Preparation for Mitigating Tin Whiskers in Alternative Lead-Free Alloys
The understanding of the growth mechanism for tin whiskers and what mitigation strategies can be employed to reduce their propagation.
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Authored By:
Mehran Maalekian, Karl Seelig, V.P. Technology, Timothy O'Neill, Technical Marketing Manager
AIM Solder
Cranston, RI USA
Summary
With the impending deadline for RoHS II and the elimination of exemptions for lead bearing solders in electronics for mission critical electronics, the issue of tin whiskers remains unresolved. Building on earlier data developed using unique test methods; the company is collaborating with universities in developing other methods of promoting whisker growth. In previous testing with specially prepared pure tin, these methods grow whiskers exponentially more quickly than the company bent wire method.
The availability of new test methods would allow for dozens of alloy combinations to be tested more rapidly and in combination, thus accelerating the development of an alloy with a demonstrable benefit in reducing tin whisker formation and the attendant risks. In addition, this work has expanded the understanding of the growth mechanism for tin whiskers and what other mitigation strategies can be employed to reduce their propagation.
Concurrent to test method development, the company has continued alloy development and has finalized several alloys that have demonstrated minimizing whisker formation when compared to widely used tin-silver-copper and tin-copper alloys. These novel alloys have also been developed within the context of manufacturability, regulatory restrictions and end user processes. These alloys meet the requirements of demonstrable reduction of tin whisker formation and would be 'drop-in' replacements for existing alloys.
Conclusions
Properties of solder alloy SBAC were studied thoroughly compared with reference SAC305.
Advantages of SBAC over SAC305:
Lower melting temperature
Smaller solidification range (lower undercooling)
Lower cost
Higher tensile strength
Finer grain structure after aging
Comparable properties:
Wetting force and wetting time
Spreading
Intermetallic compound layer thickness
Except for copper boards, voiding is in the same range. For copper plate voiding percentage is lower with SBAC.
Initially Published in the IPC Proceedings
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