whiskers do not form immediately; rather they form over time on tin surfaces. Tin is not the only alloy to whisker, but tin whiskers have the potential to short out a device if they grow long enough to bridge across those very fine pitches on leadframes.

While much is known about tin whiskers, we still do not know exactly what causes them. Thus, the ideal cure to eliminate them completely in all environments and over time is elusive.

Speakers pointed out at the very lively NEMI Tin Whisker Workshop, that occurred in conjunction with the ECTC program in June, that there are no exhaustive studies underway that would pinpoint the underlying cause of tin whisker growth.

The point was also made, however, that matte tin plating can be whisker free if the ingredients in the plating bath are controlled. This control creates a tensile-stress environment with the addition of the correct organic additives. Additionally, the parts must be annealed within 24 hours of plating. A thicker plating has been determined to be better than a thinner plate. Leadframes have to be very smooth so they will not add to the compressive stress.

Many people plate some parts with matte tin, then leave them on their desks for a couple of years, looking for whisker growth.

Speakers pointed out at the annual IPC/JEDEC International Conference on Lead free in March that this is not a sufficient whisker test. Reflowing parts on a PWB will shift the plating so that the concave portions of the leadframes will now have a thicker plating, and the convex a thinner plating, which will then be more prone to whiskering.

Applying a Nickel Underplate

Some companies are looking at applying a nickel underplate to form a barrier between the copper leadframe and the tin, preventing the intermetallic compound (IMC) growth that forms when copper and tin are in contact.

IMC is thought to play a role in the whiskering, as it leads to compressive stress. However, nickel and tin also form an IMC; in addition, nickel is difficult to plate, and cracks easily. The additional cost of a nickel undercoat is fairly inexpensive, but a new plating line usually must be installed to add the nickel plating baths.

Preplated leadframes of nickel, palladium, and a flash of gold (Ni/Pd/Au) are getting considerable attention, too. The problem with nickel is that it does crack easily; thus new trim and form equipment is required if nickel is involved.

The lead-free issue will be around for years to come, continuing to provide fodder for conferences and workshops.