Second Tin Whisker Workshop

The iNEMI held the second annual Tin Whisker Workshop the day before ECTC started in Orlando May 31, 2005. Chaired by Ron Gedney and moderated by Henning Leidecker from NASA about 100 engineers wrestled with the consequences of pure tin coatings on electronic component leads. Pure tin has become one of the common options now that the long successful Pb-Sn coatings are discouraged for environmental reasons. Results of more than 9,000 hours of testing under various stress tests were discussed. Leidecker started the session by saying this is a real problem that was responsible for destroying a space craft 8 years ago. A whisker caused an arc that became a 100 Amp event. He pointed out that the thousands of fuses in the Hubble can arc uncontrollably with only 25 V once the air leaks out of their package.
Valeska Schroeder of HP described the incubation time before the whisker growth begins. The several accelerated test methods turned out not to be all that accelerating however humidity, temperature cycling, and sustained high temperature seemed to be an accelerant. The humidity above 85% would cause condensation in a non-uniform chamber and thus corrosion which was directly correlated to whisker growth.
William J. Boettinger of NIST described his study of the correlation of compressive stress in the tin plated film with whisker growth. He also mentioned that the columnar grain structure encouraged whisker growth whereas the vertically multi-grain structure as seen in SnPb films suppresses whisker growth. For example he found tin on copper leads has x10 the compressive stress of a plated Tin/Lead finish. Adding Cu to Sn caused compression whereas Pb additions put it in tension. He recommended pulse plating of tin to attempt to get horizontal grain boundaries that slow down whisker growth.
George Galyon of IBM made a grand attempt to summarize all the results of the study groups. First he stressed that compression in the tin film is the driving force even in those few cases where whiskers appear to grow in tension. He noted that at the tin-copper interface, the copper diffused so fast that Kirkendall void developed. This diffusion could result in build up of compression in the tin film. However, if this tin film is kept thinner than 2 microns the whisker problem is usually controlled. In contrast, tin on nickel sees voiding in the tin and hence tends to put tin in tension.
Chen Xu of Cookson used X-ray data to confirm the state of stress in the tin films. He reported that the tin in Sn/Ni on copper leads are tensile after aging if not before. Also the tin directly on copper leads get more compressive during aging up to 20 MPa. The flexure beam work of S. Lal of FCI added confirmation to these results although everyone was cautioned that the stress state of the complete beam system was not the same as the stress state within the surface tin.
John Lau mentioned that when performing the high temperature surface soldering of the assembly line the resulting tension supplied by the board may wipe-out any initial compression in the isolated tin lead. The moral may be to assemble soon after tin plating. Luu Nguyen mentioned that most high volume manufacturers for consumer products prefer to keep using standard matte tin that they have already qualified at great expense despite the growing understanding from the iNEMI groups. It was also suggested that since there is no grant money chasing the whisker problem there have been few academic champions of this complex phenomena.
Asa Frye of IBM used EBSD to obtain the find the local details of grain crystal structure near the whisker base. This approach allowed one to know the stress state of the grains in question and to confirm the zonal structure analysis which tried to determine the stress of each layer in a multilayer / multiprocessed structure. This data added to the suspicion several group members have developed that unintended copper contaminants in tin plating are often the cause of unwanted stress.
Marc Dittes described the humidity tests performed. He confirmed that condensation induced corrosion also was advantageous for whisker growth. The difficulty of making chambers uniform throughout means that high humidity tests result in condensation that causes corrosion. He also confirmed that board mounting of the lead stops whisker growth. Thus, he suggested that the component should not be the level tested by the manufacturers, but rather the final system configuration.
Peng Su of Freescale stated that the potential between exposed Cu and Sn was the main driver for corrosion in condensation and that this appeared a driver for much whisker growth. With board mounted devices he had seen no whisker growth even with pure tin remaining exposed above the solder wetted region. He noticed that the bare tin was always coated with carbon compounds coming from the flux residue, thus protecting the tin. It was also noted that mechanical trimming of the leads caused local compression regions even in otherwise tensile tin and corrosion and whiskers soon followed. Sharp cutting tools caused less problem than dull.
J W Osenbach of Agere mentioned philosophically that every lead will eventually corrode since tin wants to become oxide. He stressed that coupon tests miss the important edge effects of leads. He showed how easy it was to see Kirkendall where there are none. He also mentioned that once condensed water occurred it took several weeks for it to evaporate in typical test chamber conditions. Mobile devices could be seeing lots of condensation in normal use.
Joe Smetana of Alcatel presented an engaging visual image of how the whiskers grow out of tin grains. Andre Egli of Rohm & Haas saw whisker growth as a means of reducing compressive stress when general extrusion is not allowed by the system constraints. The SnO on the surface inhibits normal surface diffusion from relieving stress. He showed how the surface morphology changed enormously from the first 30 seconds after plating to 6 days later at room temperature. In particular, all the initial screw dislocations have disappeared. He showed a sequence of under the surface changes that determined if whiskers would be encouraged or discouraged. He saw the dynamics as a race between surface "fiberous" structure formation that blended the grains versus whisker formation.