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Intermetallic Compounds in Solder Alloys: The Common Misconception
This paper describes the IMCs that occur typically in eutectic, Sn63Pb37, near-eutectic SAC305, and high-performance tin-based Pb-free solder alloys.
Technical Paper
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Authored By:
Dave Hillman,
Hillman Electronic Assembly Solutions, LLC,
IA, USA
Tim Pearson
Collins Aerospace
IA, USA
Richard Coyle, Ph.D.
Nokia Bell Labs
NY, USA
Summary
Intermetallic compounds (IMC) or intermediate phases are formed between two or more metallic elements in many metal alloy systems. During soldering, an IMC is formed at the soldered interface as the molten solder reacts with an element in the substrate. IMCs also can form within the bulk solder as the joint solidifies. IMCs have critical roles in the solder joint quality and reliability. Unlike most metal alloys, an intermetallic compound typically has a fixed stoichiometry and is in variance with the conventional phases or constituents in the metal system (e.g., alpha and beta).
An IMC has a different crystal structure than any of its constituents and seldom has all the characteristics and properties of its constituents. Ductility is an important solder joint property, and the low intrinsic ductility of IMCs has been associated with brittle behavior and reliability risk in service. However, a review of published solder field failures shows little evidence that IMC properties or IMC evolution under service conditions reduce solder joint reliability. The emphasis on the influence of IMC phases on solder joint reliability in the electronics industry is clearly overstated. Most IMC-induced solder joint failures are found to result from incorrect material specification or uncontrolled soldering processes.
This paper describes the IMCs that occur typically in eutectic, Sn63Pb37, near-eutectic SAC305, and high-performance tin-based Pb-free solder alloys. The paper also describes the potential impact of IMCs on the solder joint reliability for these alloys.
Conclusions
The emphasis on the influence of IMC phases on solder joint reliability in the electronics industry is clearly overstated and often excessive. A review of industry publications/cases clearly shows that solder joint reliability is impacted by IMC phases with root causes of incorrect soldering processes or inadequate process procedure. The list of solder joint reliability root cause failures should not include IMC phases as a primary solder joint failure mode.
Initially Published in the SMTA Proceedings
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