Letter 6083

Tin whiskers a problem with immersion tin pwb finishes? 

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I am researching the application of compliant pin /pressfitconnectors into pwb backplanes. The backplane manufacturers recommendimmersion tin as a pwb finish. What about tin whiskers. Many of thetelecommunication companies are using pressfit backplanes withimmersion tin pwb finish, (Lucent, Alcatel, Nortel etc) are they notworried about long term reliability or is there something significantto electroless/immersion tin plating processes and the deposit thatinhibit tin whiskers?

Shawn P. Thomas
Boeing - Everett, WA, USA

Tin whiskers are covered at the Goddard Space Flight Center

http://nepp.nasa.gov/whisker/

For ground hardware, the whisker blows open and dissipates.Usually no lasting damage is done to the hardware. In space a tinplasma forms in the area and allows a large enough current to flowthat can destroy even hardened electronics.

Kirk Mueller
Raytheon Systems Co. - El Segundo, CA USA

I am researching the occurance of failures in PWB assemblies that can be attributed to conductive anodic filaments (CAF) or hollow fibre migration (HFM). I am particularly interested in the possible increase risk of failure on Immersion Tin finished product.

Has anyone established a connection between specific metallic finishes and increased exposure to CAF/HFM?

This is my first excusion into the cyber world, I shall await the avalanche of collective knowledge with great interest.

Peter Norris
-Circuit Logistics Ltd - Huntingdon, UK

The correct bismuth content in an alkaline tin bath is should inhibit tin whiskers

Derek Skaro
- Blaine, Minnesota, United States

I worked in the electronics industry for several years, and the topic of tin whiskers seems to surface peiodically. Extensive studies were conducted in the '80s with the conclusion that tin whiskers "grow" as a means of stress relief form the plating operation. Reflowing the tin reduces the stress, but does not entirely cure the condition. A dip coated tin has significantly less stress than an electroplated tin, but whiskers can still develop. Over-coating and encapsulation were tried, but the whiskers can break through the coatings. So, if you have a critical application, avoid tin.

David Strawser
- Ramat Gan, Israel

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