Discussions@TMS - Tin Whisker Session Summary

I’ve compiled the following (brief) reference list [1-5] from my prior research and this conference. For more in depth background, I direct you to these studies. In particular, Tu’s paper (2005) was the most widely cited tin whisker article of this session [4]. I had not read this at all prior to the conference. Tu concludes that an adherent protective oxide is a necessary condition for whisker growth and attributes compressive stress accumulation to IMC growth. The surface of oxide free plating will serve as an effective vacancy sink which is required to accommodate the IMC formation. When an oxide layer is present, this can’t occur. When the oxide is broken, surface vacancies are made available for IMC growth and whiskers form from the broken oxide.

Several presenters presented information on whisker growth on stressed cantilevered beams at varying time, temperature, and relative humidity. One defines a whisker index based on the length and density of whiskers generated. It was found that the highest whisker index occurs at intermediate temperatures,, ~50 degrees Celsius at 10%r.h. Higher temperatures relax stress too quickly, only short hillocks grow. Lower temperatures grow a few random long whiskers, but at the intermediate temperature, many whiskers of significant length grow.

Mechanical stress is apparently not the only driving force for whisker growth. There also exists a driving force for whisker growth under an electric field and current. With a imposed current, 50-70 degrees Celsius is the worst case temperature for whisker growth.

The unfortunate thing about studying tin whiskers is that long aging times are involved. Work is still being performed on accelerating whiskers growth.

[1] Barsoum MW, Hoffman EN, Doherty RD, Gupta S, Zavaliangos A. Driving Force and Mechanism for Spontaneous Metal Whisker Formation. Physical Review Letters 2004;93:206104.
[2] Boettinger WJ, Johnson CE, Bendersky LA, Moon KW, Williams ME, Stafford GR. Whisker and Hillock formation on Sn, Sn-Cu and Sn-Pb electrodeposits. Acta Materialia 2005;53:5033.
[3] Osenbach JW, DeLucca JM, Potteiger BD, Amin A, Baiocchi FA. Sn-whiskers: truths and myths. Journal of Materials Science: Materials in Electronics 2007;V18:283.
[4] Tu KN, Li JCM. Spontaneous whisker growth on lead-free solder finishes. Materials Science and Engineering: A 2005;409:131.
[5] Eshelby JD. A Tentative Theory of Metallic Whisker Growth. Physical Review 1953;91:755.

3/9/2007 3:32 PM
Jason Walleser Posts: 3 Joined: 2/19/2007	I’ve compiled the following (brief) reference list [1-5] from my prior research and this conference. For more in depth background, I direct you to these studies. In particular, Tu’s paper (2005) was the most widely cited tin whisker article of this session [4]. I had not read this at all prior to the conference. Tu concludes that an adherent protective oxide is a necessary condition for whisker growth and attributes compressive stress accumulation to IMC growth. The surface of oxide free plating will serve as an effective vacancy sink which is required to accommodate the IMC formation. When an oxide layer is present, this can’t occur. When the oxide is broken, surface vacancies are made available for IMC growth and whiskers form from the broken oxide. Several presenters presented information on whisker growth on stressed cantilevered beams at varying time, temperature, and relative humidity. One defines a whisker index based on the length and density of whiskers generated. It was found that the highest whisker index occurs at intermediate temperatures,, ~50 degrees Celsius at 10%r.h. Higher temperatures relax stress too quickly, only short hillocks grow. Lower temperatures grow a few random long whiskers, but at the intermediate temperature, many whiskers of significant length grow. Mechanical stress is apparently not the only driving force for whisker growth. There also exists a driving force for whisker growth under an electric field and current. With a imposed current, 50-70 degrees Celsius is the worst case temperature for whisker growth. The unfortunate thing about studying tin whiskers is that long aging times are involved. Work is still being performed on accelerating whiskers growth. [1] Barsoum MW, Hoffman EN, Doherty RD, Gupta S, Zavaliangos A. Driving Force and Mechanism for Spontaneous Metal Whisker Formation. Physical Review Letters 2004;93:206104. [2] Boettinger WJ, Johnson CE, Bendersky LA, Moon KW, Williams ME, Stafford GR. Whisker and Hillock formation on Sn, Sn-Cu and Sn-Pb electrodeposits. Acta Materialia 2005;53:5033. [3] Osenbach JW, DeLucca JM, Potteiger BD, Amin A, Baiocchi FA. Sn-whiskers: truths and myths. Journal of Materials Science: Materials in Electronics 2007;V18:283. [4] Tu KN, Li JCM. Spontaneous whisker growth on lead-free solder finishes. Materials Science and Engineering: A 2005;409:131. [5] Eshelby JD. A Tentative Theory of Metallic Whisker Growth. Physical Review 1953;91:755.

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