Gallium Liquid Metal Embrittlement of Tin-based Solder Alloys
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INTRODUCTION
LIQUID metal embrittlement (LME) is defined as a phenomenon where ductile metals exposed to specific liquid metal environments may become brittle.[1,2] It has been predominantly studied as a failure mechanism to avoid in industrial processes.[3,4] Nevertheless, a limited number of investigations have leveraged the destructive nature of this phenomenon for a useful purpose, in particular for facilitating the removal of defective microelectronic devices by weakening the metallic interconnections of those devices.[5–7] While previous publications have provided detail as to how the phenomenon would be leveraged for such an application,[7,8] this paper seeks to more deeply understand the mechanisms behind the LME of the tin-silver-copper (Sn-Ag-Cu) alloy, the most prevalent material used for device interconnections.[9,10] It is generally recognized that LME is a ‘‘specific’’ phenomenon, where only certain liquid metals (LMs) can embrittle a particular solid metal (SM). Despite the fact that this phenomenon has yet to be fully explained, researchers have established non-exhaustive criteria to identify an embrittling SM/LM couple, based on empirical observations.[3,11,12] Generally, embrittling couples ELODIE NGUENA, DAVID DANOVITCH, and JULIEN SYLVESTRE are with the Institut interdisciplinaire d’innovation technologique (3IT), Universite´ de Sherbrooke, 3000 boul. de l’universite´, Pavillon P2, J1K0A5, Sherbrooke, Canada. Contact e-mail: [email protected]; [email protected] RICHARD LANGLOIS and SERGE MARTEL are with IBM Canada Ltd., 23 boul. de l’ae´roport, J2L1A3, Bromont, Canada. Manuscript submitted on February 14, 2020; accepted September 20, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
must have low mutual solubility, low electronegativity difference and not form any Intermetallic Compound (IMC), although some exceptions have been observed.[2,12] Among the SM/LM couples studied, the nontoxic liquid gallium (Ga) is one of very few LMs (along with mercury) observed to embrittle all three solid metals of interest, namely Sn, Ag and Cu.[3,4] However, there is no complete study on the embrittlement by a LM of a solid alloy, let alone the specific case of liquid Ga embrittlement of the Sn-Ag-Cu alloy. It is therefore the purpose of this paper to evaluate the changes in physical and mechanical properties of Sn-based solder alloys embrittled by liquid Ga and construct a scientifically sound model for such embrittlement. Earlier work investigating LME mechanisms has often focused on crack tip propagation.[3,13] though results often revealed that the intrusion of the liquid metal in a propagating crack can be a critical challenge for proper interpretation.[12] Other models were solely based on the responses of the SM to mechanical solicitations while exposed to a LM.[4] In this paper, a new model is proposed based not only on fracture mechanisms and mechanical properties, but also more importantly on the microstructural observations of the complex physical processes that take place during t
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