Tin Whisker Formation on Small Outline Transistors Assembled Using Bismuth-Containing Lead-Free Solder Alloys After Long-Term Ambient Temperature, High Humidity Storage Part 3: In-Depth Characterization and Mechanisms

With the introduction of environmental legislation such as the Restriction of Hazardous Substances (RoHS), lead (Pb)-free materials have made their way into the electronics manufacturing industry. One issue that has emerged is that Pb-free solder alloys can initiate and grow tin whiskers under specific conditions. These whiskers are thin, highly conductive filaments which have the potential to grow and can cause field failures in many applications. Most concerning with respect to tin whiskering are high reliability applications such as aerospace, automotive, and medical. Bismuth (Bi) is being considered for inclusion in solder alloys to replace the current industry standard (SAC 305) and provide improved thermomechanical and vibration reliability. In this paper, we discuss whisker formation of several Bi-bearing alloys after long-term (12,000 hours), ambient high humidity (25°C/85% RH) storage. Three alloys containing Bi, in addition to SAC 305 (Sn-3.0Ag-0.5Cu), were considered. These alloys were Violet (Sn-2.25Ag-0.5Cu-6.0Bi), Sunflower (Sn-0.7Cu-7.0Bi), and Senju (Sn-2.0Ag-0.7Cu-3.0Bi). The boards were fabricated with electroless nickel immersion gold (ENIG) and immersion tin (ImmSn) finishes and populated with parts having Cu and Fe42Ni alloy leads and chip parts, with half of assemblies cleaned and half cleaned and contaminated with low levels of NaCl. This paper is the third in a series of three in which we share more in-depth characterization of features of interest from the small outline transistor (SOT) inspection. Discussion regarding the role of Bi in the overall stress state of the joint will also be provided.

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Tin Whisker Formation on Small Outline Transistors Assembled Using Bismuth-Containing Lead-Free Solder Alloys After Long-Term Ambient Temperature, High Humidity Storage Part 2: Statistical Analysis

Journal of Surface Mount Technology ◽

10.37665/smt.v33i1.14 ◽

2020 ◽

Vol 33 (1) ◽

pp. 7-19

Author(s):

André Delhaise ◽

Stephan Meschter ◽

Polina Snugovsky ◽

Jeff Kennedy ◽

Zohreh Bagheri

Keyword(s):

Statistical Analysis ◽

Manufacturing Industry ◽

Galvanic Corrosion ◽

High Reliability ◽

Hazardous Substances ◽

High Humidity ◽

Whisker Formation ◽

Solder Alloys ◽

Free Solder

With the introduction of environmental legislation such as the Restriction of Hazardous Substances (RoHS), lead (Pb)-free materials have made their way into the electronics manufacturing industry. One issue that has emerged is that Pb-free solder alloys can initiate and grow tin whiskers under specific conditions. These whiskers are thin, highly conductive filaments which have the potential to grow and can cause field failures in many applications. Most concerning with respect to tin whiskering are high reliability applications such as aerospace, automotive, and medical. Bismuth (Bi) is being considered for inclusion in solder alloys to replace the current industry standard (SAC 305) and provide improved thermomechanical and vibration reliability. In this paper, we discuss whisker formation of several Bi-bearing alloys after long-term (12,000 hours), ambient high humidity (25°C/85% RH) storage. Three alloys containing Bi, in addition to SAC 305 (Sn-3.0Ag-0.5Cu), were considered. These alloys were Violet (Sn-2.25Ag-0.5Cu-6.0Bi), Sunflower (Sn-0.7Cu-7.0Bi), and Senju (Sn-2.0Ag-0.7Cu-3.0Bi). The boards were fabricated with electroless nickel immersion gold (ENIG) and immersion tin (ImmSn) finishes and populated with parts having Cu and Fe42Ni alloy leads and chip parts, with half of assemblies cleaned and half cleaned and contaminated with low levels of NaCl. This paper is the second in a series of three in which we share quantitative statistical analysis from the whisker inspection of the small outline transistor (SOT) components. It was found that on ImmSn surface finishes, the longest whiskers were found on SAC, however the longest whiskers were found on Bi-bearing alloys for ENIG. In addition, whiskers were found to generally grow in regions where the tin coverage is thin, and, on ENIG-finished assemblies, near the PCB, likely due to galvanic corrosion between the solder and the finish chemistry.

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Tin whisker formation on a lead-free solder alloy studied by transmission electron microscopy

2006 8th Electronics Packaging Technology Conference ◽

10.1109/eptc.2006.342749 ◽

2006 ◽

Cited By ~ 2

Author(s):

H. Sosiati ◽

N. Kuwano ◽

S. Hata ◽

Y. Iwane ◽

Y. Morizono ◽

...

Keyword(s):

Electron Microscopy ◽

Transmission Electron Microscopy ◽

Solder Alloy ◽

Lead Free ◽

Lead Free Solder ◽

Whisker Formation ◽

Tin Whisker ◽

Transmission Electron ◽

Lead Free Solder Alloy ◽

Free Solder

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Lead-Free Solder/Gold Metallization Interdiffusion in Electronic Interconnects – Challenges and their Control

ISTFA 2008: Conference Proceedings from the 34th International Symposium for Testing and Failure Analysis ◽

10.31399/asm.cp.istfa2008p0053 ◽

2008 ◽

Author(s):

Nausha Asrar

Keyword(s):

Solder Joint ◽

Coefficient Of Thermal Expansion ◽

Void Formation ◽

Lead Free ◽

Gold Content ◽

Lead Free Solder ◽

Whisker Formation ◽

Tin Whisker ◽

Free Solder ◽

Gold Metallization

Abstract While considerable amount of researches and investigations have been made on lead-free solder joint reliability, limited number of literatures are available on the effect of gold content on lead-free solder joint performance. The challenges of lead-free solder/gold metallization interdiffusion during high temperature application/test are: gold embrittlement, intermetallics growth, void formation, and tin-whisker formation. Tin whiskers causing system failures in earth and space-based applications have been reported. This paper illustrates a few case histories of such challenges. The results confirmed that the synergistic effects of void formation, intermetallic compounds formation due to the thick gold plating, and coefficient of thermal expansion mismatch between organic and ceramic substrates resulted in brittle fracture of the solder joint. The tin whisker formation was attributed to the compressive stress in the tin solder material, which was caused by diffusion of the end-cap metallization, formation of intermetallics, and thermal cycling of the soldered components.

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Microstructural evolution in lead-free solder alloys: Part I. Cast Sn–Ag–Cu eutectic

Journal of Materials Research ◽

10.1557/jmr.2004.0190 ◽

2004 ◽

Vol 19 (5) ◽

pp. 1417-1424 ◽

Cited By ~ 66

Author(s):

Sarah L. Allen ◽

Michael R. Notis ◽

Richard R. Chromik ◽

Richard P. Vinci

Keyword(s):

Effective Activation Energy ◽

Ternary Eutectic ◽

Eutectic Structure ◽

Lead Free ◽

Lead Free Solder ◽

Solder Alloys ◽

Controlling Mechanism ◽

Coarsening Kinetics ◽

Free Solder

Coarsening of the ternary eutectic in cast Sn–Ag–Cu lead-free solder alloys was investigated. The process was found to follow r3 ∝ t kinetics where r is the rod radius of the dispersed phase and t is time. The effective activation energy for the process is 69 ± 5 kJmol-1. The two types of intermetallic rods, Cu6Sn5 and Ag3Sn, in the eutectic structure coarsen at different rates, with each having a different rate-controlling mechanism. The overall coarsening kinetics for the Sn–Ag–Cu ternary eutectic is significantly slower than that found for the Pb-Sn eutectic, which has implications for long-term reliability of Sn–Ag–Cu solder joints.

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