Tin Whiskers…NOT the Cat’s Meow! An NPI Update. Part 2 of 2 (July 29, 2013)

Tin whisker growth noted from seal to about 20 mils from edge of solder coat. Electrical failure wa traced to a 60 mil whisker that shorted lead to case.

Thru hole oscillator.
Lead diameter 18 mils.
Bright tin finish leads and case.
Tin/lead solder dipped within 50 mils of glass seal and hand soldered to PWB.

First, let’s take a step back and better understand tin whiskers themselves. They are spontaneous hair-like growths from surfaces that use Pb-free tin (Sn) as a final finish. Tin whiskers are also electrically conductive and can grow in hours, days, weeks, or years. Both Pb-free tin-plated electronic and mechanical parts can grow whiskers. First reported by Bell Labs around 1947, tin whiskers can start growing after years of dormancy, or in a few hours. They also can start growing, stop, and then resume growing. Whisker shapes and forms vary considerably and lengths can range from a few microns to over 20 millimeters. Tin whiskers can even grow through thin conformal coatings! Now how can this menace be dealt with???

Well here’s the bad news….whisker growth mechanisms are still not known after 60+ years of study as there is conflicting evidence. There is no effective test to determine the whisker propensity of any plating. There is no single mitigation technique that provides effective protection against whisker formation except the addition of 3% or more of Pb by weight. So much for Pb-free electronics!

Last, but not least, no reliability models exist that can quantify the impact of tin whiskers. So are tin whiskers really that harmful, and why are they a major concern for aerospace engineers?

For starters, in small circuit geometries, whiskers can easily bridge between contacts and adjacent whiskers can even touch each other. Broken-off whiskers can bridge board traces and foul optics or jam MEMS. In low voltages, whiskers can handle tens of milliamps without fusing. In other cases, they can cause transient short circuits until the whisker fuses open. Tin whiskers will degrade high frequency circuit performance (6 GHz RF and above). Plasma arcing is the most destructive of all as a whisker can fuse open, but the vaporized tin may initiate a plasma that can conduct over 200 amps! This occurs in both vacuum and atmospheric conditions and can occur with less than 5V potential.

Let’s now look at some whisker mitigation techniques:

1. Using matte tin (tin with a dull low gloss finish) may be more resistant to whiskering than bright tin; however, whiskers still grow as with bright tin. Matte tin has no common definition other than “not bright.”

2. Annealing tin can reduce some of the stresses in plating that may contribute to whisker growth; however, whiskers still grow.

3. Underplating with nickel to reduce intermetallics, which had been theorized important to whisker growth, but whiskers still grow, as intermetallics do not appear to be critical for whisker growth.

4. Reflowing Pb-free tin finishes after plating introduces additional thermal exposure during assembly. The results and effectiveness is unproven in general practice.

5. The use of a Ni/Pd/Au finish avoids the tin whisker issue; however, there is an increased cost with this plating finish and it’s not available for all products.

6. Soldering/solder dipping with Sn/Pb solder can be effective but unconverted areas can still grow whiskers. Whiskers can also grow through thin Sn/Pb coatings.

7. Replating with Sn/Pb solder can be effective but it’s not possible for all components.

8. Reballing BGA/CGAs is effective but additional handling can cause damage, and fine pitch products can be particularly difficult to reball.

9. Conformal coatings can be an effective mitigant but whiskers will grow through thin coatings. Coatings may also negatively affect the performance of some circuits. Conformal coatings are the focus of many research studies including Parylene, which is whisker-tough.

So where does that leave us?

There are things that can be done to mitigate whisker risks that are inherent in Pb-free tin but no single technique is 100% effective! Multiple mitigations are necessary until additional research has been completed. Units that have been properly assembled with Sn/Pb solder and conformally coated provide reasonable protection against tin whiskers at minimal cost and system impact.

Content reference for this blog:
Lead-free Electronics Reliability -An Update, Andrew D. Kostic, Ph. D.
The Aerospace Corporation, GEOINT DEVELOPMENT OFFICE, August 2011.
www. Http://Nepp.Nasa.Gov/Whisker/Reference/Tech_papers/2011-Kostic-Pb-Free.Pdf

Photos Courtesy: NASA Electronic Parts and Packaging (NEPP) Program,
Photos by Ron Foor