Tin whiskers brush with RoHS

“The point is tin whiskers,” a Hewlett-Packard spokesman told Electronics Weekly.

Tin whiskers are needle-like crystals of tin which grow from electroplated pure tin or near-pure tin. They can be over 1mm in length and have caused short circuits in PCB assemblies.

The standard Sn-Pb coating used on integrated circuit terminals now is virtually immune from whisker formation, but the Directive will effectively ban Pb from integrated circuits for environmental reasons from the middle of next year.

The most popular Pb-free material proposed for leadframe coating is tin.

In its submission to European law makers, Hewlett-Packard seeks to have fine-pitch components “with electrical terminations spaced with centers 0.65mm or less apart” made exempt from the RoHS Pb ban. The inter-lead spacing of such components is typically 125 to 300µm, it said.

“If the coating has 15 per cent of lead, it will stop tin whiskering,” said the HP spokesman.

Should it be accepted, HP estimates that between two and 20 tonnes of additional lead will be sold in Europe each year.

Pure tin is not the only alternative to Sn-Pb leadframe coatings.

Cypress Semiconductor is moving to nickel-palladium-gold for its RoHS-compliant parts. “Our customers really like this finish,” Cypress technical sales manager Dave Hoskins told Electronics Weekly.

The physics behind tin whisker formation, which can be incredibly slow, is not yet understood, but bending and scraping the tin is known to make promote whisker formation. This accelerating factor is at the root of an exemption application by Sony.

The Japanese firm has applied for an exemption for the flexible ribbon-cable-like PCBs that are used as plug-in connectors between conventional PCBs.

It is advocating that “five to ten per cent of Pb is included in the electroplated Sn coating as whisker suppresser”.